Articulated remains of the extinct shark Ptychodus (Elasmobranchii, Ptychodontidae) from the Upper Cretaceous of Spain provide insights into gigantism, growth rate and life history of ptychodontid sharks.

Due to their cartilaginous endoskeleton and the continuous tooth replacement, the chondrichthyan fossil record predominantly consists of isolated teeth, which offer diagnostic features for taxonomic identifications, but only provide very limited information of an organism's life history. In contrast, the calcified vertebral centra of elasmobranchs (sharks, skates and rays) yield important information about ecological and biological traits that can be utilized for constructing age-structured population dynamic models of extant species and palaeoecological reconstructions of such aspects in extinct groups. Here, we describe two large shark vertebrae from the Santonian (Upper Cretaceous) of Spain, which show a unique combination of characters (asterospondylic calcification pattern, with concentric lamellae and numerous parallel bands that are oriented perpendicular) that is only known from ptychodontid sharks, a distinct, extinct group of giant durophagous sharks of the Cretaceous era. Based on linear regression models for large extant sharks a total length between 430 and 707cm was estimated for the examined specimen. Our results indicate that ptychodontid sharks were large viviparous animals, with slow growth rates, matured very late and, therefore, show typical traits for K-selected species. These traits combined with a highly specialized feeding ecology might have played a crucial role for the success but also, eventually, extinction of this group.

Introduction

More than 400 million years of evolution have shaped a diverse set of biological traits in modern elasmobranchs (sharks, skates and rays) that allowed them to occupy a variety of different niches and trophic levels. They have conquered marine and freshwater environments around the world, reaching body sizes from 0.2m (dwarf lantern shark Etmopterus perryi) to 20m (whale shark Rhincodon typus), and have developed a number of different reproductive strategies that can be roughly categorized in oviparity (egg laying), and viviparity (giving birth to live pups) [1,2]. Three general life history patterns can be found in sharks: 1) small body size, low longevity, small litters, small offspring, fast growth; 2) large body size, moderate to high longevity, large litters, small offspring, slow growth; 3) large body size, high longevity, small litters, large offspring, slow growth [3].

The application of life history traits has proven potentially useful in fisheries to determine if stocks are endangered and to estimate their chances to recover [4,5]. Additionally, life history traits of extinct taxa are vital for palaeoecological reconstructions and can give important insights into a species’ demise, perseverance, and are crucial to augment our understanding of diversity and extinction patterns [6-8]. However, the fossil record of chondrichthyans predominantly consists of isolated teeth, which only offer limited information about the biological traits of a species. Vertebrae on the other hand yield important data on the biology and ecology of fossil elasmobranchs, but only have been described for a very limited number of extinct species [9-14].

Ptychodontid sharks seemingly were giant durophagous fish that lived in the Cretaceous period from the Aptian (~113-125mya) to the Campanian (~72-83mya) [15] and are believed to have obtained body sizes of more than ten meters [16,17]. Although fossils of this group are common in Cretaceous deposits and are known from around the globe, the taxonomic placement of this group remains ambiguous and they have been discussed to be either batomorphs [18], hybodont sharks [16], or put in the new order Ptychodontiformes within the Neoselachii (sensu Compagno [19]; Elasmobranchii sensu Maisey [20]) [21]. However, the presence of calcified vertebrae [22] and a three-layered enameloid [23] support the affinity of this group to modern sharks.

Here we describe the first articulated shark remains from the Santonian of Spain, Europe. Although no teeth were found associated with the vertebrae, taxonomic placement was possible due to a unique combination of characters that is only known for ptychodontids and allowed the exclusion of any other shark taxon known from this period. Furthermore, the vertebrae yield important information about the ontogeny, growth and body size and, therefore, provide insights into the life history of this enigmatic shark group.

Geographic and geological setting

The material that forms the focus of this study comprises a portion of an axial skeleton consisting of five articulated and several disarticulated vertebral centra belonging to the same individual, which were collected 10 km west of Santander in northern Spain, from a section on the coast near the village of Soto de la Marina (Fig 1). Here, sediments ranging from the Cenomanian to Maastrichtian accumulating to 1200 m in thickness are well exposed. The section that yielded the shark remains starts with light greyish to whitish, massive and arenitic limestone beds forming the lower unit reaching a thickness of 24.7m [24]. The lower unit ranges from the late Campanian to early Santonian in age and is characterized by the occurrence of the holasteroid, Cardiaster integer (‘integer limestones’), which is a well-known species from the calcareous platforms of the Basque-Cantabrian Region, northern and southern Pyrenees, and Alpes-Maritimes [25-27].

Fig 1

Fossil locality near Santander, northern Spain where EMRG-Chond-SK-1 was recovered (indicated by a black star).

Within the ‘integer limestones’ two layers characterized by abundant occurrences of the inoceramid, Platyoceramus (Cladoceramus) undulatoplicatus were identified [24,28] and designated as undulatoplicatus events I and II. The first appearance of Platyceramus undulatoplicatus marks the base of the Santonian stage [26,27,29]. The vertebrae, which were recovered from a ca. 1.2 x 0.5m large, concretionary limestone lens coming from the upper, 2nd undulatoplicatus Event, consequently, are of earliest Santonian age.

Material and methods

Documentation and preparation

The articulated vertebral section was photographically documented in the field but not collected (Fig 2). Two disarticulated vertebral centra of varying degree of preservation were recovered and small sediment samples adjacent to the incomplete vertebral column were taken for screen-washing. The material was collected by a diploma student (Kurt Oppermann, Berlin) within a larger research project of the late Prof. G. Ernst (†25.04.2002) at the Free University of Berlin in 1996 when no additional permits were required. The centra are publicly accessible and housed in the fossil vertebrate collection of the Department of Palaeontology (University of Vienna) under the collection number EMRG-Chond-SK-1. Of one vertebral centrum, a dorso-ventrally directed thin section was prepared. The sediment samples and carbonate matrix of the other specimens were removed with 10% acetic acid and the residues sieved with a 0.25 mm mesh and sorted under a binocular. The recovered material comprises some dermal scales of the elasmobranch placoid type, which are deposited in the fossil collection of the Institute of Palaeontology, Free University Berlin without numbers. Some of these placoid scales were studied with a Jeol high-vacuum scanning electron microscope at the Institute of Geological Sciences of the Free University Berlin, Germany (S1 Fig).

Fig 2

Additional articulated (A, B) and disarticulated shark vertebrae (C,D) found in situ associated with EMRG-Chond-SK-1. Picture courtesy of K. Oppermann, 1997. Scale bars equal 20cm (plate A) and 3cm (plates B, C, D).

Size estimation

Previous studies showed a linear relationship between total length (TL) and vertebral centrum diameter (CD) in several shark species [30-34] that can be expressed with a regression equation [10,35]. Due to the fragmentary nature of the material, we measured the radius and multiplied it by two to obtain the diameter of the centrum. The larger vertebra EMRG-Chond-1b had a sedimentary infill between increment rings 25 and 26. Taking this into account, the radius was measured to the 25th increment ring and the distance between the 26th and outermost increment ring (31) was added. The distance between band pair 25 and 26 was estimated as the average value of the band intervals of the five preceding and five succeeding increment rings. As it was impossible to determine the position of the two isolated vertebrae, we conservatively regarded the vertebra with the higher centrum diameter as the largest vertebra in the entire individual. This approach ensured a minimum estimate for the total length and reduced the risk of overestimating the body size.

Most pelagic sharks have a consistent body shape [10,36], therefore, it was regarded as reasonable in previous studies to assume that the relationship between vertebral size and total body length is consistent between species with similar ecologies. However, it has been reported that this relationship can vary between species [9,10,12]. To overcome this issue, we conducted two independent approaches to estimate the total length of the examined specimen:

(1) We extrapolated the total length of EMRG-Chond-SK-1b by comparing it with the estimated total length (TL) and centrum diameter (CD) of †Ptychodus occidentalis [16].

(2) We used published regression equations for large extant shark species with comparably sized vertebrae to estimate the relationship between centrum diameter (CD; mm) and total length (TL; cm). The following species with known regression equations were used as templates: (1) the great white shark Carcharodon carcharias [35]; (2) tiger shark Galeocerdo cuvier [37]; (3) whale shark Rhincodon typus [34].

Results and discussion

Systematic palaeontology

Class CHONDRICHTHYES Huxley, 1880 [38]

Subclass ELASMOBRANCHII Bonaparte, 1838 [39]

Order incertae sedis

Family PTYCHODONTIDAE Jaekel, 1898 [18]

Genus PTYCHODUS Agassiz, 1835 [40]

Diagnosis (emended). For dental characters of this genus see Hamm [41]. Vertebral centra are well calcified, amphicoelous and circular, not dorso-ventrally compressed. Vertebrae display a calcification pattern of the asterospondylic type with four uncalcified areas radiating diagonally from the center to the bases of the neural and haemal arches. Numerous concentric lamellae extend outwards from the center. Underneath the smooth articular surfaces, parallel lamellae are oriented 360° around the center of the vertebrae and run perpendicular to the concentric lamellae.

Material. EMRG-Chond-SK-1; two vertebral centra.

Locality. Soto de la Marina, west of Santander, Cantabria, N Spain.

Age and horizon. Early Santonian, ‘integer limestone’, undulatoplicatus Events II.

Description. Both centra were found articulated with several other vertebrae, which were not recovered and left in the field (Fig 2). Vertebra centra show a marked concavity on the articular surfaces (amphicoelous) and appear nearly circular in median transverse view. The dimensions are approximately 55mm (EMRG-Chond-SK-1a) and 70mm (EMRG-Chond-SK-1b) in diameter (dorsoventrally) and the vertebral anterior-posterior length is 23mm (EMRG-Chond-SK-1a). Due to the fragmentary condition of the material, no dimensions could be measured for the mediolateral diameter.

Shark centra form a double-cone calcification with densely calcified anterior and posterior conical ends, collectively referred to as corpora calcarea. Between the corpora calcarea is the intermedialia, which is softer than the corpus calcareum. The articular facet of the corpus calcareum is weathered and expose the inner layer of the vertebrae, showing concentric calcareous rings extending outwards from the center of the vertebrae. Numerous parallel lamellae are oriented perpendicular to these concentric lamellae (Fig 3A). EMRG-Chond-SK-1b was sectioned transversely and exhibits an asterospondylic calcification pattern (Fig 3B): secondary calcification leaves four uncalcified areas (i.e. two basidorsal and two ventral cartilage wedges), which radiate diagonally from the center to the base of the neural and haemal arches. Concentric lamellae are restricted to the area between the wedges. A total of 31 growth increments can be identified in the vertebra section. Post-mortem sedimentary infilling can be observed between ring 25 and 26 which inflated the distance between those two rings (Fig 3B, S2 Fig). The dorsoventral and mediolateral radii of the first increment ring (birth ring) are 3.6 and 3.2mm, respectively.

Fig 3

Calcification pattern of the vertebral centra of EMRG-Chond-SK-1.

(A) close up view and illustration of the vertebral centra EMRG-Chond-SK-1a. The articular surface is weathered and exposes the inner layer of the vertebrae, showing concentric calcareous rings and parallel bands that run perpendicular to them; (B) vertebral section in transverse plane and illustration of EMRG-Chond-SK-1b show an asterospondylic calcification pattern with four cartilage wedges radiating diagonally from the center to the base of the neural and haemal arches.

Taxonomic remarks

In contrast to teeth, vertebral centra are thought to bear only little taxonomic information for extinct elasmobranch fishes as comparative analyses are hard to perform due to the lack of articulated material with associated teeth. Hasse [42] recognized three different calcification patterns of vertebrae in cross-section (cyclospondyl, tectospondyl, and asterospondyl), which later was revised by Ridewood [43], who stated that these three categories were not sufficient to describe the plethora of different calcification patterns that can be found in sharks, skates and rays. The vertebral centrum EMRG-Chond-SK-1b displays the asterospondylic type (sensu Hasse [42]) with four uncalcified areas radiating from the center to the bases of the neural and haemal arches, which is typical for galeomorph sharks [44].

Another character that is apparent in the cross section is the presence of concentric lamellae that are extending outwards from the center. The combination of these two features (asterospondyly with uncalcified wedges and concentric lamellae) are only known from the basking shark Cetorhinus maximus [43,45], the whale shark Rhincodon typus [31], †Ptychodus [46,47], and †Squalicorax [47,48]. Both †Ptychodus and †Squalicorax are known from Cretaceous deposits in Europe, N- and S-America, Africa and Asia [15]. Our specimen shows numerous parallel bands that are oriented 360° around the center of the vertebrae. These parallel lamellae are oriented perpendicular to the concentric lamellae, a trait only known from ptychodontid sharks and is regarded as a diagnostic feature for this group, which is absent in other sharks, including C. maximus, R. typus, and †Squalicorax [21,49]. This assumption is also supported by previous reports of ptychodontid shark vertebrae, which display this feature [22,46,50]. Rozefelds [51] reported large vertebral centra from the lower Cretaceous Toolebuc Formation of Australia, which resemble our specimens. He found associated placoid scales but no oral teeth and assigned the species tentatively to the anacoracid genus †Pseudocorax. This seems very unlikely because †Pseudocorax was a rather small shark with tooth crown heights of 1.5cm [15], which is comparable to the teeth of a two meter long †Squalicorax falcatus [9]. Furthermore, like in our specimen, parallel lamellae are visible, which is not known from anacoracid sharks. The combination of the above mentioned characters (parallel lamellae, concentric lamellae, asterospondyl centra), the size of the vertebral centra and the stratigraphic age (Cretaceous) of these species, leads us to the assumption that both specimens, EMRG-Chond-SK-1 and Rozefelds’ QMF18264, are ptychodontid sharks. Other big sharks from the Late Cretaceous are known from the order Lamniformes (e.g., †Cretalamna, †Cretodus, †Cretoxyrhina), but can easily be ruled out because they are known to have a different mineralization pattern of the vertebral centra (i.e., the vertebral centrum is strengthened by multi-branched, densely packed lamellae), have radial lamellae on the dorsoventral axis in lateral view (which our specimen does not have), and lack parallel lamellae [12,13,48,52-55]. The placoid scales from the sediment samples associated with the Spanish specimen represent three different morphotypes (see S1 Fig). Placoid scale type 3 of the Spanish specimen resembles those figured by Shimada et al. [16,17] and the ‘curved crown scale’ type of Rozefelds [51] to some extent. The other two scale types in the specimen described here have not been figured before. These morphological differences might be related to different positions of the scales on the body. As no teeth were found associated with this specimen, an exact taxonomic identification on species level remains impossible, which leaves us to refer to it as †Ptychodus sp.

Age, growth and inferred life history traits

Age estimation

Whole vertebral centra, as well as transverse and sagittally sectioned centra have been commonly used for aging elasmobranchs by counting alternating opaque and translucent band pairs (also referred to as band pairs, annuli, rings, or vertebral growth increments) that are deposited in the vertebral centra as they grow [10,21,56-58]. A number of studies suggested that these band pairs are deposited annually in several elasmobranch species e.g., in the shortfin mako shark Isurus oxyrinchus [32], scalloped hammerhead shark Sphyrna lewini [59], dusky shark Carcharhinus obscurus [60], leopard shark Triakis semifasciata [61], smalltooth sawfish Pristis pectinata [62], etc. [63-71]. However, this is questioned by recent studies [72-74] and especially in old individuals of long living species the age determination seems to be highly underestimated as the growth rate decreases with age resulting in a band width decline at the centrum edges that can become unresolvable in older individuals [56,72,75-79]. The section of EMRG-Chond-SK-1b revealed 31 band pairs (birth mark + 30 band pairs), suggesting, under the assumption of an annual growth band deposition, that EMRG-Chond-SK-1 was around 30 years old. These band pairs were well distinguishable, indicating that EMRG-Chond-SK-1 had not reached the maximum length when it died, because no compressed arrangement of band pairs is preserved at the edges.

Body size estimation

Based on the previously published centrum diameter and estimated total length of †Ptychodus occidentalis [16] we calculated an estimated total length of 887-1183cm for EMRG-Chond-SK-1. However, this estimation should be taken with caution, as the TL-CD relationship of †P. occidentalis is based on a single vertebral centrum which not necessarily represents the largest vertebra in this specimen and, therefore, can result in overestimated size approximations. Therefore, we recommend taking the previously estimated TL of 13m for †P. rugosus [16], which was also based on this TL-CD relationship, with much caution. Shimada et al. [16] also compared the anterior-posterior length of the teeth of †P. occidentalis and †P. rugosus and concluded that †P. rugosus might have reached a body size of even 14.4m. However, it is important to note here that the total length for †P. occidentalis, on which both calculations are based on, is unknown and was estimated based on the length of the lower jaw. Therefore, an erroneous size estimation for †P. occidentalis would also bias all subsequent calculations.

Further indication of overestimated body sizes for †Ptychodus is given by our calculations of the total length of EMRG-Chond-SK-1 based on regression equations for large extant shark species. In contrast to the above-mentioned extrapolation, this approach has the advantage, by assuming EMRG-Chond-SK-1b to be the largest vertebra, to offer minimum size estimations and, therefore, reducing the risk of overestimating the size. Using the vertebral diameter of 70 mm and the regression equations for the great white shark Carcharodon carcharias [35], tiger shark Galeocerdo cuvier [37] and whale shark Rhincodon typus [34], the minimum total length of EMRG-Chond-SK-1 is calculated to be 430cm, 539cm, and 707cm respectively (Fig 4). The use of regression equations for three different species (from three different orders) has shown significant variations in estimated body sizes and, thus, indicates that previous assumptions of the more or less consistent body forms in pelagic sharks resulting in similar size estimations [36] were oversimplified. In fact, a variety of different factors (e.g., phylogenetic affiliations, lifestyle, trophic level, etc.) might contribute to the relationship between vertebral diameter and total body length and, therefore, affect size estimations. However, it seems reasonable to assume that the total length of EMRG-Chond-SK-1 lies within the estimated range of 430-707cm as the three template species of the regression equations cover a wide range of different ecologies (microphagous and macrophagous sharks) and are not closely related to each other (diversification of these three groups occurred in the Early and Middle Jurassic, respectively [8]). Given that our specimen most likely has not yet reached maturity and therefore represents a subadult, previous size estimations of around 10m [17] seem possible for this group. Although more accurate maximum size estimations need to wait until a complete specimen can be analyzed, our study agrees with previous work that †Ptychodus was one of the largest durophagous vertebrates ever to have lived.

Fig 4

Estimated total length for the examined ptychodontid shark from Spain.

Hypothetical outlines of †Ptychodus sp. showing the minimum and maximum size estimations for the specimen EMRG-Chond-SK-1.

Growth rate

The intervals between adjacent band pairs remain more or less stable from the innermost band pair to the outermost band pair, although some variability does exist (Table 1). This differs from previous studies [10-13], which reported the intervals to decrease from the innermost to the outermost band pairs and indicates that the growth rate of EMRG-Chond-SK1 has not decreased until its death (Fig 5A). Attempts to fit the data to a von Bertalanffy growth model resulted in erroneous maximum size estimations (TLmax>2000cm). Plotting the centrum radius CR of each growth ring against the growth ring count (“age”) resulted in a graph that followed a linear model which, because of the linearity between CR/CD and TL, follows the same trend if TL is plotted against growth ring count (or “age”) (Fig 5B). Most growth models for fish are nonlinear [80] and shark growth models are usually best described by an asymptotic growth model (e.g., von Bertalanffy growth model) [58]. To date, all examined sharks and rays show asymptotic growth during ontogeny and it therefore seems justified to assume that †Ptychodus did so as well. Our data suggest that EMRG-Chond-SK1 has not reached the plateau of the asymptote of the growth curve yet and, consequently, not its maximum length. Furthermore, clear inflections of the growth curve, that indicate a decreased growth rate (e.g., after reaching maturity when energy from somatic growth is diverted to gonadal growth), are absent in the data set from specimen EMRG-Chond-SK1 and, thus, suggests that this individual has not reached maturity at band pair 25 with an estimated body size between 369 and 607cm. These estimations are reasonable when compared to modern giant sharks (“gigantism” sensu Pimiento et al. [81] refers to sharks with body sizes exceeding six meters), which show similar sizes at maturity, e.g., the great white shark, Carcharodon carcharias, at 350-500cm (TL about 600cm), basking shark, Cetorhinus maximus, at 400-800cm (TL more than 1000cm), whale shark, Rhincodon typus, at 600-800cm (TL 1700-2100cm) [2]. When compared to big macropredatory sharks (i.e., great white shark Carcharodon carcharias and Cretoxyrhina mantelli), it is apparent that the slope of the growth curve of †Ptychodus is less steep and more similar to the microphagous basking shark Cetorhinus maximus (Fig 5C). Therefore, the growth rate of †Ptychodus is assumed to be lower than those of apex predatory sharks. Under the assumption of an annual growth band deposition, †Ptychodus matured very late (after more than 25 years) and showed great longevity, similar to the giant filter-feeding sharks that live today [34,82].

Table 1

Measurements and derived estimations taken from vertebra EMRG-Chond-SK-1b.

BN	CR (mm)	BI (mm)	pCD (%)	TLCC (cm)	TLGC (cm)	TLRT (cm)
1	3.7	-	10.51%	64.92	88.25	107.23
2	4.4	0.7	12.50%	73.04	98.28	120.57
3	5.3	0.9	15.06%	83.48	111.16	137.72
4	6.1	0.8	17.33%	92.76	122.61	152.97
5	6.9	0.8	19.60%	102.04	134.06	168.22
6	8.1	1.2	23.01%	115.96	151.24	191.10
7	9.5	1.4	26.99%	132.20	171.28	217.79
8	10.5	1	29.83%	143.80	185.59	236.85
9	12	1.5	34.09%	161.20	207.06	265.44
10	12.9	0.9	36.65%	171.64	219.94	282.60
11	14.2	1.3	40.34%	186.72	238.55	307.38
12	15.6	1.4	44.32%	202.96	258.59	334.06
13	16.8	1.2	47.73%	216.88	275.77	356.94
14	18	1.2	51.14%	230.8	292.95	379.81
15	19.3	1.3	54.83%	245.88	311.55	404.59
16	20.4	1.1	57.95%	258.64	327.30	425.56
17	21.4	1	60.80%	270.24	341.61	444.62
18	22.5	1.1	63.92%	283.00	357.36	465.59
19	24.1	1.6	68.47%	301.56	380.26	496.09
20	25.3	1.2	71.88%	315.48	397.44	518.97
21	26.4	1.1	75.00%	328.24	413.18	539.94
22	27.2	0.8	77.27%	337.52	424.63	555.19
23	28.1	0.9	79.83%	347.96	437.52	572.34
24	29	0.9	82.39%	358.40	450.40	589.50
25	29.9	0.9	84.94%	368.84	463.28	606.65
26	N/A	N/A	N/A	N/A	N/A	N/A
27	N/A	0.8	N/A	N/A	N/A	N/A
28	N/A	0.7	N/A	N/A	N/A	N/A
29	N/A	1.1	N/A	N/A	N/A	N/A
30	N/A	1.1	N/A	N/A	N/A	N/A
31	*35.2	0.8	100.00%	430.32	539.15	707.68

Abbreviations: BN, band number; CR, centrum radius; BI, band interval; pCD, percent centrum diameter from the center of the vertebra TL, total length estimation based on the regression equation of Carcharodon carcharias [35]; TL, total length estimation based on the regression equation of Galeocerdo cuvier [37]; TL,total length estimation based on the regression equation of Rhincodon typus [34].

*reconstructed value; see “Materials and methods” for more details on its computation.

Fig 5

Growth ring profile of EMRG-Chond-SK-1b.

(A) centrum size profiles of EMRG-Chond-SK-1b follow the same trend for radii in dorsal, ventral and mediolateral direction. (B) data points of the centrum size profile closely follow a linear function. (C) growth ring distances of EMRG-Chond-SK-1b compared to †Cretoxyrhina mantelli [10] and Cetorhinus maximus [82].

Inferred life history

Vertebral centra of sharks have proven useful for estimating body sizes [13,36], growth rates [10-13,32,33,69] and, therefore, allow the reconstruction of biological and ecological traits, as various of these aspects of an organism are correlated with body size [83-86]. Although we could not determine the identity of EMRG-Chond-SK-1 on species level, it gives us valuable insights into the ontogeny and ecology of ptychodontid sharks. Our analysis on the vertebral growth indicates that †Ptychodus was slow growing, late maturing and seemingly long-living, all of which are key traits for a K-selected species. Based on the radius of the birth ring (3.7mm) we estimated a total length of 65-107cm for our specimen at the time of birth. Offspring with similar sizes are known from a variety of large viviparous sharks, e.g. great hammerhead shark Sphyrna mokarran (50-70cm), tiger shark Galeocerdo cuvier (51-76cm), great white shark Carcharodon carcharias (110-160cm), basking shark Cetorhinus maximus (150-170cm), and whale shark Rhincodon typus (55-64cm) [2]. Similar sizes at birth have not been reported for oviparous sharks (usually not exceeding 15-25cm), which leads us to the assumption that †Ptychodus also was a viviparous shark that put a lot of resources into the development of large offspring.

To date, we can only speculate about the reasons for the extinction of this group. However, K-selected species are characterized by specific adaptations (slow growing, late maturity, large body, small size of litter) that make such sharks more prone to environmental changes and have been correlated with increased extinction risk compared to oviparous (r-selected) sharks [87]. Our results strongly suggest that extinct ptychodontid sharks had K-selected traits, which in combination with a highly specialized trophic niche (durophagy) might have been major intrinsic contributors to the demise of this group.

Placoid scales found associated with vertebrae of EMRG-Chond-SK-1.

(A) type 1, “six-keeled scales”; (B) type 2, “three-keeled scales”; (C) type 3, “knob-like scales”. Scale bar equals 100μm.

(TIF)

Click here for additional data file.

Transverse section of EMRG-Chond_SK-1b, showing the number of growth increments.

(TIF)

Click here for additional data file.

18 Mar 2020

PONE-D-20-05473

First articulated remains of the extinct shark, Ptychodus (Elasmobranchii, Ptychodontidae) from the Upper Cretaceous of Spain provide insights into gigantism, growth rate and life history of ptychodontid sharks

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Reviewer #1: Dear Editor and Authors,

The study reports some interesting findings about the enigmatic genus Ptychodus from the Cretaceous of Spain and has a clear paleobiological approach. Despite the fossil material is fragmentary, it displays peculiar and useful characters. These characters have never been remarked in other papers and allow the authors to make some very interesting paleobiological inferences. Data are presented in an appropriate fashion and statistics are adequate, even though the Von Bertalanffy growth model was not applicable (but the authors clearly state the reason of its unsuitability). The inferences are well-supported and the language is fluent. All in all, this study is of international interest and provides novel and significant information.

There are only some minor points I would like to raise:

- The title deals with first articulated specimens from Spain, but there is no mention in the manuscript to other findings of the genus Ptychodus from Spain. It would be useful to provide some other references to these findings and compare with the specimens reported herein.

- The diagnostic vertebral characters reported here should be included in the diagnosis of the genus (see Hamm, 2020 and please emend).

- Please reconsider the structure of the manuscript and the issues with the images and other minor indications addressed in the annotated version of the manuscript attached.

I would be pleased to answer any question from the authors and be open to any discussion.

Best regards,

Jacopo Amalfitano, PhD

Reviewer #2: The manuscript of Jambura & Kriwet is an interesting and important contribution to the knowledge of the paleobiology of the enigmatic extinct shark Ptychodus. The manuscript is of high-impact, well written, and provides sufficient data to suggest new hypotheses about the palaeobiology of Ptychodus. However, the manuscript still presents some issues that the authors should address before the publication. A further proofcheck is also needed to avoid small typos and errors. This is why I consider the manuscript worth of publication after minor revision. In particular:

- In the title and several times in the text (e.g. line 70) the authors emphasize that these are the first articulated Ptychodus remains. However, there are other specimens of Ptychodus described in the literature that can be considered as articulated, and these include nearly complete tooth sets (e.g. Amadori et al. 2019), and even articulated vertebrae (Hamm 2010, fig. 6B). This is why authors should remove ‘first’ from the title and subsequent statements in the manuscript.

- The authors used regression equations for extant galeomorph sharks to estimate the relationships between centrum diameter and total length of Ptychodus. However, the authors should clearly state, in Material and Methods, the reason why they use galeomorph sharks as comparative taxa, and not squalomorphs, or other extinct groups. As far as we know the affinities of Ptychodus are far from being clear. Is the presence of asterospondylic vertebrae enough to detect its close relationship with galeomorph sharks? Are there other hypotheses in the literature to be considered? If so why are these hypotheses discarded?

- Moreover, the authors use Carcharodon carcharias, Galeocerdo cuvier, Rhincodon typus as the only representatives of galeomorph sharks for comparisons. However, there are other extinct and living galeomorphs for which the relationship between centrum diameter and total body length are known, as reported by the authors themselves in the literature, that should be considered. For example: the extant Prionace glauca, Isurus oxyrhincus, Carcharhinus limbatus, and the extinct Carcharodon megalodon, and Cretoxyrhina mantelli (see Stevens 1975; Killam & Parsons 1989; Gottfried et al. 1996; Ribot-Carballal et al. 2005; Shimada 2008). There are probably even more taxa. In my opinion, the authors should consider for comparison the higher number of galeomorph taxa as possible and discuss, in any case, the results obtained. Otherwise, they should clearly state in Material and Methods the reason why they limited their comparisons to these three particular taxa.

- Although it is not a common rule, several journals suggest to avoid the Saxon genitive in scientific papers being this mostly used in colloquial and informal sentences. I would suggest to avoid the Saxon genitive also here (see lines 27, 39, 57, 282, 312, 351).

- Line 46. Replace “reproduction strategies” with “reproductive strategies”.

- Line 47. Some lamniform and carcharhiniform sharks and some rays (e.g. electric rays and stingrays) are actually ovoviviparous (aplacental viviparity). In my opinion this should be considered a different, third type of reproductive mode since it is quite different from the pure viviparity in that there is no placental connection and the unborn young are usually nourished by egg yolk.

- Line 50. Do you mean ‘slow growth’ instead of ‘small growth’?

- Line 104 to 107. This sentence is unclear and/or seems incomplete. Please re-write it.

- Line 134. I would write ‘conservative body shape’ instead of ‘consistent body form’, being ‘shape’ only related to the ‘outline’ but not to size.

- Line 141. It is unclear to me how the authors extrapolate the equation (1), or if this is based on a published paper. If the goal is using the following proportion:

TLEmrg-Chond-SK-1b : TLP.occidentalis = CDEmrg-Chond-SK-1b : CDP.occidentalis

you should consider that the product of the means (TLP.occidentalis x CDEmrg-Chond-SK-1b ) equals the product of the extremes (TLEmrg-Chond-SK-1b x CDP.occidentalis ) and then you must employ this equation:

TLemrg-Chond-SK-1b = (TLp.occidentalis x CDemrg-Chond-SK-1b) / CDp.occidentalis

Please also show somewhere which are the values from the published literature that you use for the equations. That means, just replace abbreviations (TL, CD) with numbers.

- Line 222-223. Should it maybe be ‘…and lack parallel lamellae ON vertebrae’ ?

- Line 255-256. ‘single vertebral centrum’ and ‘the largest vertebra’

- Line 258. In Shimada et al. (2009) it seems that the body size estimate for P. rugosus is not calculated based on vertebral centrum diameter, but rather on the antero-posterior tooth crown length. Please check.

- Line 290. Please spell the first time what CR is.

- In the whole manuscript, please be consistent in using meters or centimetres to indicate the total body length; and millimetres or centimetres for the radius/diameter of the centra.

- Line 346-347. A bit unclear. What about ‘However, K-selected species are characterized by specific adaptations… etc’

- Line 347/348. Replace ‘changing environments’ with ‘environmental changes’.

- Line 349. Be careful. You DID NOT demonstrate unambiguously that ptychodontids had K-selected traits (although you can assume/hypothesize it) because your hypothesis has been inferred based on indirect evidences and/or comparisons with living representatives, not with statistical demonstration or direct observations. Maybe better the sentence as 'We suggest/ can infer/ hypothesize that..."

- In figure 3, the names of the anatomical features should be in ‘lower case’.

- Line 498. The first author surname is ‘Larocca Conte’.

Suggested literature:

Hamm SA. 2010. The Late Cretaceous shark, Ptychodus rugosus , (Ptychodontidae) in the Western Interior Sea. Transactions of the Kansas Academy of Science, 113: 44-55.

Killam KA., Parsons, GR. 1989. Age and growth of the blacktip shark, Carcharhinus limbatus, near Tampa Bay, Florida. Fishery Bulletin. U.S. 87: 845-857.

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Reviewer #1: Yes: Jacopo Amalfitano

Reviewer #2: Yes: Giuseppe Marrama'

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Submitted filename: PONE-D-20-05473_reviewer JA.pdf

Click here for additional data file.

25 Mar 2020

Dear Editor and reviewers.

First, let me thank you, also in the name of my co-author, Jürgen Kriwet, for all the work and effort that you put into this manuscript to make it ready for publication. We went through all your comments and, except for a few issues, agreed with your suggestions and modified the manuscript accordingly. Please find more detailed answers to how we addressed your points below. Although a minor revision, we saw how the comments of the reviewers significantly improved the quality and intelligibility of the manuscript and, therefore again, want to express our gratitude.

Sincerely,

Patrick L. Jambura, MSc

Department of Palaeontology - Geozentrum

University of Vienna

Althanstrasse 14 - 1090 Vienna, AUSTRIA

E-mail: patrick.jambura@gmail.com

Reviewer #1: Dear Editor and Authors,

The study reports some interesting findings about the enigmatic genus Ptychodus from the Cretaceous of Spain and has a clear paleobiological approach. Despite the fossil material is fragmentary, it displays peculiar and useful characters. These characters have never been remarked in other papers and allow the authors to make some very interesting paleobiological inferences. Data are presented in an appropriate fashion and statistics are adequate, even though the Von Bertalanffy growth model was not applicable (but the authors clearly state the reason of its unsuitability). The inferences are well-supported and the language is fluent. All in all, this study is of international interest and provides novel and significant information.

There are only some minor points I would like to raise:

Reply: We want to let you know that we really appreciate your assessment. You raised a number of issues, which combined with your very helpful suggestions, significantly improved the quality of the manuscript. Please find more detailed responses on how we addressed your comments below.

- The title deals with first articulated specimens from Spain, but there is no mention in the manuscript to other findings of the genus Ptychodus from Spain. It would be useful to provide some other references to these findings and compare with the specimens reported herein.

Reply: Thank you for your suggestion. Due to comments of reviewer 2 we decided to remove this from the title. The manuscript now has its sole focus on the palaeecology of Ptychodus.

- The diagnostic vertebral characters reported here should be included in the diagnosis of the genus (see Hamm, 2020 and please emend).

Reply: thank you very much for this very helpful suggestion. A brief diagnosis paragraph on this genus’ vertebrae structure was added.

”Diagnosis (emended). For dental characters of this genus see Hamm [41]. Vertebral centra are well calcified, amphicoelous and circular, not dorso-ventrally compressed. Vertebrae display a calcification pattern of the asterospondylic type with four uncalcified areas radiating diagonally from the center to the bases of the neural and haemal arches. Numerous concentric lamellae extend outwards from the center. Underneath the smooth articular surfaces, parallel lamellae are oriented 360° around the center of the vertebrae and run perpendicular to the concentric lamellae”

- Please reconsider the structure of the manuscript and the issues with the images and other minor indications addressed in the annotated version of the manuscript attached.

Reply: We appreciate your suggestions and adopted them in our manuscript accordingly. Please find a detailed point-by-point response below.

- Line 10. alternatively consider "life history traits"

Reply: Reconsidered and changed accordingly.

- Line 71-73. The paper has clearly a paleobiological approach, but these characters has never been clearly evidenced in previous papers. This should be better remarked, maybe emending the diagnosis of the genus adding the characters evidenced in the analysis. For dental characters of the genus please refer to and complete the diagnosis by Hamm 2020. New Mexico Museum of Natural History and Science Bulletin 81.

Reply: We are very thankful for your suggestion and added a diagnosis paragraph accordingly.

- Line 78. I think that this subparagraph should be placed outside the Matherial and Methods as a single paragraph after the Introduction

Reply: We are aware that the Geological Setting often (but not exclusively) is part of the introduction. We, therefore, followed your suggestion and put it there.

- Line 86. I would recommend the insertion of an image to better illustrate the stratigraphic framework of the section and the location of the specimen

Reply: Unfortunately, the section was measured by a diploma student back in the 1990s. The section is reproduced in his thesis but the labelling is in German. Moreover, we were not able to attain the original file for modification or permission by the author to modify the graph by ourselves. Unfortunately, the section on the coastline also differs slightly from other sections in the area (e.g., Gallemí et al. 2007), which we therefore can’t use. We consequently included the reference of the diploma thesis, which is available on request from the Free University of Berlin library service.

- Line 94. Please provide also more recent references. For example, Lamolda, M. A., Paul, C. R. C., Peryt, D., & Pons, J. M. (2014). The global boundary stratotype and section point (GSSP) for the base of the Santonian Stage," Cantera de Margas", Olazagutia, northern Spain. Episodes, 37(1), 2-13.

Reply: We followed your advice and added Gallemí J, López G, Martínez R, Pons JM. Macrofauna of the Cantera de Margas section, Olazagutia: Coniacian/Santonian boundary, Navarro-Cantabrian Basin, northern Spain. Cret Res. 2007;28: 5-17. doi: 10.1016/j.cretres.2006.05.014 and Lamolda et al. 2014.

- Line 169-171. It would be useful a brief introduction to the general structure of the shark centrum (corpus calcareum, intermedialia, etc.) (see e.g., Newbrey et al., 2015: p. 879)

Reply: We added a brief explanation of the centra composition that should help to better follow our description. We really appreciate your suggestion; the paragraph should be much clearer now.

“Shark centra form a double-cone calcification with densely calcified anterior and posterior conical ends, collectively referred to as corpora calcarea. Between the corpora calcarea is the intermedialia, which is softer than the corpus calcareum. The articular facet of the corpus calcareum is weathered and expose the inner layer of the vertebrae, showing concentric calcareous rings extending outwards from the center of the vertebrae. Numerous parallel lamellae are oriented perpendicular to these concentric lamellae (Fig 3A).”

- Line 169-171. Does the outer pattern of the corpus calcareum reflect the inner calcification pattern of the intermedialia? Apparently yes. Please take in consideration to better describe the external aspect for taxonomic comparison (Is it smooth or has some other pecularities? In lamniform sharks the external aspect is considered diagnostic for some species, e.g., the presence of papillose circular ridges on the surface of the corpus calcareum in Cardabiodon ricki)

Reply: We want to thank you for pointing this out. Actually, the described pattern cannot be seen on the surface of Ptychodus vertebrae but lay underneath a smooth articular surface (see for example Woodward 1911, Everhart & Caggiano 2004, Hamm 2019) and could only be seen because of the weathered condition of our material that exposed the inner layer. We modified our description to better reflect this, also the diagnosis paragraph that you suggested and was implemented, should further clarify this.

“Anterior and posterior facets of the vertebral centra are weathered and expose the inner layer of the vertebrae, showing concentric calcareous rings extending outwards from the center of the vertebrae. Numerous parallel lamellae are oriented perpendicular to these concentric lamellae (Fig 3A).”

- Line 175. the area between the wedges, sensu Ridewood (1921)

Reply: Thank you very much for your suggestion. However, since this term has become a standard anatomical term in the last 100 years, it seemed redundant to us to explicitly state its origin in the manuscript and hasn’t been done in previous published work on shark vertebrae either. Therefore, we decided not to include the reference here (which is mentioned at a later point in the manuscript).

- Line 244. An image showing the band count would be useful to better illustrate the estimation.

Reply: Done. An image showing the band count has been included in the supplementary material.

- Line 274. an image illustrating upper and lower limit with a scale reference could potentially improve the visual effect of the paragraph (e.g., see Marramà et al., 2017: fig.5) doi.org/10.1080/08912963.2017.1341503

Reply: We appreciate your suggestion and agree with you on it better visualizing this paragraph. We, therefore, added such a figure (figure 4) to the paragraph.

- Line 329. PlosOne does not request a mandatory conclusion paragraph, but please consider making a paragraph remarking the most important points of this study. The title is "First articulated skeletal remains" but there is no reference to other findigs of Ptychodus in Spain and comparison with their kind of preservation (isolated teeth? isolated vertebral centra?). And the fact that these are the first articulated skeletal remains should be better remarked in the discussion or in the conclusions (there is a reference to this only in the title and in the introduction).

Reply: Thank you for your suggestion. As mentioned above, we followed the advice of reviewer 2 and modified the title and got rid of the “first articulated skeletal remains” part. This paper now solely focuses on the palaeoecological aspect of the find, which most important points are summarized in the last paragraph (even if it is not explicitly stated as “conclusion” paragraph, the “Inferred life history” paragraph basically represents the conclusions of this study).

- Fig 1. Please check the font and even out with the subsequent images

Reply: We very much appreciate your advice and standardized font and font size for all figures, using Times 10 and 18.

- Fig 2. I would recommend providing a scale bar at the bottom right corner of the four sections of the image

Reply: Done.

- Fig 3. I would recommend to center the scale reference in the middle of the scale bar, and I would suggest to make a thinner scale bar. Please check the font, even out with previous and next images

Reply: Done.

- Fig 4. Please even out the font of words and letters with that of previous images and check also previous images. PLOS indications: Arial, Times, or Symbol font only in 8-12 point.

Reply: Done.

Reviewer #2: The manuscript of Jambura & Kriwet is an interesting and important contribution to the knowledge of the paleobiology of the enigmatic extinct shark Ptychodus. The manuscript is of high-impact, well written, and provides sufficient data to suggest new hypotheses about the palaeobiology of Ptychodus. However, the manuscript still presents some issues that the authors should address before the publication. A further proofcheck is also needed to avoid small typos and errors. This is why I consider the manuscript worth of publication after minor revision. In particular:

Reply: We would like to express our gratitude for your assessment. We adapted most of your suggestions and hope to properly have addressed the remaining raising points. Please find a point-by-point response below.

- In the title and several times in the text (e.g. line 70) the authors emphasize that these are the first articulated Ptychodus remains. However, there are other specimens of Ptychodus described in the literature that can be considered as articulated, and these include nearly complete tooth sets (e.g. Amadori et al. 2019), and even articulated vertebrae (Hamm 2010, fig. 6B). This is why authors should remove ‘first’ from the title and subsequent statements in the manuscript.

Reply: We are sorry for causing confusion here: we are aware of other articulated remains of Ptychodus, this find however is the first report of such from Spain. To avoid any confusion, we removed “first” from the title. We kept it in the last paragraph of the introduction though, because here it should be clear that “first” refers to the locality, not the material (“Here we describe the first articulated shark remains from the Santonian of Spain, Europe.”)

- The authors used regression equations for extant galeomorph sharks to estimate the relationships between centrum diameter and total length of Ptychodus. However, the authors should clearly state, in Material and Methods, the reason why they use galeomorph sharks as comparative taxa, and not squalomorphs, or other extinct groups. As far as we know the affinities of Ptychodus are far from being clear. Is the presence of asterospondylic vertebrae enough to detect its close relationship with galeomorph sharks? Are there other hypotheses in the literature to be considered? If so why are these hypotheses discarded?

Reply: The choice of these three taxa was not based on their phylogenetic position, but on their large body size. We did not intend to suggest a close relationship between galeomorph sharks and Ptychodus (even if a similar mineralization pattern of the centra might indicate such a relation). However, squalomorph sharks generally are rather small species, with very few exceptions, like the six-gill shark Hexanchus griseus (TL 3-5m) and the sleeper sharks Somniosus spp. (TL 3-7m), but there are no regression models available for these species, making it impossible to use them as template species for this study. The reason why we choose not to pick regression models for smaller species can be found in our reply below. To prevent any inferences of the taxonomic position of Ptychodus, we chose to refer to our template species as “large extant sharks” instead of “large galeomorph sharks”.

- Moreover, the authors use Carcharodon carcharias, Galeocerdo cuvier, Rhincodon typus as the only representatives of galeomorph sharks for comparisons. However, there are other extinct and living galeomorphs for which the relationship between centrum diameter and total body length are known, as reported by the authors themselves in the literature, that should be considered. For example: the extant Prionace glauca, Isurus oxyrhincus, Carcharhinus limbatus, and the extinct Carcharodon megalodon, and Cretoxyrhina mantelli (see Stevens 1975; Killam & Parsons 1989; Gottfried et al. 1996; Ribot-Carballal et al. 2005; Shimada 2008). There are probably even more taxa. In my opinion, the authors should consider for comparison the higher number of galeomorph taxa as possible and discuss, in any case, the results obtained. Otherwise, they should clearly state in Material and Methods the reason why they limited their comparisons to these three particular taxa.

Reply: Thank you for raising this point. The thought behind our decision to choose these three taxa was simple – we wanted to compare our specimen with extant species with similar vertebrae diameter. Following this approach we are expecting the most reliable size estimations for Ptychodus. Smaller sharks, like cat sharks that exhibit r-selected life history traits like fast growth can be expected to follow very different regression models, compared to large, K-selected sharks and therefore bias the estimated body size significantly. To test this hypothesis, we calculated the total length of our specimen based on the regression model for the blacktip sawtail catshark Galeus sauteri, which resulted in an estimated size of almost 9m (8,69m) – a value that is 20% higher than our suggested upper bound (and more than 200% of our lower bound). It is apparent that this is the result of different life history traits and lifestyles. Therefore, our decision to compare our specimen with sharks with similar sized vertebrae seems justified. Furthermore, these three template species are covering different groups, physiologies and ecologies, which resulted in a large interval of possible sizes and, therefore, already resembles a very careful and conservative interpretation. Fossil taxa like megalodon and Cretoxyrhina were not considered here, because TL for megalodon was calculated based on the ratio observed in white sharks and the TL of Cretoxyrhina was reconstructed (Shimada 2008 used “measurements” from Shimada 1997, which are partly reconstructed). We, therefore, followed your suggestion and added a brief note why we chose these three species in the M&M section.

“(2) We used published regression equations for large extant shark species with comparably sized vertebrae to estimate the relationship between centrum diameter (CD; mm) and total length (TL; cm). The following species with known regression equations were used as templates: (1) the great white shark Carcharodon carcharias [35]; (2) tiger shark Galeocerdo cuvier [37]; (3) whale shark Rhincodon typus [34].”

- Although it is not a common rule, several journals suggest to avoid the Saxon genitive in scientific papers being this mostly used in colloquial and informal sentences. I would suggest to avoid the Saxon genitive also here (see lines 27, 39, 57, 282, 312, 351).

Reply: Thank you very much for raising this point. We followed your suggestion and limited the use of the Saxon genitive to a minimum (1; line 27).

- Line 46. Replace “reproduction strategies” with “reproductive strategies”.

Reply: Done.

- Line 47. Some lamniform and carcharhiniform sharks and some rays (e.g. electric rays and stingrays) are actually ovoviviparous (aplacental viviparity). In my opinion this should be considered a different, third type of reproductive mode since it is quite different from the pure viviparity in that there is no placental connection and the unborn young are usually nourished by egg yolk.

Reply: We agree that the division into viviparity and oviparity is somehow superficial and does not cover the plethora of reproductive modes that can be found in elasmobranchs (extended oviparity, retained oviparity, aplacental viviparity, placental viviparity, etc.). However, neither would a subdivision into three categories, and a number of authors have actually abandoned the term ovoviviparity, as it comprises various modes of viviparity (e.g., Compagno 1990, Conrath & Musick 2012, Klimley 2013). Furthermore, a comparative data set of relative body size of the offspring of ovoviviparous and viviparous sharks is missing and would not allow any further interpretation of our data on this level. Therefore, we feel that a rough division into viviparity and oviparity best suits the interpretation of our data. Reconstructing if ptychodontid sharks were aplacental or placental viviparous is not possible at this point and would need an extensive review of reproductive modes and relative body sizes of offspring, which would represent a study in its own and is not within the scope of this manuscript.

- Line 50. Do you mean ‘slow growth’ instead of ‘small growth’?

Reply: Indeed, slow growth was meant here. Thank you very much for pointing this out.

- Line 104 to 107. This sentence is unclear and/or seems incomplete. Please re-write it.

Reply: Thanks a lot for highlighting this mistake. We changed this part accordingly and it now reads as follows: “Two disarticulated vertebral centra of varying preservational degree were collected and small sediment samples adjacent to the incomplete vertebral column were taken for screen-washing. The centra are housed in the fossil vertebrate collection of the Department of Palaeontology (University of Vienna) under the number EMRG-Chond-SK-1.”

- Line 134. I would write ‘conservative body shape’ instead of ‘consistent body form’, being ‘shape’ only related to the ‘outline’ but not to size.

Reply: Form was changed to shape. However, we are reluctant to call it “conservative”, since stem group elasmobranchs and early crown group elasmobranchs lived in shallow marine habitats and shared a similar body shape with today’s cat sharks. The pelagic realm was conquered later, therefore, the body shape associated with this lifestyle is rather derived than conservative.

- Line 141. It is unclear to me how the authors extrapolate the equation (1), or if this is based on a published paper. If the goal is using the following proportion:

TLEmrg-Chond-SK-1b : TLP.occidentalis = CDEmrg-Chond-SK-1b : CDP.occidentalis

you should consider that the product of the means (TLP.occidentalis x CDEmrg-Chond-SK-1b ) equals the product of the extremes (TLEmrg-Chond-SK-1b x CDP.occidentalis ) and then you must employ this equation:

TLemrg-Chond-SK-1b = (TLp.occidentalis x CDemrg-Chond-SK-1b) / CDp.occidentalis

Reply: The equation in question is a simple extrapolation that was established the same way you suggested it. The only difference is that we did not include parentheses, because the order of operations is irrelevant in this case, as multiplications and divisions follow the same priority rule and can be executed in any order. Therefore,

TLemrg-Chond-SK-1b = (TLp.occidentalis x CDemrg-Chond-SK-1b) / CDp.occidentalis

gives the same results as our equation

TLemrg-Chond-SK-1b = TLp.occidentalis x CDemrg-Chond-SK-1b/ CDp.occidentalis.

If TLp.occidentalis and CDemrg-Chond-SK-1b were added up or subtracted, this would be a whole other story of course.

Please also show somewhere which are the values from the published literature that you use for the equations. That means, just replace abbreviations (TL, CD) with numbers.

Reply: Thank you for your suggestion. The numbers were added to the extrapolation equation separately to better illustrate it. However, we had to use the variable X for TLp.occidentalis, because there are two values for the total length. To illustrate this, we added X€{150cm; 200cm}. The other equations were linear regression models, for which it is not applicable to replace abbreviations with numbers.

- Line 222-223. Should it maybe be ‘…and lack parallel lamellae ON vertebrae’ ?

Reply: Hamm (2010) did use “parallel lamellae vertebrae” as anatomical term in his work on Ptychodus rugosus to describe this structure. However, he never really defined this structure and we, therefore, decided to just describe this structure rather than naming it.

- Line 255-256. ‘single vertebral centrum’ and ‘the largest vertebra’

Reply: Done.

- Line 258. In Shimada et al. (2009) it seems that the body size estimate for P. rugosus is not calculated based on vertebral centrum diameter, but rather on the antero-posterior tooth crown length. Please check.

Reply: Shimada et al. (2009) did use both, the vertebral centra diameter and the anterior-posterior length of the teeth to extrapolate the total length of P. rugosus. However, both approaches were based on the same, estimated TL of P. occidentalis and bear great risk to be erroneous. To clearly drive home our point here, we rewrote this paragraph to make these ambiguities clear to the reader.

“Based on the previously published centrum diameter and estimated total length of †Ptychodus occidentalis [16] we calculated an estimated total length of 887-1183cm for EMRG-Chond-SK-1. However, this estimation should be taken with caution, as the TL-CD relationship of †P. occidentalis is based on a single vertebral centrum which not necessarily represents the largest vertebra in this specimen and, therefore, can result in overestimated size approximations. Therefore, we recommend taking the previously estimated TL of 13m for †P. rugosus [16], which was also based on this TL-CD relationship, with much caution. Shimada et al. [16] also compared the anterior-posterior length of the teeth of †P. occidentalis and †P. rugosus and concluded that †P. rugosus might have reached a body size of even 14.4m. However, it is important to note here that the total length for †P. occidentalis, on which both calculations are based on, is unknown and was estimated based on the length of the lower jaw. Therefore, an erroneous size estimation for †P. occidentalis would also bias all subsequent calculations.”

- Line 290. Please spell the first time what CR is.

Reply: Done.

- In the whole manuscript, please be consistent in using meters or centimetres to indicate the total body length; and millimetres or centimetres for the radius/diameter of the centra.

Reply: We followed your advice and used cm for TL and mwm for CR/CD.

- Line 346-347. A bit unclear. What about ‘However, K-selected species are characterized by specific adaptations… etc’

Reply: Thank you very much for this suggestion. We adapted it and hope the sentence became clearer this way.

- Line 347/348. Replace ‘changing environments’ with ‘environmental changes’.

Reply: Done.

- Line 349. Be careful. You DID NOT demonstrate unambiguously that ptychodontids had K-selected traits (although you can assume/hypothesize it) because your hypothesis has been inferred based on indirect evidences and/or comparisons with living representatives, not with statistical demonstration or direct observations. Maybe better the sentence as 'We suggest/ can infer/ hypothesize that..."

Reply: We appreciate your suggestion and changed the sentence to tone it down a bit: “Our results strongly suggest that extinct ptychodontid sharks had K-selected traits, which in combination with a highly specialized trophic niche (durophagy) might have been a major intrinsic contributor to this group’s demise.”

- In figure 3, the names of the anatomical features should be in ‘lower case’.

Reply: Done.

- Line 498. The first author surname is ‘Larocca Conte’.

Reply: Thank you a lot for pointing this out. Corrected.

Suggested literature:

Hamm SA. 2010. The Late Cretaceous shark, Ptychodus rugosus , (Ptychodontidae) in the Western Interior Sea. Transactions of the Kansas Academy of Science, 113: 44-55.

Killam KA., Parsons, GR. 1989. Age and growth of the blacktip shark, Carcharhinus limbatus, near Tampa Bay, Florida. Fishery Bulletin. U.S. 87: 845-857.

Reply: Thank you very much for your suggestions. Hamm 2010 was already incorporated in the previous version of this manuscript. We agree that Killam and Parsons 1989 is a great study on the life history of Carcharhinus limbatus, and added it to our manuscript.

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26 Mar 2020

Articulated remains of the extinct shark, Ptychodus (Elasmobranchii, Ptychodontidae) from the Upper Cretaceous of Spain provide insights into gigantism, growth rate and life history of ptychodontid sharks

PONE-D-20-05473R1

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30 Mar 2020

PONE-D-20-05473R1

Articulated remains of the extinct shark, Ptychodus (Elasmobranchii, Ptychodontidae) from the Upper Cretaceous of Spain provide insights into gigantism, growth rate and life history of ptychodontid sharks

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