http://orcid.org/0000-0002-4361-725X
Figures
Abstract
Late Cretaceous members of Peritresius belong to a diverse clade of marine adapted turtles currently thought to be some of the earliest representatives of the lineage leading to modern hard-shelled sea turtles (Pan-Cheloniidae). Prior studies have suggested that Peritresius was monospecific, with a distribution restricted to Maastrichtian deposits in North America. However, new Peritresius specimens identified from Alabama and Mississippi, USA, show that the genus contains two taxa, Peritresius ornatus, and a new species Peritresius martini sp. nov. These two taxa are characterized by the presence of a generally cordiform carapace with moderately serrated peripherals, well-developed ventral flanges beginning at the third peripheral, squarish umbilical and lateral plastral fontanelles, and a narrow bridge formed by the contact between the hyoplastron and hypoplastron. Peritresius martini sp. nov. can be distinguished by its lack of dermal ornamentation and the presence of a ‘rib-free’ 10th peripheral. These new specimens represent the first occurrences of Peritresius from the Late Cretaceous Mississippi Embayment and extend the temporal range of this genus back to the early Campanian. When tested within a global phylogenetic context, Peritresius is placed on the stem of Cheloniidae (Pan-Cheloniidae) along with Ctenochelys and Allopleuron hofmanni. The heavily vascularized and uniquely sculptured dermal elements of P. ornatus are interpreted here as potentially relating to thermoregulation and therefore may have been one of the key factors contributing to the survival of Peritresius into the Maastrichtian, a period of cooling when other lineages of Campanian marine turtles (e.g., Protostegids, Toxochelys, and Ctenochelys) went extinct.
Citation: Gentry AD, Parham JF, Ehret DJ, Ebersole JA (2018) A new species of Peritresius Leidy, 1856 (Testudines: Pan-Cheloniidae) from the Late Cretaceous (Campanian) of Alabama, USA, and the occurrence of the genus within the Mississippi Embayment of North America. PLoS ONE 13(4): e0195651. https://doi.org/10.1371/journal.pone.0195651
Editor: Thierry Smith, Royal Belgian Institute of Natural Sciences, BELGIUM
Received: December 14, 2017; Accepted: March 22, 2018; Published: April 18, 2018
Copyright: © 2018 Gentry et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the paper and its Supporting Information files.
Funding: The authors received no specific funding for this work.
Competing interests: The authors have declared that no competing interest exist.
Abbreviations: ALMNH, Alabama Museum of Natural History, Tuscaloosa, USA; AMNH, American Museum of Natural History, New York City, NY, USA; ANSP, Academy of Natural Sciences of Drexel University, Philadelphia, PA, USA; ChM, Charleston Museum, Charleston, SC, USA; MMNS, Mississippi Museum of Natural Sciences, Jackson, USA; MSC, McWane Science Center, Birmingham, AL, USA; NJSM, New Jersey State Museum, Trenton, NJ, USA; USNM, United States National Museum, Washington D.C., USA
Introduction
Cretaceous marine turtle fossils are abundant within Santonian to Campanian marine deposits in the southeastern United States, and have been reported from Alabama, Arkansas, Georgia, Mississippi, and Tennessee [1–4]. Although extensive Maastrichtian surface deposits are present in these southern states, few marine turtle specimens have been recovered from these units [5]. To date, the only well-described marine turtle known definitively from the Maastrichtian of the southeastern United States is Peritresius ornatus Baird, 1964 [4], a taxon reported previously from only the Navesink and Redbank Formations of New Jersey and the Ripley Formation of Georgia ([4,6]). The most frequently recovered Cretaceous marine turtle taxa from the Cretaceous of the southeastern U.S. are Toxochelys Cope, 1873 [7], Ctenochelys Zangerl, 1953 [2] and Prionochelys Zangerl 1953 [2], with each genus represented by dozens, or in the case of Toxochelys, hundreds of specimens. Despite their abundance in the southeastern U.S., these genera appear absent from the Maastrichtian components of the Hornerstown and Navesink formations along the Atlantic Coast, and are seemingly absent entirely from Maastrichtian deposits in North America (Zangerl 1953 [2]). The relative paucity of Maastrichtian pan-chelonioids (i.e., P. ornatus, Catapleura Cope, 1868 [8], Euclastes Cope, 1867 [9]) and their relationship to well-known Santonian and Campanian taxa (i.e. Toxochelys latiremis Cope, 1873 [7] and Ctenochelys stenoporus Zangerl, 1953 [2]), are both issues of particular interest to any attempt to resolve the phylogeny and biogeography of Late Cretaceous Pan-Chelonioidea.
Recently, the remains of several marine turtles referred to the genus Peritresius Leidy, 1856 [10], including those of a new species, were identified in the collections at the Alabama Museum of Natural History in Tuscaloosa, USA, McWane Science Center in Birmingham, Alabama, USA, and the Mississippi Museum of Natural Sciences in Jackson, USA. Presented herein are descriptions of these specimens along with comments on their taxonomy. We also provide remarks on chelonioid diversity and paleobiogeography during the Late Cretaceous.
Geologic setting
The specimens examined in this study were collected from 10 distinct localities spread across seven counties in eastern Mississippi and central and western Alabama, USA (Fig 1). The specimens were derived from four Upper Cretaceous formations, the Mooreville Chalk, Demopolis Chalk, Ripley Formation, and Prairie Bluff Chalk, which span from the lower Campanian through the upper Maastrichtian (Fig 2).
Upper Cretaceous surface exposures in both Alabama and Mississippi and the localization of discussed Peritresius specimens.
Stratigraphy follows that of Mancini et al. [11] and Dockery [12]. Planktonic foraminiferal zones after Caron [13].
Although the lithologies of these formations vary considerably, their depositional settings are similar as all are interpreted to represent outer neritic to nearshore environments ([14–15]; see Table 1).
(AL) Alabama. (MS) Mississippi. Lithologic descriptions and depositional environments follow Raymond et al. [14] and Puckett [15].
The holotype specimen described herein (ALMNH 6191) was surface collected from Ripley Formation exposures at site ALn-8, a creek locality located in Lowndes County, Alabama (Fig 1). In Alabama, an unconformity exists within the Ripley Formation, dividing it into upper and lower components (see Fig 2). The vertebrate remains from site ALn-8 fall were collected from the Gansserina gansseri (Bolli, 1951 [16]) Planktonic Foraminiferal Interval Zone, indicating they were recovered from the lower Ripley Formation, making them upper Campanian in age. The surface geology at site ALn-8 was described by Hall and Savrda [17], and this site is known for producing large numbers of fossil-bearing phosphatic concretions that contain crabs, spiny lobsters, and occasionally vertebrate remains. At the same time, however, vertebrate remains not encased in concretions occur only intermittently within the exposures.
Across Alabama, numerous Late Cretaceous vertebrate taxa have been reported from the upper Campanian to lower Maastrichtian Ripley Formation including sharks (i.e. Brachyrhizodus sp., Cretalamna sp., Ginglymostoma sp., Pseudocorax laevis (Leriche, 1906 [18]), Scapanorhynchus texanus (Roemer, 1849 [19]), and Squalicorax pristodontus (Agassiz, 1843 [20]), bony fishes (i.e. Anomoeodus sp., Xiphactinus audax Leidy, 1870 [21], Enchodus ferox Leidy, 1855 [22], and Enchodus petrosus Cope 1874 [23], a crocodile (i.e. Deinosuchus rugosus Emmons, 1858 [24]), mosasaurs (i.e. Mosasaurus maximus Cope, 1869 [25] and Plioplatecarpus sp.), and marine turtles (i.e. Ctenochelys sp. and Protostega gigas Cope 1871 [26]) [5,27]. As part of this study, two additional marine turtles have been identified within this formation, Peritresius ornatus (Baird, 1964 [4]) and a new taxon described herein, Peritresius martini sp. nov. Although this new taxon is currently known only from the type locality, here we report the occurrence of P. ornatus from nine additional late Campanian to lower Maastrichtian localities within Alabama and Mississippi. Information regarding the strata exposed at these localities is listed in Table 1.
Within this study, all localities are referenced by standard Alabama and Mississippi site file numbers. All cited localities are located on private property; however, permission was obtained by the ALMNH, MMNS, and MSC to collect at these locations. All specimens are legal property of the specific museums. Precise locality information is not provided herein though this information is fully available to qualified researchers and is on file at the ALMNH, MMNS, and MSC.
Nomenclatural acts
The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature, and hence the new names contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix "http://zoobank.org/". The LSID for this publication is: urn:lsid:zoobank.org:pub:815E2FD7-146C-4E13-896E-080BC9B683B2. The electronic edition of this work was published in a journal with an ISSN, and has been archived and is available from the following digital repositories: PubMed Central and LOCKSS.
Materials and methods
Cladistic methods and taxonomy
The character-taxon matrix used in this study (S1 File) follows that of Cadena and Parham [28], and was modified to include scores for Peritresius and recently discussed character adjustments for Ctenochelys and Toxochelys latiremis ([29], see S2 File). The character state of ‘absent’ was removed from character 133 (rib-free peripherals) to reflect the original states proposed for this feature (see [30], ch. 21). The 37 ordered characters used in Cadena and Parham [28] were also used in the present study. Terminal operational taxonomic units (OTUs) were limited to individual species. Cretaceous fossil taxa were restricted to only those that could be adequately incorporated into the matrix (more than 30% of characters coded). The only excluded Cretaceous marine turtle meeting this minimum threshold was Euclastes wielandi (Hay, 1908 [31]). This was due to the fact that the inclusion of Euclastes wielandi caused all crown cheloniids to collapse into a polytomy. The fact that Euclastes wielandi is known exclusively from cranial material may contribute to its behavior as a rogue taxon in our matrix. The matrix was analyzed with PAUP* v4.0 with all characters considered equally weighted using the heuristic search function and the subtree pruning/regrafting method of rearrangement. Bootstrap values were based on 1000 replicates and decay indices were calculated by retaining trees with sequentially higher steps than the most parsimonious strict-consensus tree until all bipartitions had collapsed. The positions of extant taxa were constrained by an incorporated ‘molecular scaffold’ (S3 File) taken from global phylogenomic studies of turtles [32]. Phylogenetic nomenclature and definitions follow Joyce [33] and Joyce et al. [34]. Osteological terminology largely follows that of Gaffney [35], but includes recent adjustments to the terminology for the carotid arteries [36]. Numbers in parentheses refer to characters used in the phylogenetic analyses and their corresponding scores.
Systematic Paleontology
Reptilia Laurenti, 1768 [37]
Testudines Batsch, 1788 [38]
Cryptodira Cope, 1868 [8]
Chelonioidea Baur, 1893 [39]
Pan-Cheloniidae Joyce, Parham, and Gauthier, 2004 [40]
Genus Peritresius Leidy, 1856 [10]
Type species
Peritresius ornatus Baird, 1964 [4], figs 1–8, Navesink Formation (upper Maastrichtian), Burlington County, New Jersey, USA.
Amended diagnosis
Cretaceous pan-cheloniid differentiated from Allopleuron hofmanni in having a more broadly rounded carapace, a decreased distance between the axillary and inguinal notches of the plastron, a lack of elongate, finger-like lateral projections of the hypoplastron, and the relatively constant width of peripheral elements 3–11. Differentiated from pan-cheloniids such as Ctenochelys by a greatly expanded contact between the left and right epiplastra, significantly reduced contact between the hyo- and hypoplastron due to the presence of large central and lateral plastral fontanelles, and a highly domed carapace as evidenced by the broad angle (90°-120°) formed by the dorsal and ventral facets of peripherals 3–8. Specimens can be diagnosed as Peritresius by the following combination of features: generally cordiform carapace having peripheral elements with moderate lateral serrations; a single mid-sagittal keel on the dorsal surface of the carapace (ch.116/3) consisting of 7 keeled neurals (ch.126/1) with epineural ossifications situated at the junctures of neurals 3–4, 5–6, and 7-suprapygal 1; reduction in peripheral height moving posteriorly from peripheral 4; ratio between the axillary-inguinal distance of the plastron and the length of hyo-hypoplastral contact >2.5:1 (plastral index is this value * 100); and thyroid fenestra subdivided by pronounced contact between the pubes and ischia (ch.224/1).
Peritresius martini sp. nov.
urn:lsid:zoobank.org:act:B876072A-57AE-4603-AAFC-D169B325E204
(1) carapace in dorsal view and plastron in ventral view; (2) left peripherals 3–6, 9, & 11 in posterior view; (3) 10X magnified view of the dorsal surface of right peripheral 10; (4) hypothetical reconstruction of the complete shell with the preserved elements shown in gray. Abbreviations: p, peripheral; pyg, pygal; spg, suprapygal.
(1) Plastron in ventral view; (2) hypothetical reconstruction of the plastron with the preserved elements shown in gray. Abbreviations: epi, epiplastron; hyo, hyoplastron; hypo, hypoplastron; xiphi, xiphiplastron.
Pelvis in dorsal view. Abbreviations: isc, ischium; pip, posterior iliac process; tf, thyroid fenestra.
Etymology
martini: for the discoverer and initial preparator of the holotype specimen, Mr. George Martin of Auburn, Alabama.
Differential diagnosis
As for genus but can be distinguished from Peritresius ornatus by a lack of sculpturing on the dermal surfaces of the carapacial elements, a less pronounced lateral keel of the anterior peripherals, and a ‘rib-free’ 10th peripheral (ch.133/2).
Description of P. martini sp. nov.
Carapace
The preserved region of the carapace allows us to interpret a morphology typical for Cretaceous pan-cheloniids in having a broadly cordiform general outline and moderately serrated peripherals (Fig 3). ALMNH 6191 has an estimated total carapace length in excess of 90 cm, far exceeding the largest described specimen of Peritresius ornatus (NJSM 11051) the only other member of the genus. The greatest width of the carapace is interpreted as approximately 75 cm along a line between the posterior margins of the sixth peripherals. Prominent scale sulci can be seen on the dorsal face of the peripherals of ALMNH 6191 and appear to closely resemble the arrangement seen in P. ornatus (Fig 3; [4], figs 3–4). Unfortunately, an accurate rendering of the scutes across the entirety of the carapace is not possible given the partial nature of specimen. The dorsal surface of the carapace of P. martini lacks the vermiculate arrangement of papillae and rugosities characteristic of P. ornatus, however there appears to be some evidence of vascular innervations in the outermost cortical lamellae of the carapacial elements similar to that seen on Corsochelys haliniches Zangerl, 1960 [3] and certain Tertiary pan-cheloniids such as Carolinochelys wilsoni Hay, 1923 [41], (ChM PV4792, [42], fig 5).
Peripherals
Although a complete peripheral series is not preserved with the holotype, enough of the peripherals are preserved that this portion of the carapace can be reconstructed with a reasonable degree of confidence. The peripheral series is typical for the genus (Fig 3) with each element marked by an interscutal sulcus on its dorsal surface and has a moderately serrated lateral margin similar to that of Ctenochelys stenoporus (Hay 1905 [43]) ([2], p. 240, fig 108). The lateral serrations begin with a large, circular boss on the first peripheral and they become more distinct and acute on the subsequent peripherals. Unlike P. ornatus, the lateral keel of P. martini is much less pronounced on the anterior peripherals. Along the medial and posterior peripherals, the keel crests anterior to the interscutal sulcus, similar to Ctenochelys. When viewed in anterior or posterior profile, the dorsal and ventral facets of peripherals 3–11 create a medially oriented trough that increases in height moving posteriorly from peripherals 3–5. The height increases until the fifth peripheral only to diminish posteriorly and terminate at the pygal (Fig 3). This arrangement is similar to that of Allopleuron hofmanni Gray 1831 [44] ([45], fig pl. 1, p. 41), however, unlike A. hofmanni, in dorsal view the peripherals of P. martini do not decrease in width moving posteriorly along the peripheral series. Beginning at the third peripheral, the medial trough is marked by shallow indentations that serve as insertion points for the distal ends of the adjacent costals.
As with P. ornatus, costal ribs 1–7 articulate with peripherals 3–9 but costal rib 8 articulates with peripheral 11 instead of 10. There is no indication of a rib-end insertion into the medial facet of the 10th peripheral implying the presence of a rib-free peripheral between the 7th and 8th costal rib (ch. 133/2). This feature has been previously noted as a synapomorphy of the clade containing pan-cheloniids such as Puppigerus Cope, 1871 [26] and extant cheloniids [29,46]. The pygal is slightly notched at its posterior midpoint and transversely arched (Fig 3). The anterior margin of the pygal is marked by a large, circular articulation site for the posterior end of the 2nd suprapygal.
Plastral elements
The plastron of ALMNH 6191 is more complete than any previously described Peritresius specimen, lacking only the entoplastron, right hypoplastra and left epiplastra (Fig 4). There is no evidence of the dermal sculpturing observed on P. ornatus, though the narrow hyo-hypoplastral suture, large central and lateral plastral fontanelles, and orientation of the scute sulci make the plastron of P. martini more similar to that of P. ornatus than to any of the other closely related pan-cheloniids (i.e. C. stenoporus—[47], USNM 357166, fig 13C; [2], USNM 6013, fig 108; Allopleuron hofmanni—[45], pl. 33). However, it should be noted that the most intact plastra yet described of C. stenoporus ([47], USNM 357166; [2], USNM 6013) both belong to sub-adult individuals. Furthermore, greater than 50% of the P. ornatus plastron described by Baird ([4], NJSM 11051, figs 7–8) is plaster reconstruction.
The finely pointed interdigitations marking the lateral margins of the hyo- and hypoplastron are larger than those found on C. stenoporus, but are smaller than those of A. hofmanni. These interdigitations are indicative of a fully ligamentous connection between the carapace and plastron (ch. 148/1) as seen on juvenile Lepidochelys specimens (see [48], fig 83). The lateral and central plastral fontanelles are more expansive than those of other comparably sized Cretaceous pan-chelonioids (i.e. Toxochelys and Ctenochelys), a result of a diminished contact between the hyo- and hypoplastron (ch.153/1). Rather than flaring broadly both anteriorly and posteriorly (as in Toxochelys and Ctenochelys), the inguinal buttresses of the hypoplastron of P. martini are narrow and lie at nearly right angles to the midline, similar to that seen on P. ornatus. The orientation of the axillary and inguinal buttresses, along with the enlarged lateral fontanelles, create a bifid plastral connection with the carapace. The epiplastra are narrow and elongate, similar to those of Ctenochelys, and appear to have been suturally connected at their medial contact (ch. 160/1). The xiphiplastra are elongate and lack any significant medial sutural contact (ch. 169/2). The distal margins of the xiphiplastra are not as medially curved as in Toxochelys and Ctenochelys, but are instead quite straight and give the xiphiplastra an almost triangular appearance in ventral view, similar to the xiphiplastra of A. hofmanni ([45], pl. 32)
Pelvis
Preserved with ALMNH 6191 is a nearly intact pelvic girdle (Fig 5). The arrangement of the girdle elements is typical of Cretaceous pan-cheloniids, such as Ctenochelys, and has large pubes and proportionally diminutive ischia. The thyroid fenestra is subdivided by an osseous contact between the posterior edge of the medial pubic processes and the anterior-most margin of the ischia (ch. 224/1), as seen on A. hofmanni ([45], pl. 43).
The medial processes of the ischia of ALMNH 6191 are less developed than those of Toxochelys but more so than those of Cenozoic cheloniids (ch. 233/1). The flattened lateral processes of the pubes are well developed (ch. 230/1) and slightly angled dorsally away from the medial plane of the symphyseal portion of the pubes, more so than observed in either Toxochelys or Ctenochelys. The posterior processes of the ilia are elongate and medially rugose, as on Ctenochelys, but also possess a dorsolateral rugosity potentially homologous with the ilio-carapacial contact observed in modern cheloniids. The remaining portions of the pelvis are indistinguishable from those of Ctenochelys.
Remarks
A confluent thyroid fenestra has been suggested as a derived characteristic of crown cheloniids based on the subdivided thyroid fenestra of many early cryptodires and the absence of such a division in fossil chelonioids like Toxochelys latiremis and Lophochelys spp. Zangerl 1953 [2] ([49]). However, the presence of a divided thyroid fenestra in Peritresius spp., A. hofmanni, and certain extant cheloniids such as Caretta caretta ([48], figs 106a and 106b) may indicate this feature was lost early in pan-chelonioid evolution and later reacquired in select lineages of pan-cheloniids. It is also possible that Late Cretaceous sea turtles, such as Toxochelys and Peritresius, represent distinct radiations of marine adapted turtle potentially due to multiple invasions of marine environments by Testudines during the latter half of the Cretaceous, with the plesiomorphic condition retained in one lineage (Peritresius) and lost in another (Toxochelys). Testing the latter scenario using only morphology based phylogenetics would require an extensive review of the pelvic elements of fossil and extant Testudines in order to ensure that any character set or coding strategy regarding the arrangement of these elements was sufficiently inclusive to provide meaningful resolution between members of clades containing highly convergent lineages (i.e. marine turtles). Such a review is beyond the scope of the present study but is certainly an area of chelonioid evolution in need of further examination.
Peritresius ornatus Leidy, 1856 [10]
(1) 10X magnified view of dermal sculpturing present on all preserved elements. (2, 4) slab in dorsal view. (3, 5) slab in ventral view. Abbreviations: C, costal; N, neural.
(1) MMNS 5876; (2) ALMNH 5497; (3) MMNS 5274; (4) MMNS 5710; (5) MMNS 8632.4. All elements shown in (A) anterior, (B) ventral, (C) dorsal, and (D) 10X magnified views.
(1) MMNS 4547 in: (A) dorsal, (B) ventral, and (C) posterior views; (2) MMNS 4003 in dorsal view; (3) ALMNH 6256 in: (A) dorsal, (B) ventral, and (C) posterior views; (4) RMM 5741 in dorsal view; (5) MMNS 5102 in: (A) dorsal and (B) ventral views; (6) MMNS 5533 in: (A) dorsal and (B) ventral views.
1856 Chelone ornata Leidy [10]: 105, pl. 18, fig 10.
1869 Peribresius [sic, errore] ornatus. Cope in Cook [50]: 735.
1869 Peritresius ornatus Cope [51]: 88; 1870: 150.
1870 Prochonias nodosus Cope [52]: 158, 159.
1870 Taphrosphys nodosus Cope [52]: 167, 244, pl. 1, fig 16.
1908 Peritresius ornatus = ? Taphrosphys nodosus Hay [31]: 122, 210.
1955 Peritresias [sic] ornatus Miller [53]: 908.
1964 Peritresius ornatus Baird [4]
Referred Specimens
ALMNH 3900 –Various highly fragmented carapacial and plastral elements. ALMNH 3780 –Plastral fragment. ALMNH 5497 –Nearly complete neural. ALMNH 5887 –Partial left costal missing distal rib-end, right hypoplastron, anterior half of one neural, and several peripheral/costal fragments. ALMNH 6256 –Carapace fragments including the dorsal face of an anterior peripheral (4–6?). ALMNH 8988 –Several dissociated costals and four anterior neurals. AMNH 1410 –Isolated carapacial scraps. AMNH 1480 –Two partial costals, an anterior neural, and numerous carapacial fragments. MMNS 4003 –Costal fragment. MMNS 4407 –Costal fragment. MMNS 4546 –Partial neural. MMNS 4547 –Single peripheral. MMNS 5102 –Pieces of several costals. MMNS 5274 –Partial neural (juvenile?). MMNS 5533 –Large costal fragment. MMNS 5710 –Partial neural. MMNS 8632.4 –Single neural of a large individual; MSC 5741 –Small individual; three articulated anterior peripherals, possibly peripherals 3–5 of the left side.
Description of new material
Carapace.
Though none of the newly identified specimens of P. ornatus possess an intact carapace, the Alabama and Mississippi material can be assigned to P. ornatus based on the presence of pronounced vermiculate sculpturing consisting of irregular grooves and channels located on the outer surfaces of all costals, peripherals, and neurals (Figs 6–8). The seemingly random distribution of papillae and rugosities formed by the sculpturing found on these specimens differs greatly from the pitting found on the surfaces of the dermal elements of trionychid turtles, but appears identical to that described on P. ornatus (see [4]).
Costals.
Though numerous costal pieces can be identified from among the specimens in our sample, there is very little diagnostic information that can be derived from many of these elements due to their high degree of fragmentation. However, one specimen (ALMNH 8988) possesses six nearly intact costals and the medial portions of four others (Fig 6). The degree to which the costal plates extend laterally along the length of each element and the sizes of the resulting fontanelles created between the distal rib-ends of each costal pair appear identical to those of the holotype ([4], figs. 12 and 13). The carapacial fontanelles of P. ornatus are pronounced, even in large, presumably adult, individuals (such as ALMNH 8988) and are more similar to those of P. martini and A. hofmanni than to other well-described Cretaceous pan-cheloniids such as Ctenochelys stenoporus. The robust nature of these elements is also noteworthy with the anterior costals of ALMNH 8988 being nearly 3 cm thick, owing primarily to the dense layer of cortical bone covering the surfaces of each element.
Neurals.
Although the preserved neurals exhibit a considerable amount of variation in overall size, each possesses the distinct deep sculpturing inherent to carapacial material belonging to P. ornatus (Fig 7). The largest of the neurals is slightly wider (7.9 cm) than long (7.4 cm), generally hexagonal, and despite being preserved with significant dorsoventral compression, retains a mid-dorsal keel indicative of ‘lophechelyine’-grade taxa such as Prionochelys and Ctenochelys (ch.125/1).
The smallest neural in our sample, which measures 2.5 cm in length and 1.8 cm in width, is interpreted here as representing the first juvenile material recovered for this taxon (Fig 7.3). The relative dimensions of this element implies that the neurals of P. ornatus increased more in width than in length as the turtle matured. The posterior third of the mid-dorsal keel of this neural is excavated into an ovoid depression, presumably the insertion point for the first epineural, indicating that this is neural 3. The neurals of P. ornatus are relatively thicker than those of Ctenochelys with a gradual decrease in thickness from the anterior to posterior positions.
Peripherals
The peripherals of P. ornatus are generally longer than wide and possess a moderate lateral serration (Fig 8). Due to the isolated nature of the peripherals described here, very little can be ascertained regarding the total number of these elements, or their position, relative to the remainder of the carapace.
Discussion
Phylogenetic placement of Peritresius
Phylogenetic analyses resulted in 3 equally parsimonious trees with a length of 307 steps. The strict consensus tree places Peritresius spp. within Pan-Cheloniidae as a sister group to Ctenochelys spp. and A. hofmanni (Fig 9). The hypothetical sister relationship between Ctenochelys and Peritresius, first proposed by Baird [4] and again supported by Hirayama [1] based on the presence of epineurals, is also seen here based on additional postcranial characters (see below). Although epineurals have also been observed in other species of fossil marine turtle (e.g. Archelon ischyros Wieland 1896 [54]), and as an ontogenetically variable characteristic in the extant cheloniid Lepidochelys kempii (Garman 1880 [55]), the general arrangement of these elements in Peritresius more closely resembles the neural-epineural conformation of Ctenochelys than those of any other fossil or extant marine turtle. Hirayama’s [1] monophyletic grouping of Ctenochelys and Peritresius also included species belonging to the North American genus Prionochelys, but due to a lack of described material for members of this genus and the resulting confusion surrounding their taxonomy, no species of Prionochelys could be adequately incorporated into the present matrix.
Bootstrap values (left) and decay indices (right) are shown for each node; CI = 0.586; RI = 0.671; Branch lengths for crown cheloniid species taken from Cadena and Parham [28].
The present study did not recover a single unambiguous postcranial synapomorphy uniting the species of Peritresius, and only one unambiguous postcranial synapomorphy was identified for Pan-Cheloniidae which is the presence of a rib-free peripheral between the ribs of the 7th and 8th costals (ch. 133/2). Peritresius spp. is grouped with Ctenochelys spp. and A. hofmanni based on the following characteristics: medial line of keels on the dorsal surface of the carapace (ch. 116/3), posteromedial nuchal fontanelles (ch. 123/1), and an extreme reduction in lateral costal ossification resulting in the dorsal exposure of the distal rib ends in almost every costal series (ch. 132/3). All of these characters are found in other chelonioids. The shared character states among distinct lineages highlights the homoplastic morphology of chelonioids and the importance of additional descriptions of Cretaceous chelonioid specimens, especially those possessing both cranial and postcranial elements.
Biostratigraphy and paleobiogeography of Late Cretaceous chelonioids sensu stricto
The discovery of Peritresius remains from the Campanian of Alabama closes the temporal gap noted by Baird [4] between Peritresius and other ‘toxochelyid’-grade taxa and makes Peritresius the only Cretaceous pan-chelonioid genus known to cross the Campanian-Maastrichtian boundary [2,56]. Several authors have proposed the continuation of the Toxochelys latiremis lineage from the Campanian into the Maastrichtian [1,57] based on the synonymy of T. latiremis and Toxochelys weeksi Collins, 1951 [58] from the Ripley Formation of Tennessee. This synonymy was originally proposed by Nicholls [59] based on an expansion in the accepted range of intraspecific variability with regard to proportions of the plastron. T. weeksi is represented only by the holotype (USNM.V.20110—previously UT K20) which consists of a partial plastron and three associated posterior peripherals. This specimen was illustrated by both Collins [58] and Zangerl [2], however neither author provided photographs of the specimen. Nicholls [59] noted in her taxonomic reassessment of Toxochelys latiremis that the holotype of Toxochelys weeksi was the only specimen she did not personally examine. Recently, photographs of the holotype of Toxochelys weeksi were made available online by the USNM Department of Paleobiology which show that the precise size of at least the lateral plastral fontanelle of this specimen is impossible to adequately determine due to missing pieces of bone at the posterolateral margin of the hyoplastron (Fig 10).
Plastron in ventral view with associated measurements. Dashed lines represent areas historically interpreted as fontanelles and the black arrow indicates a broken edge. Courtesy of Smithsonian Institution. Photograph by A. Millhouse.
Nicholls [59] accurately notes that the plastral measurements reported by Zangerl ([2], p. 174) for this specimen are wrong and once the correct measurements are taken (Fig 10), the values much more closely align with those of the holotype of T. moorevillensis (FMNH 27330) than to any referred specimen of T. latiremis. The xiphiplastron of USNM.V.20110 also more closely resembles the xiphiplastron of T. moorevillensis than that of T. latiremis. Additionally, the Coon Creek Tongue of the Ripley Formation of Tennessee from which USNM.V.20110 was recovered, previously thought to be early Maastrichtian in age, has more recently been interpreted to fall within the latest Campanian [60]. Given the questionable morphology and horizon of USNM.V.20110, T. weeksi should no longer be considered a junior synonym of T. latiremis and is here referred to T. moorevillensis. The youngest specimens of T. latiremis are herein considered to be those from the Late Campanian Pierre Shale of Kansas and South Dakota.
The presence of Peritresius in the Maastrichtian of the northeastern Atlantic Coast (i.e. New Jersey and Maryland) makes it just the third Cretaceous pan-cheloniid known from this area (along with Euclastes and Catapleura). A large humerus belonging to the Campanian marine turtle Atlantochelys mortoni Agassiz, 1849 [61] was recovered from the Mount Laurel Formation of New Jersey (ANSP 9234) but given that the species is known from a single element, it is impossible to generate a large enough suite of characters to adequately incorporate A. mortoni into a phylogenetic analysis. As a result, its cladistic affinities cannot be determined with any confidence. Based solely on humeral morphology, A. mortoni has been hypothesized as a member of either Protostegidae [1,62] or Cheloniidae [63] but until more material of this species is recovered and the species’ phylogenetic placement formally tested, we conservatively exclude this specimen from any discussions pertaining to the cladistics or paleobiogeography of Cretaceous chelonioids sensu stricto given the possibility that A. mortoni may not belong to this clade.
The Peritreus material described herein makes this genus the only late Cretaceous marine turtle known from both the Mississippi Embayment and the northeastern Atlantic Coast despite the prevalence of marine turtle fossils in both areas. Based on the fossil material currently known for the genus, it seems that the unsculptured species of Peritresius (P. martini) did not disperse beyond the Mississippi Embayment and unlike the predominantly Maastrichtian P. ornatus, is known exclusively from Campanian deposits (Fig 11). The limited distribution of P. martini fits the previously noted pattern of endemic speciation common among Cretaceous chelonioids sensu stricto [1].
Localities and taxon ranges for fossil occurrences were taken from the literature as follows: Nichollsemys baieri from Brinkman et al. [64], Porthochelys laticeps from Hirayama [1], Toxochelys latiremis from Hirayama [1] and this study, Ctenochelys stenoporus from Hirayama [1], Prionochelys nauta from Hirayama [1], Toxochelys moorevillensis from Hirayama [1], Ctenochelys acris from Gentry [29], Thinochelys lapisossea from Hirayama [1], Zangerlchelys arkansaw from Hirayama [1], Peritresius martini from this study, Peritresius ornatus from Baird [4] and this study, Euclastes wielandi from Parham [65], and Catapleura repanda from Hirayama [66]. Age justifications are provided as supporting information (S4 File). Base map obtained and modified from the USGS National Map Viewer.
Sculptured vs. unsculptured Peritresius
The existence of an unsculptured species of Peritresius (P. martini.) makes Peritresius the first genus of marine turtle to contain both a sculptured and unsculptured form. The irregular, dermal sculpturing of P. ornatus differs from that of trionychids and adocids in that there is no discernible pattern in the arrangement of the ridges and papillae that form the sculpturing, other than the occasional appearance of brief channels created by the alignment of seemingly random dorsally protruding trabeculae (Figs 6–8). The vermiculate sculpturing of P. ornatus seems to more closely resemble the condition observed in the Eocene cheloniid Osonachelus decorata de Lapparent de Broin, Murelaga, Farres, and Altimiras 2014 [67], the Oligocene cheloniids Ashleychelys palmeri Weems and Sanders, 2014 [42] and Carolinochelys wilsoni, and the Neogene pan-cheloniid Trachyaspis lardyi Meyer 1843 [68] (= Syllomus aegyptiacus [Lydekker 1889 [69]] fide Villa and Raineri 2015 [70]).
Though the morphology of the sculpturing found on certain fossil marine turtles has received some attention [4,67,70,71], little effort has yet been made to identify potential functions of this feature. Any hypothesis regarding the functional basis of the dermal sculpturing of fossil marine turtles would have to rely heavily upon inferences from modern taxa, but unfortunately, even though slight carapacial sculpturing has been described in Natator depressus Garman, 1880 [55] (see [72]), no extant cheloniid exhibits the high degree of ornamentation found in P. ornatus. Several groups of non-marine turtles are ornamented, including various pan-trionychians, all solemydids, and most pleurosternids. The shell histology of these taxa is well documented [73–76] but we are not aware of any published studies about the function of the shell sculpturing. However, highly ornamented, irregular sculpturing of the dermal elements is present among other animals, such as squamates [77–79] and basal tetrapods [80], where this feature is the result of highly vascularized bones necessary for thermal regulation via the alteration of blood flow to and from the dermis.
Peritresius ornatus is the only Campanian marine turtle known to persist from the early Campanian into the Maastrichtian (Fig 11), an interval where there is strong isotopic evidence that ocean temperatures were dropping [81,82]. This cooling and other environmental perturbations have already been plausibly linked to a dip in the diversity of large marine tetrapods during this time (e.g. mosasaurs; [83]) and may also have been responsible for the apparent drop in chelonioid diversity during the Maastrichtian [1,6]. If shell sculpturing does relate to thermoregulation, then it is possible that the persistence of P. ornatus into the cooler Maastrichtian may have been facilitated by this feature. If so this would be one of the few examples of fossilized characters relating to thermoregulation in marine turtles (e.g. [84]).
Conclusions
A new species of Cretaceous marine turtle from the southeastern United States (Peritresius martini sp. nov.) is herein described based on material collected from the upper Campanian of Alabama, USA. Peritresius martini sp. nov. differs from Peritresius ornatus in having a ‘rib-free’ 10th peripheral, a less pronounced lateral keel on the anterior peripherals, and an unsculptured carapace and plastron. The heavily vascularized and sculptured dermal elements characteristic of P. ornatus are interpreted here as potentially indicative of a thermoregulatory capability and may have been one of the key factors contributing to the survival of Peritresius into the Maastrichtian, a period of cooling when other lineages of Campanian marine turtles (e.g., Protostegids, Toxochelys, and Ctenochelys) went extinct.
Supporting information
S1 File. Character-taxon matrix in Mesquite format.
https://doi.org/10.1371/journal.pone.0195651.s001
(NEX)
S4 File. Fig 11 supporting information.
Including (I) Locality age estimates; (II) Literature cited.
https://doi.org/10.1371/journal.pone.0195651.s004
(PDF)
Acknowledgments
The authors would like to thank George Phillips (MMNS) and David Parris (NJSM) for access to the specimens in their care, as well as Thomas Jorstad and Amanda Millhouse with the Department of Paleobiology at USNM, for providing the photograph of the Toxochelys weeksi holotype in Fig 10. The authors also thank Kenneth Whetstone for donating hard-copies of many historical fossil turtle manuscripts to the ALMNH along with Jason Robinson and L. Majure for their donations of Peritresius ornatus specimens to MMNS. We would especially like to thank George Martin for his donation of the Peritresius martini holotype to ALMNH and T. Lynn Harrell Jr. for the additional prep work performed on this specimen following its donation. We thank Sandy Ebersole of the Geological Survey of Alabama for providing the GIS shape files necessary to produce Fig 1. Walter Joyce, Don Brinkman, and Adan Perez-Garcia provided crucial review commentary that greatly improved the quality of the manuscript and figures. Finally, the authors thank Ed Hooks who initiated the study into the type specimen of P. martini and then graciously allowed us to continue and complete his work. Travel to the FMNH was funded in part by a Catherine P. Ireland Travel Grant to ADG from the University of Alabama at Birmingham.
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