One of the most diverse clades of Mesozoic and Cenozoic turtles is Pelomedusoides, with fossils worldwide distributed and extant representatives restricted to southern hemisphere (Ferreira et al., 2018; Gaffney et al., 2011; Gaffney, Tong & Meylan, 2006; Hermanson et al., 2020; Vlachos, Randolfe & Sterli, 2018). Recent molecular phylogenetic hypotheses suggest that they split from Chelidae during the Late Jurassic at 161.7 Ma (149.3–168.9 Ma) (Pereira et al., 2017), and total-evidence tip-dating (TE TD) suggest even older date for this splitting during the Early Jurassic at 172.6 Ma (Holley, Sterli & Basso, 2019). However, at present, the earliest indisputable fossil record of Pelomedusoides is the bothremydid Atolchelys lepida (Romano et al., 2014), from the upper Barremian of Brazil; meaning approximately 36 Ma of ghost-lineage.
Almost a decade ago, I described some fragmentary material from the Early Cretaceous (Valanginian) Rosa Blanca Formation of Colombia, which I attributed as potentially belonging to Podocnemidoidea (one of the subclades inside Pelomedusoides) (Cadena, 2011). This occurrence has been questioned and considered dubious by Romano et al. (2014), arguing that the presence of an inguinal buttress that medially extends onto the ventral surface of costal 5 is highly variable within Pelomedusoides, even within all of Testudines. Here, I present new material from a locality nearby to the one from where the material described in 2011 came from; from the same segment of the Rosa Blanca Formation (Fig. 1). This new fossil material allows me to revise the hypothesis proposed back in 2011, and present new evidence and comparisons that support the occurrence of Pelomedusoides during the Valanginian in northern South America.
Materials & Methods
Fossil material. I found the fragmentary material described here in 2016. Recently I added it to the emerging Paleontological Collection of the Facultad de Ciencias Naturales from Universidad del Rosario, in Bogota, Colombia. Its collection identification number is UR-CP-0025. I obtained permit from the Ethics committee of the Universidad del Rosario to execute this study via the DVO005 672-Cv1066 communication. Local landowner Roberto Serrano verbally authorized the paleontological exploration of the zone and collection of the specimen.
Institutional abbreviations. CRI, Chelonian Research Institute, Oviedo, Florida, USA; IPN, Museo Geológico Nacional José Royo y Goméz, Bogoté, Colombia; MNHN, Muséum National d’Histoire Naturelle, Paris, France;UR-CP, paleontological collection, Facultad de Ciencias Naturales, Universidad del Rosario, Bogoté, Colombia; USNM, herpetological collection, Smithsonian Natural History Museum, Maryland, USA.
Carapace length estimation. In order to establish an estimation of the total length of the carapace to which the fossil fragment belongs, I measured fifteen specimens of extant podocnemidids that I have examined in recent years, as well as the extinct taxa Francemys gadoufaouaensis from Pérez-García (2019) and Atolchelys lepida from Romano et al. (2014) (Data S1). Using the software Image J2 (Rueden, Schindelin & Hiner, 2017), I set the scale to the one provided in the photos or figures of the specimens and measured the maximum length of both mesoplastra, peripherals 6, 7, and the total length of the carapace (according to their individual preservation). I established the simple linear regression and its equation using Microsoft-Excel (Data S1), and used it to estimate the maximum length of the carapace of UR-CP-0025.
Comparisons. For comparisons with other pan-pleurodires including several Pelomedusoides, I created a comparative figure redrawing only the right bridge region from figures or photographs of previous literature or from direct examination of specimens as follow: Platychelys oberndorferi, Notoemys laticentralis, and Notoemys zapatocaensis from Cadena & Joyce (2015); Notoemys oxfordiensis from De La Fuente & Iturralde-Vinent (2001); Francemys gadoufaouaensis from Perez-Garcia (2019); Bonapartemys bajobarrealis from Lapparent de Broin & de la Fuente (2001); Mendozachelys wichmanni from De La Fuente et al. (2017); Prochelidella cerrobarcinae from De La Fuente et al. (2011); Euraxemys essweini and Cearachelys placidoi from Gaffney, Tong & Meylan (2006); Araripemys barretoi from Meylan (1996); Dortoka vasconica from Lapparent de Broin & Murelaga (1996); Pelomedusa subrufa CRI-5200, Podocnemis expansa USNM-29476 and Chelus fimbriata MNHN-2581A from personal reference photo gallery; and UR-CP-0025 from this study.
|PLEURODIRA Cope, 1864|
|PELOMEDUSOIDES Cope, 1868|
Referred material.—UR-CP-0025, a portion of the right shell bridge including the mesoplastron, peripheral 6, portions of peripherals 5 and 7, as well as the lateral most portions of right hyoplastron and hypoplastron. From Cadena (2011): IPN 16 EAC-14012003-1A, left partial costal 5; IPN 16 EAC-14012003-1B, posterior peripheral bone.
Locality and Age.—I collected UR-CP-0025 from a locality nearby the Laguna del Sapo (6°50′34″N, −73°14′17.3″W), approximately 1.5 km southwest of the Pico de la Vieja road locality where I found the material reported in Cadena (2011) (Figs. 1A–1B). The Laguna del Sapo locality is northeast of Zapatoca, Santander Department, Colombia; and it is part of the recently defined Carrizal Member of the shallow marine Rosa Blanca Formation (Etayo-Serna & Guzmen-Ospitia, 2019), correlated to the base of the late Valanginian ( ∼135 Ma) based on the occurrence of the ammonite Saynoceras verrucosum, according to the biochronostratigraphic framework of Ogg, Ogg & Gradstein (2016). I found UR-CP-0025 at the base of a calcareous yellow siltstone layer (Fig. 1C).
Remarks.—UR-CP-0025 is attributed as belonging to Pelomedusoides based on having an equidimensional mesoplastron (Fig. 3). IPN 16 EAC-14012003-1A (Cadena, 2011, fig. 1) is also reassigned as potentially belonging to Pelomedusoides based on the massive inguinal buttress scar extending medially onto it, as well as ventrally projected (Character 150, Gaffney, Tong & Meylan, 2006). Romano et al. (2014) ignored that the character used is dealing with the degree of extension of the inguinal buttress as clearly pointed out in Cadena (2011 fig. 1) with examples in different Testudines. IPN 16 EAC-14012003-1B is similar in size to the peripherals of UR-CP-0025 described herein, much larger than the peripherals of Notoemys zapatocaensis (Cadena, Jaramillo & Bloch, 2013), making it also potentially belonging to Pelomesudoides.
Description. UR-CP-0025 constitutes a portion of the right shell bridge, preserving the mesoplastron, the most posterolateral corner of the right hyoplastron, the most anterolateral portion of the right hypoplastron, peripheral 6, and portions of peripherals 5 and 7. In ventral view (Figs. 2A–2B), the mesoplastron exhibits an almost equidimensional circular-shape, and it is in lateral and posterolateral contact with peripherals 5 and 6 respectively. Medially and posteromedially is in contact with the right hyoplastron and right hypoplastron respectively. The most lateral portion of peripherals 5 to 7 is missing (natural breaking). Also in this view, there is evidence of some of the sulci, particularly between marginals and of these with the abdominal scute. There is not indication that the pectoroabdominal sulcus reached the anterolateral corner of mesoplastron. In dorsal and lateral views (Figs. 2C–2F), the peripheral 6 is the most complete of the three preserved, showing a rectangular shape with its most medial margin (contact with costals) missing. The sulci between marginals are poorly preserved, however there is enough evidence that they were restricted to the peripherals, without reaching the costoperipheral sutural margin. The sutural contact between peripherals shows a medial indentation (Figs. 2E–2F), however this seems to be due to that the bone is naturally cut and cancellous tissue exposed. In anterior view (Figs. 2G–2H), the peripheral 5 and mesoplastron contact is well defined, and the bridge angle formed between the peripherals and the plastron indicates that the shell was probably low to moderate dome-shaped. Also in this view is evident the considerable thickness of these bones. A close-up of the margin of peripheral 5 (Figs. 2I–2J) shows a very thin external bone cortex and abundance of large pores at the cancellous bone. A large (88.17 cm, carapace length) of the extant Podocnemis expansa USNM-29476 is shown in Fig. 2K for comparison and anatomical location of UR-CP-0025 in a turtle shell.
Mesoplastra of Pan-Pleurodira. The mesoplastra bones have exhibited important modifications along turtle evolution. In basal Pan-Testudines as for example Odontochelys semitestacea they were two separate bony plates meeting medially (Li et al., 2008). In basal Testudines as for example Kayentachelys aprix there was a reduction in the number of mesoplastra, being only one pair extended medially reaching the central fontanelle (Joyce, 2007, fig 11). Another transformation of mesoplastra occurred in the both major groups of turtles, with their complete lost in Cryptodires and being one pair but they do not contact one another medially in Pan-Pleurodira (Cadena & Joyce, 2015; Joyce, 2007). Inside Pan-Pleurodira the mesoplastra have exhibited additional transformations from the primitive condition exhibited by Platychelyidae and Cretaceous-Paleocene members of Pan-Chelidae (Figs. 3A–3C, 3G–3I) of being almost triangular in shape, much wider than long to the condition exhibited by almost all Pelomedusoides of having almost equidimensional mesoplastra (Figs. 3E–3F, 3J–3L, 3P). An equidimensional mesoplastron was considered by Gaffney, Tong & Meylan (2006) as characteristic of a Nanorder that they defined as Eupleurodira (Cheloides = Pan-Chelidae plus Pelomedusoides). As I show in Fig. 3, the condition in pan-chelids who have mesoplastra is similar to the one exhibited by platychelids, which allow me to suggest that instead this is a characteristic of Pelomedusoides, shared by UR-CP-0025 described herein (Fig. 3E). Another transformation of mesoplastra inside Pan-Pleurodira is their complete lost in some Dortokidae, some Araripemydidae, crown-Chelidae, and Pelusios spp. (Gaffney, Tong & Meylan, 2006) (Figs. 3M–3O).
Carapace size estimation of UR-CP-0025.Using the simple linear regression equation (y = 6.5362x + 5.5002, x corresponding to maximum mesoplastron length, and y maximum carapace length) obtained from specimens of extant podocnemidids and some fossil pelomedusoids (Data S1). I estimated that the length of the carapace of UR-CP-0025 was of ∼40.59 cm, indicating a much larger size in contrast to the exhibited by Jurassic and Early Cretaceous platychelids, which fluctuated between 20 to 27 cm (Cadena, Jaramillo & Bloch, 2013, table 8.1). This suggests that the increase in shell size was a characteristic exhibited by early representatives of Pelomedusoides; a trend that continued during the Late Cretaceous (Hermanson, Ferreira & Langer, 2016) and the Cenozoic, with the giant pelomedusoids from the Paleocene of Colombia (Cadena, Bloch & Jaramillo, 2012; Cadena et al., 2012), and the Miocene Stupendemys geographicus from northern South America (Cadena et al., 2020).
Implications of UR-CP-0025 for Pelomedusoides history understanding. With the description of UR-CP-0025 and its attribution as belonging to Pelomedusoides (see above), I show that they inhabited northern South America during the Early Cretaceous. A hypothesis that is in agreement with recent molecular phylogenetic hypotheses that suggest they split from Chelidae during the Jurassic (Holley, Sterli & Basso, 2019; Pereira et al., 2017), therefore their fossil record should be expected to occur in Late Jurassic and Early Cretaceous sequences (Fig. 3Q). However, it is important to point out that UR-CP-0025 and the material previously described also from Rosa Blanca Formation (Cadena, 2011) are still too fragmentary to be recognized as the earliest indisputable record of the group, which it is not intention of this study. With this study, I once again showed that the Rosa Blanca Formation is still being a very productive and promising rock sequence in northern South America for future paleontological studies and the understanding of the Early Cretaceous faunas, including the evolution of Pelomedusoides turtles.
With this study, I once again showed that the Rosa Blanca Formation is still being a very productive and promising rock sequence in northern South America for future paleontological studies and the understanding of the Early Cretaceous faunas, including the evolution of Pelomedusoides turtles. Despite the taxonomic uncertainty that fragmentary material as UR-CP-0025 has, it exhibits the characteristic equidimensional mesoplastron of Pelomedusoides turtles, supporting the occurrence of this group of turtles in northern South America during the Early Cretaceous.