Paleontología Mexicana

Manuscript received: October 22, 2021.

Manuscript corrected: December 6, 2021.

Manuscript accepted: December 6, 2021.

1. Introduction

Platychelyidae includes five species belonging to two genera: Platychelys and Notoemys (de la Fuente and Iturralde-Vinent, 2001; Cadena and Gaffney, 2005). Platychelyids are known from Europe, South America, North America, and Caribbean Islands, and their fossil record spans from the Upper Jurassic to the Lower Cretaceous (Cadena and Joyce, 2015).

The first described member of the Platychelyidae was Platychelys oberndorferi (Upper Jurassic, Kimmeridgian–Tithonian of Germany and Switzerland) (Wagner, 1853). The Argentinian Notoemys laticentralis was the first platychelyid to be found outside Europe (Upper Jurassic, Tithonian) (Cattoi and Freiberg, 1961). The Cuban Notoemys oxfordiensis is the oldest known platychelyid (Upper Jurassic, Oxfordian), this species represents also the first platychelyid found in the Caribbean Islands (de la Fuente and Iturralde-Vinent, 2001). The Colombian Notoemys zapatocaensis is the only platychelyid known from the Lower Cretaceous (Valanginian) (Cadena and Gaffney, 2005). Lately, the last member of platychelyid was described from Mexico, Notoemys tlaxiacoensis (Upper Jurassic, Kimmeridgian) (López-Conde et al., 2017). All the above mentioned species form a monophyletic group within the stem Pleurodira, named Platychelyidae.

The Sabinal Formation is exposed in the areas of Tezoatlan, Huajuapan de León, Chalcatongo, Huamuxtitlan and Tlaxiaco, in the north-eastern part of the state of Oaxaca (Meneses-Rocha et al., 1994). The locality of Llano Yosobé is a small outcrop of the Sabinal Formation (Upper Jurassic, Kimmeridgian), it comprises a sequence of marine bituminous shales. This site it is important because it has the highest concentration and diversity of Jurassic marine vertebrates in Mexico (e.g. fishes, ichthyosaurs, plesiosaurs, marine crocodyliforms and turtles) (Barrientos-Lara et al., 2015, 2016, 2018; Barrientos-Lara and Alvarado-Ortega, 2018; Alvarado-Ortega and Brito, 2016; López-Conde et al., 2017). Notoemys tlaxiacoensis is the only known taxon turtle in the Upper Jurassic (Kimmeridgian) of Mexico (López-Conde et al., 2017). The presence of Jurassic turtles in other Mexican localities is unknown, leaving the Llano Yosobé as the only Jurassic locality to continue to provide new materials. Herein, a new turtle remains from this locality is reported. It corresponds to an insolate partial shell that presents similar characteristics observed in Notoemys tlaxiacoensis (López-Conde et al., 2017).

2. Geological setting

The fossiliferous outcrop of the Llano Yosobé (17°15’18’’N – 97°42’04’’W) is located 3.5 km southwest of Tlaxiaco downtown, in the Mixteca region of Oaxaca, southern Mexico (Figure 1). The fossiliferous sediments in the northern part of Yosobé are 60 m thick, at the base of which is a 30 – 40 cm thick marl layer containing trigonids, other bivalves, and ammonites (Alvarado-Ortega et al., 2014; Barrientos-Lara et al., 2015, Alvarado-Ortega and Brito, 2016). During the Jurassic, the Tlaxiaco Basin was active, receiving both continental and marine shelf sediments (Rueda-Gaxiola et al., 2007). López-Ticha (1985) named the Sabinal Formation comprising a sequence of marine bituminous shales exposed along the north-eastern of Oaxaca State. Meneses-Rocha et al. (1994) described the Sabinal Formation as a sequence of mudstone and wackestone clay, marl and dark grey to black bituminous shale strata with abundant calcareous concretions and ammonites, arranged in thin laminar layers of 5, 10, and 20 cm. López-Ticha (1969) estimated the age of the Sabinal formation based on its ammonite assemblage. The fossils of Yosobé were collected primarily from shale. Some of the fossils are preserved within nodules and others occur at the contacts between shale layers. The different types of preservation suggest different diagenetic processes. All the fossils preserved in nodules, including ostracodes and wood, are three-dimensional but most of the vertebrates and invertebrates preserved between layers of shale are severely flattened and commonly carbonized. These observations suggest that the nodules were formed relatively quickly after burial. The nodules commonly contain complete invertebrate carcasses, a process that seems to have facilitated three-dimensional preservation (Alvarado-Ortega et al., 2014).

Although the large vertebrates in Yosobé are commonly disarticulated, complete, or partially complete skeletons have been collected. The skeletal specimens are generally preserved by an incomplete process of nodule-formation, in which one or more adjacent nodules contains 3-D structures, but around them, the shale strata include strongly compressed structures of the same individual; therefore, it is very uncommon to obtain complete skeletons, especially when the nodules have been displaced from their original position, regardless the extent of the displacement. Vertebrate remains are generally black and relatively soft, but when weathered these can be white or red and are usually harder (Alvarado-Ortega et al., 2014).

3. Material and methods

The specimen described in this paper is housed in the Colección Nacional de Paleontología “María del Carmen Perrilliat”, Instituto de Geología, Universidad Nacional Autónoma de México, in Mexico City. The specimen corresponds to an isolate partial shell embedded on a shale matrix. The rocky matrix preserved on the bones was removed with pneumatic tools and dental needles. The shell fragment was hardened by applying a layer of plexygum dissolved in cyanoacrylate. In this study, the specimen was observed and measured directly, and photographs were obtained under different conditions. A comparative study was carried out with Notoemys tlaxiacoensis, observing the characteristics in the plates and scutes of the carapace and plastron.

Institutional abbreviations: MACN, Museo Argentino de Ciencias Naturales "Bernardino Rivadavia", Buenos Aires, Argentina; MOZ, Museo Olsacher de Zapala, Neuquén, Argentina; MGJRG, Museo Geológico Nacional "José Royo y Gómez", Instituto Colombiano de Geología y Minas-Ingeominas, Bogotá, Colombia; MNHNCu, Museo Nacional de Historia Natural, La Habana, Cuba; IGM, Colección Nacional de Paleontología, Instituto de Geología, UNAM, Ciudad de México.

Anatomical nomenclatures: We based our comparisons on the descriptions published by López-Conde and Alvarado-Ortega, 2017 and López-Conde et al., 2017.

Material reviewed: Notoemys oxfordiensis (MNHNCu-P), La Jagua Formation (Upper Jurassic, Oxfordian); Notoemys tlaxiacoensis (IGM-4861), Sabinal Formation (Upper Jurassic, Kimmeridgian); Notoemys laticentralis (MACN-18043 and MOZ-PV-2487), Vaca Muerta Formation (Upper Jurassic, Tithonian); Notoemys zapatocaensis (MGJRG IPN 15-EAC), El Caucho Formation (Lower Cretaceous, Valanginian).

4. Systematic Paleontology

Testudines Batsch, 1788

Pan-Pleourodira Joyce et al., 2004

Platychelyidae Bräm, 1965

Notoemys Cattoi and Freiberg, 1961

Notoemys tlaxiacoensis López-Conde, Sterli, Alvarado-Ortega and Chavarría-Arellano, 2017

Referred specimen. IGM-12994 (Figure 2); one partial shell.

Occurrence. Llano Yosobé (17°15’18’’N – 97°42’04’’W), Oaxaca, México. Sabinal Formation, Upper Jurassic (Kimmeridgian).

Description. This new material corresponds to an isolate shell fragment with the presence of elements of the carapace and plastron. Only four costals (costal two to costal five) are preserved. They are wider than long and the remaining part of these plates are rectangular in shape, in the same way as all Notoemys species. The preserved scutes in the carapace are two vertebrals and three pleurals. Vertebrals 2 and 3 are similar in shape to those of Notoemys tlaxiacoensis (hexagonal), and they are wider than long. The vertebral 2 covers the costals 2 and 3; contacts laterally with the pleurals 1 and 2. The sulcus between pleurals 1 and 2 is on the costal 2, and the sulcus between the vertebrals 2 and 3 is on the costal 3. The vertebral 3 covers the costals 3, 4 and 5; contacts laterally with the pleurals 2 and 3; and the sulcus between the pleurals 2 and 3 is on the costal 4. The preserved plates of the plastron are the hypoplastron, mesoplastron, and xiphiplastron; these plates are damage and the most part were not preserved. We can observe the inguinal area. The ischium can be observed, but no further description of this element can be made.

Remarks. Notoemys is an enigmatic group of Upper Jurassic-Lower Cretaceous turtles, and has four representatives: Notoemys oxfordiensis (de la Fuente and Iturralde-Vinent, 2001) from the Upper Jurassic (Oxfordian) of Cuba; Notoemys tlaxiacoensis (López-Conde et al., 2017) from the Upper Jurassic (Kimmeridgian) of Mexico; Notoemys laticentralis (Cattoi and Freiberg, 1961) from the Upper Jurassic (Tithonian) of Argentina and Notoemys zapatocaensis (Cadena and Gaffney, 2005) from the Lower Cretaceous (Valanginian) of Colombia (Figure 3). These species are known mainly for their shells, and they were collected in transitional and marine stratigraphic sequences, with abundant marine invertebrates and reptiles (Bräm, 1965; Fernández and de la Fuente, 1994; de la Fuente and Iturralde-Vinent, 2001; Cadena-Rueda and Gaffney, 2005; López-Conde et al., 2017). With the description of N. tlaxiacoensis, a gap was filled in the Kimmeridgian (Upper Jurassic). The fossil record of this taxon shows one gap in the Berriasian (Lower Cretaceous) (Figure 3).

Notoemys tlaxiacoensis, is the oldest turtle from Mexico. With the description of this new taxon, the stratigraphic reach of turtles was extended, previously the oldest record corresponded to the Lower Cretaceous (Albian) from the Tlayúa quarry (Reynoso, 2006).

5. Discussion and conclusions

The specimen IGM-12994 was recovered from the vicinity of the occurrence of Notoemys tlaxiacoensis the isolated shell fragment does not present any set of characters unique. Our observations suggest that the specimen IGM-12994 represents the remains of Notoemys tlaxiacoensis based on the similar characteristics observed in the shape and contacts of the costals plates and the vertebrals and pleurals scutes. The following diagnostic character supports our taxonomic assignment of IGM-12994 as a new N. tlaxiacoensis specimen: neural 3 hexagonal (being slightly octagonal in N. laticentralis and N. zapatocaensis and rectangular in N. oxfordiensis). The isolated shell fragment does not present any set of characters unique that indicates sexual or ontogenetic variability.

The paleogeography importance of this taxon corresponds to that it has only been collected until now in America. The fossil record of platychelyids on both continents during the Upper Jurassic and Lower Cretaceous reinforces the proposed connection between the Tethys and the Palaeopacific through the Hispanic Corridor. Notoemys inhabited shallow waters with tolerance to marine environments which helped them to disperse along the coastline through the Hispanic Corridor (López-Conde and Alvarado-Ortega, 2017) (Figure 4).

The discovery of turtles from other Jurassic localities is required to improve our knowledge about fossil turtles from Mexico. New remains of turtles collected in Llano Yosobé will be revealed, other materials of Jurassic turtles are collected and according to a preliminary result, they could correspond to a new Jurassic turtle. Therefore, it is important to continue the studies in this locality, to increase the knowledge of Jurassic vertebrates in Mexico.

Acknowledgements

We are deeply indebted to the local people of Tlaxiaco who helped us in multiple tasks during the field works. We also thank all students and paleontologists involved in the field work. The first author sincerely thanks María Guadalupe Conde Gutierrez for her confidence and support. We would like to thank anonymous reviewers whose comments have greatly improved the manuscript. Financial support for this research was provided by the UNAM (DGAPA-PAPIIT IN110920). The first author extends the acknowledgements to the Posgrado en Ciencias Biológicas, UNAM for their support, in addition to the CONACyT scholarship 545082. MSc. María Luisa Chavarría-Arellano took the photographs included in this article.

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Figure 1. Map of the locality. A, general map of Mexico showing the state of Oaxaca; B, close up to the state of Oaxaca and location of Tlaxiaco municipality; C, Geological map of The Llano Yosobé. This map includes the surrounding area of this locality and deposit belonging to the Sabinal Formation (Modified from Servicio Geológico Mexicano, 2000).

Figure 2. IGM-12994 corresponding to shell fragment of Notoemys tlaxiacoensis, from the Upper Jurassic (Kimmeridgian), from Llano Yosobé, Oaxaca, Mexico. A, dorsal view; B, ventral view; C, drawing of the dorsal view; D, drawing of the ventral view. Abbreviations: c, costal; hyp, hypoplastron; is, ischium; me, mesoplastron; Pl, pleural; V, vertebral; xi, xiphiplastron. Grey areas: sediment. Scale bar equals 2 cm.

Figure 3. Stratigraphic reach of Notoemys calibrated in time showing the relationships within Platychelyidae (Modified from López-Conde et al., 2017).

Figure 4. Distribution of Notoemys during the Upper Jurassic and Lower Cretaceous. A: Notoemys oxfordiensis. B: Notoemys tlaxiacoensis. C: Notoemys laticentralis. D: Notoemys zapatocaensis (Modified from López-Conde et al., 2017).