Ophthalmosauridae, Baur, 1887
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publication ID |
https://doi.org/ 10.26879/454 |
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persistent identifier |
https://treatment.plazi.org/id/8131E10D-1D2A-E86D-CE6E-FBF82049F984 |
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treatment provided by |
Felipe |
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scientific name |
Ophthalmosauridae, Baur, 1887 |
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Family Ophthalmosauridae, Baur, 1887
Genus and species undetermined
Figure 10.4-6 View FIGURE 10
Referred material. IGM 9325, four caudal vertebral centra from Yosobé ( Figures 10.4-6 View FIGURE 10 ). It was originally preserved within a nodule from the shale layer between limestone strata L3 and L4 ( Figure 4 View FIGURE 4 ).
Occurrence. Yosobé, Tlaxiaco, Oaxaca. Kimmeridgian-Tithonian shales of the Sabinal formation.
Description. Although strongly eroded, in this specimen four centra from the anterior caudal region are recognized. These are deeply amphicelic centra, ovoid in anterior-posterior view, rectangular laterally, and about 1.8 times higher than long ( Figure 10.4-6 View FIGURE 10 ). The cavities on the anterior and posterior surfaces of these centra are just slightly tilted near the borders and vastly prominent near the center.
In all these centra the floor of neural canal is recognizable, it is practically flat, rectangular in shape, and laterally adjacent to two prominences that represent the facets of the autogenous neural arches. There is a foramen in the middle of the ventral (hemal) surface of these centra. In the ventrolateral area of these centra there is a single ovoid rib-facet that is located in the middle of the transverse process, which is extended through the entire length of the centra.
Remarks. Among marine reptiles, ichthyosaurs are unique in that they have amphicelic and higher than wide vertebra ( Andrew, 1910). As the IGM 9325 specimen shows these characteristics, its taxonomic determination as an ichthyosaur is consistent. Unfortunately, the characters of the anterior caudal vertebral centra are poor elements to achieve a more accurate taxonomical identification; however, it is possible to suggest that IGM 9325 belongs to Thunnosauria, an unranked clade erected by Montani (1999) to include Stenopterygius Jaekel, 1904 , Ichthyosaurus De la Beche and Conybeare, 1821 , and all their younger relatives ( McGowan and Motani, 2003).
Fisher et al. (2013, figure 2) noted three major radiation events along the evolution of ichthyosaurs. The Thunnosauria is the only natural group that survived until the Middle Jurassic of those originated from the “Neoichthyosaurian Radiation” occurred at the end of the Triassic. At the beginning of the Middle Jurassic, the Thunnosauria experienced a diversification event, the so called “Ophthalmosaurid Radiation,” in which different lineages emerged but apparently disappeared in the same epoch except for the family Ophthalmosauridae , which survived until the Cretaceous. This family includes the subfamilies Ophthalmosaurinae and Platypterygiinae. Ophthalmosaurinae was the most diverse and abundant subfamily in the Middle Jurassic, but began to decline in the Kimmeridgian and disappeared before the beginning of the Cretaceous. Platypterygiinae underwent the third radiation event of ichthyosaurs during the Kimmeridgian, the “Platypterygiine Radiation,” from which the groups that survived into the Cretaceous evolved. Fisher et al. (2013) described the only non-Ophtalmosauridae ichthyosaur that survived the Jurassic-Cretaceous boundary as Malawania anachronous . The description of this species was based on a single and incomplete specimen recovered from Hauterivian–Barremian marine strata in Iraq.
The only known specimen of Malawania anachronous lacks the caudal vertebrae, making the morphological comparison between this species and the specimen IGM 9325 impossible. Moreover, the synapomorphies of the Thunnosauria and its subgroups revealed in recent phylogenetic analyses (e.g., Fernández, 2007; Maxwell, 2010; Fisher et al., 2012, 2013; Druckenmiller and Maxwell, 2014) are characters related to the skull, limbs, and teeth; since none of these elements is preserved in IGM 9325, also it is not possible to make a comparison between these taxa and this specimen. Therefore the temporal occurrence of IGM 9325 within the Kimmeridgian suggests the inclusion of IGM 9325 within the family Ophtahlmosauridae . Members of this family were extremely diverse, abundant, and reached a worldwide distribution in the Kimmeridgian. In contrast, for now M. anachronous is a sole species restricted to a single locality in Iraq that probably represents the last basal and non-opthalmosaurid thunnosaurians, which were considered extinct before the Upper Jurassic (Fisher et al., 2013).
The caudal vertebrae preserved in IGM 9325 show a possible primitive condition also present in Ophthalmosaurinae and Platypterygiinae, in which these vertebrae are 3.5 times or less higher than long. Among the ophthalmosaurids only Ophthalmosaurus incenicus Seeley, 1874 , Arthropterygius chrisorum Maxwell, 2010 , show the possible homoplasic derived condition that is also present in some non-ophthalmosaurid ichthyosaurs, whose posterior dorsal and anterior caudal vertebrae are 4 or more times higher than long (see Massare et al., 2006; Fischer et al., 2012, appendices 1-4, character 23). In this context, it is not possible to conclude whether IGM 9325 belongs in Ophthalmosaurinae or Platypterygiinae; in consequence it is considered an undetermined member of the Ophthalmosauridae family.
Buchy (2007) acknowledged the occurrence of only two Late Jurassic ophtalmosaurids ichthyosaurs in Mexico, Ophthalmosaurus Seeley, 1874 and Brachypterygius Huene, 1922 , both collected in the north of Mexico, far from Yosobé. Although at present it is not feasible to provide new data on the diversity and paleobiogeography of ophtalmosaurids via the study of IGM 9325; this specimen is remarkable because it represents the southernmost record of this group throughout North America to date. It is expected that further work in the preparation of already collected and more complete additional specimens from Yosobé (skull fragments, jaws, and fins) will help to better understand the taxonomical diversity of ichthyosaurs in Mexico and their corresponding implications on other paleontological fields.
Other Vertebrates from Yosobé
The fossil diversity in the Yosobé locality is much higher than the sample presented here, since a large number of new specimens have already been collected (approximately 50 vertebrate specimens). This assemblage contains additional osteichthyan and reptile taxa whose preparation and precise taxonomical determinations require additional time: numerous ovoid cycloid scales, highly compressed, isolated, and disarticulated small fish bones, and articulated skeletons and isolated bones preserved tridimensionally within nodules, skull fragments, jaws, trunks, fins, and vertebrae in general.
Undoubtedly, among these the most abundant and best preserved correspond to pycnodonts, probably mostly Gyrodus . Liston et al. (2013) started the study of some remains of Pachycormidae fishes from Yosobé. Additionally, some extremely delicate and badly preserved isolated scales of rectangular shape with a height greater than the length, which have been observed as carbonized remains in the shales strata of Yosobé, resemble the typical scales of the longirostrine fishes grouped in the Family Aspidorhynchidae .
Moreover, other four taxa of marine Jurassic reptiles are present in Yosobé. Aside from the crocodylomorphs and ichthyosaurs already referred here, a few plesiosaurid remains from this locality have been preliminarly studied by Barrientos-Lara (2013), who is currently preparing their formal descriptions. In addition, two almost complete shells of possible sea turtles, collected in Yosobé by the two first authors during the 2009 field season, are yet to be prepared. However partially studied to date, the vertebrate fossil assemblage from Yosobé appears to be the most diverse and abundant among the Late Jurassic faunas in the south of North America and the Caribbean.
Non-vertebrate Fossils from Yosobé and La Lobera
The Jurassic invertebrate assemblage described, illustrated, and identified by Felix (1891, p. 141) from his “Cerro de Titania” was probably partially collected from La Lobera, where we have identified a rich association of invertebrates. Although these fossils are part of a comprehensive taxonomic and biostratigraphic study in progress; it can be noticed that most of them are referable to one of the 14 species described by Felix (1891), some of which have been recently placed in different genera. This assemblage includes the bivalves Myophorella (Trigonia) sologureni , Gryphaea mexicana , Astarte microphyes , the calcispongea Stellispongia (Ceriospongia) bernensis (Étallon in Thurmann and Étallon, 1864), the crinods Angulocrinus (Millericrinus) polyclonos , the brachiopod Animonithyris (Terebratula) dorenbergi , the annelids Glomerula (Serpula) gordialis ( Schlotheim, 1820) , and Mucroserpula (Serpula) tricarinata ( Sowerby, 1829) [non Goldfuss (1831) as cited by Felix]. The fossil assemblage from La Lobera also contains the echinoids previously reported by Buitrón (1970) from the “Caliza con Cidaris ” unit, including Cidaris submarginata and Acrocidaris nobilis Agassiz, 1840 .
Specimens of Lima comatulicosta Felix, 1891 are yet to be collected in La Lobera. Fragments of large ostreoids that probably belong in Exogyra Say, 1820 , have been collected in this locality; nevertheless, it is not possible to recognize among them any of the three species reported by Felix (1891). This observation suggests that the Cerro de Titania locality is a longer area in relation to the small area defined here as La Lobera. Despite this fact, several specimens of taxa not identified by previous authors have been collected in La Lobera, including an additional species of sea echinoids, remains of gasteropods, undetermined bivalves, and small fragments of corals that may represent a single species. In this site the ammonites are exceptionally scarce and extremely poorly preserved.
In Yosobé, the invertebrates also constitute an abundant and diverse assemblage. The marl thick strata at the bottom of its stratigraphical sequence ( Figure 4 View FIGURE 4 ) bears specimens attributable to a few of the taxa described by Felix (1891), including the bivalves Myophorella sologureni , Astarte microphyes and Gryphaea mexicana . Only the last two of these bivalves are distributed along the upper strata of Yosobé, where numerous large undetermined bivalves of very thin shell appear together with other unidentified bivalves that probably represent the ostreoids Exogyra . In the shale strata of this site there are numerous and diverse ammonites and aptychi that today are part of taxonomical and biostratigraphical study in progress by A.B. Villaseñor-Martínez (Instituto de Geología, UNAM). Finally, in this locality few fragments of corals have been collected, ostracodes and carbonized weed remains also are well preserved within the nodules.
| IGM |
Geological Institute, Mongolian Academy of Sciences |
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