Basilemys

''Basilemys'' () is a large, terrestrial trionychoid turtle from the Upper Cretaceous. In Greek, the word "Basil" means royal or kingly and the word "Emys" means turtle. Therefore, ''Basilemys'' means King Turtle. ''Basilemys'' has been found in rocks dating to the Campanian and Maastrichtian subdivisions of the Late Cretaceous and is considered to be the largest terrestrial turtle of its time. ''Basilemys'' has solely been found in North America. The family Nanhsiungchelyidae, which is the family ''Basilemys'' belongs to, made its first appearance in the Lower Cretaceous, in what we now call Asia. Because of ''Basilemys,'' we know that this family appeared in North America in the Upper Cretaceous. It is possible that ''Basilemys'' and other nanhsiungchelyids are immigrants from Asia. They might have arrived in North America by passing through what we now call the Bering Strait somewhere during the Cretaceous. In an analysis made by Sukhanov et al. on a new Nansiunghelyid turtle from the Upper Cretaceous of Mongolia, it was demonstrated that Asian nanhsiungchelyids gave rise to the North American nanhsiungchelyids. ''Basilemys'' shares some traits with another member of nanhsiungchelyidae, '' Zangerlia'', which is similar to ''Basilemys'' in, for example, skull proportions. However, ''Basilemys'' has a more complex triturating surface that includes well-defined pockets on the dentary, and it also has tooth-like projections on the triturating surface of the maxilla.

From the species in nanhsiungchelyidae, ''Basilemys'' is considered to be most similar to tortoises. Many paleontologists have described the behaviors of ''Basilemys'' to likely be comparable to that of tortoises, due to living in terrestrial habitats and consuming tough plants. Moreover, the complex triturating surface of ''Basilemys'' indicates that they are similar to tortoises in being terrestrial herbivores. ''Basilemys'' is easily distinguishable from other fossil turtles due to how thick its shell is, the intricate sculpture of rows of triangular tubercles separated by pits, and its reduced inframarginal scales. The fossil record is abundant with material from the shell and with appendicular, but cranial and cervical material is quite rare for ''Basilemys''.

Geological Information and Discovery

Turtles were prominent members of the Upper Cretaceous and thus, their specimens found throughout North America are useful in defining biogeographic patterns. In 1924, a partial skeleton and crushed skull of ''Basilemys'' was collected by C.M. Sternberg from the Horseshoe Canyon Formation. However, this specimen was poorly preserved and information of the skull could not be analyzed. In another part of Canada, a ''Basilemys'' specimen from the Frenchman Formation of Saskatchewan retained most of the neck and cranial fragments. This was groundbreaking because of how scarce cranial and cervical material is for ''Basilemys''. Most of the well-preserved specimens of ''Basilemys'' are of the carapace, the hard upper shell of a turtle. Apart from the type species ''Basilemys variolosa'', five other species have been described to date. These include ''B. gaffneyi'', ''B. morrinensis'', ''B. nobilis'', ''B. praeclara'' and ''B. sinuosa.'

In 2018, a nearly complete shell of ''B. morrinensis'' was found from the Horseshoe Canyon Formation of Alberta. ''Basilemys'' specimens have also been found from the Oldman Formation and Dinosaur Park Formation.

Other shell fragments of ''Basilemys'' have been found in the El Gallo Formation, located in Baja California. Like the Horseshoe Canyon Formation and the Frenchman Formation, the El Gallo Formation is a rich fossiliferous geological unit. The three shell fragments that were found in the El Gallo Formation represent the first record of ''Basilemys'' from the Upper Cretaceous of Mexico. Due to the ornamentation of the shell fragments, researchers were able to identify them as ''Basilemys''. These ornamentations include shallow pits and small pyramidal elevations. ''Basilemys'' remains have also been recovered from the Aguja Formation, which stretches from Texas to Mexico. Here, shell fragments of ''Basilemys'' were found that are similar to the shell fragments found in the El Gallo Formation, also possessing the previously mentioned pits.

Description and Paleobiology

Skull

The proportions of ''Basilemys''' skull are very similar to those of '' Adocus'', '' Baptemys'' and ''Zangerlia''. These genera have cheek and temporal regions that are deeply emarginated. The cheek emargination is short and deep and reaches above the level of the ventral edge of the orbit. The ventro-posterior corner of ''Basilemys''' skull lacks a posterior projection into the temporal emargination which is unlike the genus ''Adocus''. Additionally, the temporal emargination of ''Basilemys'' projects forward and reaches the anterior edge of the cheek emargination. The deep cheek and temporal emarginations found in ''Basilemys'' are not seen in the genus '' Nanhsiungchelys'', which is part of the same family ''Basilemys'' belongs to; Nanhsiungchelyidae. Near the center of individual bones, the skull roof of ''Basilemys'' is developed and there are striations that extend outwards from these central regions. In contrast, the skull roof of ''Nanhsiungchelys'' is covered by sculpture that matches the carapace. Meanwhile, in ''Adocus'', the skull roof bones are smooth.

Resembling both ''Adocus'' and ''Zangerlia'', ''Basilemys'' has a small external narial opening. ''Basilemys'' has a deep premaxillary border that is just below the external narial opening. In addition, the premaxillae of ''Basilemys'' are paired instead of fused to form a single element like in the Trionychia family. Similar to other trionychoids, ''Basilemys''' orbits have large openings. At the antero-ventral edge of the orbit, a groove on the external surface of the maxilla borders it. The orbit is also extensively floored by the palate which is a condition that is seen in ''Adocus'', ''Baptemys'', and '' Dermatemys''.

The triturating surface of the maxilla is visible on both sides of the skull of ''Basilemys'' and has a well developed maxillary tooth. The tooth is elongated and has a blade-like structure. It also borders a deep, circular cavity medially. A deep labial ridge is also present. The lower jaw of ''Basilemys'' is short and deep. A sharp symphyseal hook is present on the lower jaw. The coronoid process of Basilemys is low and is located near the posterior end of the jaw.

Carapace and plastron

Turtle shells are a key feature in the identification and differentiation of turtles. The shells are made up of two portions: the carapace and the plastron. The carapace is the hard dorsal shell of a turtle while the plastron is the ventral surface of the shell. Turtle shells are important because they, to varying degrees, protect them from harm. The surface texture of a ''Basilemys'' carapace consists of many small, shallow pits that are arranged in a chain-link pattern. These shallow pits are bordered by low, tetrahedal protrusions. In ''Nanhsiungchelys'', the pits are arranged more tightly and are in transverse rows over the costals, which heavily contrasts the arrangement seen in ''Basilemys''. The pattern of the carapace in ''Basilemys'' can be described as being nearly isotropic.

The researchers who found the carapace of ''B. morrinensis'' from the Horseshoe Canyon Formation were able to reconstruct it through drawings and distinguish the various features of the carapace. The midline of the carapace consists of vertebral scales. On ''B. morrinensis'' there are five of these scales. On the lateral sides of the vertebral scales, there are costals and pleural scales that make up another portion of the carapace. The borders of the carapace are made up of the peripherals and marginal scales. The pygal bone sits at the posterior end of the shell, and the suprapygal is right above it.

''Basilemys''' plastron is octagonal and elongated. The various parts that make up the plastron can be seen in the reconstion made by the researchers who recovered the ''B. morrinensis'' specimen from the Horseshoe Canyon Formation. Like the carapace, the plastron is made up of many bony elements, and the plastron can be divided into five distinct parts. At the anterior end of the plastron, the first part is the epiplastron. Following the epiplastron is the entoplastron. The next two divisions after the entoplastron are the hyoplastron and the hypoplastron. From the reconstructed images of the plastron of ''B. morrinensis'', the hyoplastron and hypoplastron appear to be the largest sections of the plastron. At the posterior end of the plastron, the last division is the xiphiplastron. The scales that make up the plastron include the humeral-, axillary-, pectoral-, abdominal-, femoral- and anal scales.

Histology

In a histological analysis of the shell bones of two groups of stem-trionychians, Adocidae and Nanhsiungchelyidae, it was found that the shell bones of the Nanhsiungchelyids have a diploe structure and cortical bone layers that frame the interior cancellous bone. The osteoderms of ''Basilemys'' were found to have pronounced and elaborate sculpturing patterns. The sample from the North American ''Basilemys'' showcased a highly organized “spindle-shaped” ornamentation pattern. In addition, the sculpturing pattern of ''Basilemys'' is made of irregular grooves and pits of external bone surface which is what is typically described as the “pock-mark” surface.

Classification

Based on the specimens found from the family Nanhsiungchelyidae, researchers have been able to create cladograms based on their findings. The placement of ''Basilemys'' puts it closest to the genera ''Nanhsiungchelys'' and ''Zangerlia''. Both within its family, Nanhsiungchelyidae. With close examination of shell histology, along with skull and neck proportions, it was determined that, of the two, Basilemys is more closely related to ''Zangerlia''. Based on similar characteristics, ''Adocus'' was recovered as relatively closely related to ''Basilemys''. However, most cladograms place ''Adocus'' outside of Nanhsiungchelyidae.

References

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{{Taxonbar, from=Q21224338

Trionychia
Nanhsiungchelyidae
Cryptodira
Emys
Extinct turtles
Campanian genus first appearances
Maastrichtian genus extinctions
Late Cretaceous turtles of North America
Cretaceous Mexico
Fossils of Mexico
Cretaceous United States
Fossils of the United States
Hell Creek fauna
Kaiparowits Formation
Laramie Formation
Fossil taxa described in 1902