Basilemys

Basilemys is a large, terrestrial trionychoid turtle that was from the Upper Cretaceous time period. In Greek, the "Basil (name)" means royal or kingly and the word "Emys" means turtle. Therefore, "Basilemys" means King Turtle. The stratigraphic subdivisions of the Upper Cretaceous include Cenomanian, Turonian, Coniacian, Santonian, Campanian, and Maastrichtian. "Basilemys" was mostly from the Campanian and Maastrichtian subdivisions of the Cretaceous time period and is considered to be the largest terrestrial turtle of its time. This extinct genus of land turtles belongs to the family Nanhsiungchelyidae. Occurrences of "Basilemys" have largely been reported in the North America region. It is interesting to note that the family Nanhsiungchelyidae made its first appearance in the Lower Cretaceous in Asia and we know from "Basilemys" that this family appeared in the Upper Cretaceous in North America. The North American populations of Basilemys are considered to be immigrants from Asia through the Beringia during the Upper 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. One genus from the family Nanhsiungchelyidae, "Zangerlia", is similar to "Basilemys" in terms of skull proportions. However, "Basilemys" has a more complex triturating surface that includes well-defined pockets on the dentary. "Basilemys" also has tooth-like projections on the triturating surface of the maxilla.
From the turtle species, Basilemys is described to be most similar to tortoises. Many paleontologists have described the behaviors of Basilemys to be similar to tortoises due to living in terrestrial habitats and the consumption of tough plants. Moreover, the complex triturating surface that is a feature of Basilemys, indicates that they are similar to tortoises in being terrestrial and herbivores. Basilemys is easily identifiable due to how thick its shell is, the intricate sculpture of rows of triangular tubercles separated by pits, and reduced inframarginal scales. The fossil record is abundant with shell and appendicular material for Basilemys, but cranial and cervical material is quite rare.
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 specimens that have been well-preserved for researchers to study Basilemys are the carapace, the hard upper shell of a turtle. Apart from the discovery of the genus Basilemys, there have been new discoveries of subtaxa such as Basilemys gaffneyi, Basilemys imbricaria, Basilemys morrinensis, Basilemys nobilis, Basilemys praeclara, Basilemys sinuosa, and Basilemys variolosa. In 2018, Basilemys morrinensis, was found due to a nearly complete shell 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 which is 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 is 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 belonging to Basilemys. The fragments have shallow pits and in between them, there are small pyramidal elevations. Another area where Basilemys remains have been recovered is the Aguja Formation. Similar to the shell fragments found in the El Gallo Formation, shell fragments of Basilemys were found in the Aguja Formation and they had large pits, the pits were 1 mm in depth, the pits had a diameter of 4 mm, and there were approximately 4 depressions per centimeter.
Description and Paleobiology.
Skull.
The skull proportions of Basilemys are very similar to the following genera: Adocus, Baptemys and Zangerlia. These genera have cheek and temporal regions that are deeply emarginated. The cheek emargination is short and deep, it also reaches above the level of the ventral edge of the orbit. The ventro-posterior corner of Basilemys 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 family 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 Adocus and Zangerlia, the external narial opening of Basilemys is small. 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, the orbits of Basilemys 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 palatine which is a condition that is seen in the following genera: Adocus, Baptemys, and Dermatemys.
On both sides of the skull of Basilemys, the triturating surface of the maxilla is visible. The triturating surface has a well developed maxillary tooth. The tooth is elongated, blade-like in structure, and it 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 identifying feature of turtles. The shells are made up of two portions: the carapace and the plastron. The carapace is the hard upper shell of a turtle while the plastron is the ventral surface of the shell. Turtle shells are phylogenetically important because it protects the animal. 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 prominences. In contrast, in Nanhsiungchelys, the pits are arranged more tightly and are in transverse rows over the costals. The pattern of the carapace in Basilemys can be described as being nearly isotropic.
The researchers that found the carapace of Basilemys 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 Basilemys morrinensis there are five vertebral scales. On the lateral sides of the vertebral scales, there are costals and pleural scales that make up this portion of the carapace. What borders the carapace are the peripherals and marginal scales. At the posterior end of the shell, the pygal bone sits here and right above it is the suprapygal.
The plastron of Basilemys is octagonal and elongated. The various parts that make up the plastron can be seen in the reconstructed version from the researchers that recovered the Basilemys morrinensis specimen from the Horseshoe Canyon Formation. Like the carapace, the plastron is made up of many bony elements. The plastron is divided into five 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 Basilemys morrinensis, the hyoplastron and hypoplastron appear to be the biggest sections of the plastron itself. The posterior end of the plastron, the last division is the xiphiplastron. The scales that make up the plastron include the humeral scale, the axillary scale, the pectoral scale, the abdominal scale, the femoral scale, and the anal scale.
Histology.
In an analysis of the shell bone histology of two stem trionychians, the 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. Osteoderms of the Basilemys have pronounced and elaborate sculpturing patterns. The sample from the North America Basilemys showed a highly organized “spindle-shaped pattern” of ornamentation. 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. Placement of Basilemys on where it should be in terms of the genera Nanhsiungchelys and Zangerlia. With close examination of shell histology, along with skull and neck proportions, it was determined that Basilemys is more closely related to Zangerlia. Based on similar characteristics, a group that seems to be closely related to Basilemys is Adocus. However, most cladograms place Adocus as an outgroup in relation to Basilemys.