| Basilemys | |
|---|---|
 | |
| B. variolosa skeleton,Royal Tyrrell Museum | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Testudines |
| Suborder: | Cryptodira |
| Family: | †Nanhsiungchelyidae |
| Genus: | †Basilemys Hay 1902 |
| Type species | |
| †Basilemys variolosa (Cope, 1876) | |
| Species | |
Basilemys ("king turtle" in Greek) is a large, terrestrialnanhsiungchelyidturtle from theUpper Cretaceous of North and Central America.[1]Basilemys has been found in rocks dating to theCampanian andMaastrichtian subdivisions of the Late Cretaceous and is considered to be the largest terrestrial turtle of its time.[2] In an analysis made by Sukhanov et al. on a nansiunghelyid turtle from the Upper Cretaceous of Mongolia, it was demonstrated that Asian nanhsiungchelyids gave rise to the North American nanhsiungchelyids.[3]
Amongst nanhsiungchelyids,Basilemys is considered to be most similar totortoises.[4] Many paleontologists have described the behaviors ofBasilemys to likely be comparable to that of tortoises, due to living in terrestrial habitats and consuming tough plants.[4]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.[2] The fossil record is abundant with material from the shell, but cranial and cervical material is quite rare forBasilemys.[1]
Turtles were prominent members of the Upper Cretaceous and thus, their specimens found throughout North America are useful in defining biogeographic patterns.[5] In 1924, a partial skeleton and crushed skull ofBasilemys was collected by C.M. Sternberg from theHorseshoe Canyon Formation.[1] However, this specimen was poorly preserved and information of the skull could not be analyzed.[1] In another part of Canada, aBasilemys specimen from theFrenchman Formation of Saskatchewan retained most of the neck and cranial fragments.[1] Most of the well-preserved specimens ofBasilemys are of thecarapace, the hard upper shell of a turtle. Apart from the type species,Basilemys variolosa, five other species have been described to date. These includeB. gaffneyi,B. morrinensis,B. nobilis,B. praeclara andB. sinuosa.[1] In 2018, a nearly complete shell ofB. morrinensis was found from the Horseshoe Canyon Formation of Alberta.[4]Basilemys specimens have also been found from theOldman Formation andDinosaur Park Formation.[6]
Other shell fragments ofBasilemys have been found in theEl Gallo Formation, located in Baja California.[5] Like the Horseshoe Canyon Formation and the Frenchman Formation, the El Gallo Formation is a rich fossiliferous geological unit.[5] The three shell fragments that were found in the El Gallo Formation represent the first record ofBasilemys from the Upper Cretaceous ofMexico.[5]Basilemys remains have also been recovered from theAguja Formation, which stretches fromTexas to Mexico.[7] Here, shell fragments ofBasilemys were found that are similar to the shell fragments found in the El Gallo Formation, identified by shallow pits and small pyramidal elevations on the carapace.[5][7]
The proportions ofBasilemys' skull are very similar to those ofAdocus,Baptemys andZangerlia.[1] These genera have cheek and temporal regions that are deeply emarginated.[1] The cheek emargination is short and deep and reaches above the level of the ventral edge of the orbit.[1] The ventro-posterior corner ofBasilemys' skull lacks a posterior projection into the temporal emargination which is unlike the genusAdocus.[1] Additionally, the temporal emargination ofBasilemys projects forward and reaches the anterior edge of the cheek emargination.[1] Near the center of individual bones, the skull roof ofBasilemys is developed and there are striations that extend outwards from these central regions.[1] In contrast, the skull roof of the relatedNanhsiungchelys is covered by sculpture that matches the carapace.[1]
Resembling bothAdocus andZangerlia,Basilemys has a small external narial opening.[1]Basilemys has a deep premaxillary border that is just below the external narial opening.[1] In addition, the premaxillae ofBasilemys are paired instead of fused to form a single element like in theTrionychia superfamily.[1] Similar to other trionychoids,Basilemys' orbits have large openings.[1] At the antero-ventral edge of the orbit, a groove on the external surface of the maxilla borders it.[1] The orbit is also extensively floored by the palate which is a condition that is seen inAdocus,Baptemys, andDermatemys.[1]
The triturating surface of the maxilla is visible on both sides of the skull ofBasilemys and has a well developed maxillary tooth.[1] The tooth is elongated and has a blade-like structure, bordering a deep, circular cavity medially.[1] A deep labial ridge is also present.[1] The lower jaw ofBasilemys is short and deep, with a sharp symphyseal hook is present on the lower jaw.[1] The coronoid process of Basilemys is low and is located near the posterior end of the jaw.[1]
Basilemys shares some traits with another nanhsiunchelyid,Zangerlia, which is similar toBasilemys in, for example, skull proportions.[6] 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.[6]
Turtle shells are a key feature in the identification and differentiation of turtles. The surface texture of aBasilemys carapace consists of many small, shallow pits that are arranged in a chain-link pattern.[4] These shallow pits are bordered by low,tetrahedal protrusions.[4] InNanhsiungchelys, the pits are arranged more tightly and are in transverse rows over the costals, which heavily contrasts the arrangement seen inBasilemys.[4] The pattern of the carapace inBasilemys can be described as being nearly isotropic.[4]
The researchers who found the carapace ofB. 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.[4] On the lateral sides of the vertebral scales, there are costals and pleural scales that make up another portion of the carapace.[4] The borders of the carapace are made up of peripheral and marginal scales.[4] The pygal bone sits at the posterior end of the shell, and the suprapygal sits right above it.[4]
Basilemys' plastron is octagonal and elongated.[4] 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, followed by the entoplastron.[4] The next two divisions are the hyoplastron and the hypoplastron; from reconstructed images of the plastron ofB. 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.[4] The scales that make up the plastron include the humeral-, axillary-, pectoral-, abdominal-, femoral- and anal scales.[4]
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.[8] Theosteoderms ofBasilemys were found to have pronounced and elaborate sculpturing patterns.[8] The sample from the North AmericanBasilemys showcased a highly organized "spindle-shaped" ornamentation pattern.[8] In addition, the sculpturing pattern ofBasilemys is made of irregular grooves and pits of external bone surface which is what is typically described as the "pock-mark" surface.[8]
Based on the specimens found from the family Nanhsiungchelyidae, researchers have been able to create cladograms based on their findings. The placement ofBasilemys puts it closest to the generaNanhsiungchelys andZangerlia. With close examination of shell histology, along with skull and neck proportions, it was determined that, of the two,Basilemys is more closely related toZangerlia.[9]