A new xinjiangchelyid turtle from the Middle Jurassic of Xinjiang, China and the evolution of the basipterygoid process in Mesozoic turtles

Abstract

BackgroundMost turtles from the Middle and Late Jurassic of Asia are referred to the newly defined clade Xinjiangchelyidae, a group of mostly shell-based, generalized, small to mid-sized aquatic froms that are widely considered to represent the stem lineage of Cryptodira. Xinjiangchelyids provide us with great insights into the plesiomorphic anatomy of crown-cryptodires, the most diverse group of living turtles, and they are particularly relevant for understanding the origin and early divergence of the primary clades of extant turtles.ResultsExceptionally complete new xinjiangchelyid material from the ?Qigu Formation of the Turpan Basin (Xinjiang Autonomous Province, China) provides new insights into the anatomy of this group and is assigned to Xinjiangchelys wusu n. sp. A phylogenetic analysis places Xinjiangchelys wusu n. sp. in a monophyletic polytomy with other xinjiangchelyids, including Xinjiangchelys junggarensis, X. radiplicatoides, X. levensis and X. latiens. However, the analysis supports the unorthodox, though tentative placement of xinjiangchelyids and sinemydids outside of crown-group Testudines. A particularly interesting new observation is that the skull of this xinjiangchelyid retains such primitive features as a reduced interpterygoid vacuity and basipterygoid processes.ConclusionsThe homology of basipterygoid processes is confidently demonstrated based on a comprehensive review of the basicranial anatomy of Mesozoic turtles and a new nomenclatural system is introduced for the carotid canal system of turtles. The loss of the basipterygoid process and the bony enclosure of the carotid circulation system occurred a number of times independently during turtle evolution suggesting that the reinforcement of the basicranial region was essential for developing a rigid skull, thus paralleling the evolution of other amniote groups with massive skulls.

Allaeochelys libyca, a New Carettochelyine Turtle from the Middle Miocene (Langhian) of Libya

“…The anterior suture with the palatines is preserved and broadly convex. The sella turcica is broad and forms an overhang that covers the enlarged and widely spaced foramina anterior canalis carotici cerebralis (sensu Rabi et al 2013). The clinoid processes and the rostrum are damaged, and therefore, their morphology cannot be assessed.…”

A Review of the Fossil Record of Turtles of the CladeBaenidae

“…The internal carotid artery enters the skull halfway along the contact between the pterygoid and basisphenoid, but splits immediately within the skull into the palatine and cerebral branches (Gaffney 1975). It is therefore a question of semantics whether a true foramen posterius canalis carotici interni is present or, rather, tightly spaced foramina posterius canalis carotici palatinum and cerebralis (sensu Rabi et al 2013). The quadrates form a large cavum tympani and low mandibular condyles.…”

“…The monophyly of Paracryptodira is currently under debate, as some analyses retrieve them as monophyletic (e.g., Joyce 2007;Anquetin 2012;Gaffney et al 2007;Rabi et al 2013), but others as paraphyletic (e.g., Sterli et al 2013). All analyses agree that baenids diverged very close to the base of crown Testudines, but there is disagreement if they are situated at the base of Pan-Cryptodira (e.g., Joyce 2007), just outside of crown Testudines (e.g., Sterli et al 2013) or in an unresolved polytomy (e.g., Anquetin 2012).…”

A Review of the Fossil Record of Turtles of the CladeThalassochelydia

“…Joyce (2007) included an expanded sample by scoring Plesiochelys etalloni, Portlandemys mcdowelli, Jurassichelon oleronensis (his "Thalassemys" moseri), and Solnhofia parsonsi as terminal taxa, but they were found in a paraphyletic arrangement. More recent global phylogenetic analyses of turtles continued to include these species as separate terminal taxa Parham 2006, 2008;Sterli 2010;Anquetin 2012;Rabi et al 2013;Sterli et al 2013;Zhou et al 2014;Zhou and Rabi 2015), but none found them to form a monophyletic group.…”

“…Jurassichelon oleronensis is based on a single specimen (PIMUZ A/III 514) from the Tithonian of the Isle of Oléron, France (Rieppel 1980). This specimen represents one of the few skull-shell associations known for thalassochelydians and is therefore regularly included into phylogenetic analyses (e.g., Gaffney and Meylan 1988;Joyce 2007;Sterli 2010;Anquetin 2012;Rabi et al 2013). Rieppel (1980) originally referred this specimen to Thalassemys moseri Bräm, 1965, but pointed out that this species was probably more closely related to Plesiochelys than to Thalassemys hugii.…”