Pantestudines or Pan-Testudines is the group of all reptiles more closely related to turtles than to any other living animal. It includes both modern turtles (crown group turtles, also known as Testudines) and all of their extinct relatives (also known as stem-turtles).[2] Pantestudines with a complete shell are placed in the clade Testudinata.
The identity of the ancestors and closest relatives of the turtle lineage was a longstanding scientific mystery, though new discoveries and better analyses in the early 21st century began to clarify turtle relationships. They had traditionally been considered relatives of the captorhinids, which also possessed an anapsid skull configuration. Later, the consensus shifted towards Testudinata's placement within Parareptilia, another "anapsid" clade.[3]
Analysis of fossil data has shown that turtles are likely diapsid reptiles, most closely related either to the archosaurs (crocodiles, bird, and relatives) or the lepidosaurs (lizards, tuatara, and relatives). An early proponent of this scenario was Goodrich (1916), who defended a diapsid origin of turtles based on morphological evidence.[4] Genetic analysis strongly favors the hypothesis that turtles are the closest relatives of the archosaurs, though studies using only fossil evidence often continue to recover them as relatives of lepidosaur or as non-diapsids. Studies using only fossils, as well as studies using a combination of fossil and genetic evidence, both suggest that sauropterygians, the group of prehistoric marine reptiles including the plesiosaurs and the often superficially turtle-like placodonts, are themselves stem-turtles.[1] This hypothesis had been previously investigated in the 19th century.[5]
Lee (2001) found that forcing the turtle group to cluster with archosauromorphs resulted in Rhynchosauria becoming Testudinata's sister clade. Forcing a relationship with lepidosaurs resulted in turtles being close relatives of sauropterygians within Lepidosauromorpha. The anapsid hypothesis was still better supported, although an archosauromorph affinity could not be rejected.[6]
Although morphology-based analyses usually do not support a turtle-archosaur clade (Archelosauria), Bhullar & Bever (2009) identified a laterosphenoid bone, typical of Archosauriformes, in the stem-turtle Proganochelys. It may serve as a synapomorphy for this proposed clade.[7]
The cladogram shown below follows the most likely result found by an analysis of turtle relationships using both fossil and genetic evidence by M.S. Lee, in 2013. This study found Eunotosaurus, usually regarded as a turtle relative, to be only very distantly related to turtles in the clade Parareptilia. However, Lee discusses the necessity to investigate the possibility that parareptiles are actually archelosaurs instead of non-saurian sauropsids.[8]