Abstract
BackgroundProterochersis robusta from the Late Triassic (Middle Norian) of Germany is the oldest known fossil turtle (i.e. amniote with a fully formed turtle shell), but little is known about its anatomy. A newly prepared, historic specimen provides novel insights into the morphology of the girdles and vertebral column of this taxon and the opportunity to reassess its phylogenetic position.ResultsThe anatomy of the pectoral girdle of P. robusta is similar to that of other primitive turtles, including the Late Triassic (Carnian) Proganochelys quenstedti, in having a vertically oriented scapula, a large coracoid foramen, a short acromion process, and bony ridges that connect the acromion process with the dorsal process, glenoid, and coracoid, and by being able to rotate along a vertical axis. The pelvic elements are expanded distally and suturally attached to the shell, but in contrast to modern pleurodiran turtles the pelvis is associated with the sacral ribs.ConclusionsThe primary homology of the character “sutured pelvis” is unproblematic between P. robusta and extant pleurodires. However, integration of all new observations into the most complete phylogenetic analysis that support the pleurodiran nature of P. robusta reveals that this taxon is more parsimoniously placed along the phylogenetic stem of crown Testudines. All current phylogenetic hypotheses therefore support the basal placement of this taxon, imply that the sutured pelvis of this taxon developed independently from that of pleurodires, and conclude that the age of the turtle crown is Middle Jurassic.
Highlights
Proterochersis robusta from the Late Triassic (Middle Norian) of Germany is the oldest known fossil turtle, but little is known about its anatomy
The glenoid is fused, peanut-shaped, lacks a distinct neck, and consists of a flat facet formed by the coracoid and a flat facet formed by the scapula that are arranged at an angle of 120 degrees relative to one another (Figure 1B)
The right and left scapulacoracoids were removed from the block during preparation, but the scapular processes were oriented vertically, the distal end of the acromion was only separated by a small gap from the plastron, and the coracoid blades were oriented horizontally essentially mirroring the condition seen in all extant turtles
Summary
Proterochersis robusta from the Late Triassic (Middle Norian) of Germany is the oldest known fossil turtle (i.e. amniote with a fully formed turtle shell), but little is known about its anatomy. A series of recent papers have explored whether the Early Jurassic turtle Kayentachelys aprix is best interpreted as the oldest known stem cryptodire [12,13] or a stem turtle [14-16] These different interpretations imply a significantly different age of the turtle crown, which in return. The concurrent debate regarding the phylogenetic placement of the oldest known shelled amniote, Proterochersis robusta from the Late Triassic (Middle Norian) of Germany, is of equal importance. This taxon has traditionally been thought to have a pelvis that is sutured to the inside the shell (i.e., a “sutured pelvis”) and to be an early stem pleurodire, as this is traditionally believed to be an unambiguous apomorphy of the group [12,20,21]. The specimen is of particular importance because it helps clarify the orientation of the scapula among basal turtles, confirms the unambiguous presence of a sutured pelvis in P. robusta, and provides an abundance of other character information that further corroborates its basal position of this taxon along the turtle stem lineage
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