Prosauropoda

Prosauropods were once thought to be the early, primitive forebears of sauropods. Now, the group is commonly reckoned to be too specialized to have been directly ancestral to sauropods; rather, they retain many of the characters of the common ancestor of prosauropods and sauropods, an unidentified animal that likely must have lived in the Late Triassic. Prosauropods are monophyletic, and thus united by a suite of diagnostic characters. Recent phylogenetic work indicates that Prosauropoda can be subdivided into two monophyletic groups, with a few genera falling outside these subclades (Figure 8.19).

Figure 8.19. Cladogram of a monophyletic Prosauropoda. Derived characters include: at 1, lateral lamina on the maxilla, strap-like ventral process of the squamosal, ridge on lateral surface of the dentary, elongate posterior dorsal centra, distal carpal I transversely wider than metacarpal I, phalanx I on manual digit I with a proximal heel, a 45° twisting of the large thumb claw; at 2, separate opening for vena cerebralis media above the trigeminal foramen, axis centrum that is three times longer than high, short and robust metacarpal I, an acetabulum that is completely open medially, subtriangular distal end of ischium, increased robustness of metatarsals II and III; at 3, prefrontal length approximately that of the frontal, frontal excluded from supratemporal fossa, at least five premaxillary teeth, forelimb length greater than 60% hindlimb length, straight femoral shaft, fourth trochanter is displaced to the caudomedial margin of the shaft, hour-glass-shaped proximal end of metatarsal II; at 4, long retroarticular process on the lower jaw, longest postaxial cervical centrum at least three times as long as high, a dorsal vertebra added to the sacrum, proximal carpals present, large obturator foramen.

Prosauropoda Sauropodomorpha

Figure 8.19. Cladogram of a monophyletic Prosauropoda. Derived characters include: at 1, lateral lamina on the maxilla, strap-like ventral process of the squamosal, ridge on lateral surface of the dentary, elongate posterior dorsal centra, distal carpal I transversely wider than metacarpal I, phalanx I on manual digit I with a proximal heel, a 45° twisting of the large thumb claw; at 2, separate opening for vena cerebralis media above the trigeminal foramen, axis centrum that is three times longer than high, short and robust metacarpal I, an acetabulum that is completely open medially, subtriangular distal end of ischium, increased robustness of metatarsals II and III; at 3, prefrontal length approximately that of the frontal, frontal excluded from supratemporal fossa, at least five premaxillary teeth, forelimb length greater than 60% hindlimb length, straight femoral shaft, fourth trochanter is displaced to the caudomedial margin of the shaft, hour-glass-shaped proximal end of metatarsal II; at 4, long retroarticular process on the lower jaw, longest postaxial cervical centrum at least three times as long as high, a dorsal vertebra added to the sacrum, proximal carpals present, large obturator foramen.

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