At its base, Theropoda is the wellspring of the three major groups of descendants: Coelophysoidea (named after Coelophysis and including some related, less well-known forms), Neoceratosauria (named after one of its members, the Jurassic Ceratosaurus and including some other bad boys, including the formidable Cretaceous-aged Carnotaurus) and Tetanurae (tetanus - stiff; uro - tail). It was in Tetanurae that some of the most remarkable theropod evolution took place.
Members of this group, whose record extends from the Middle Jurassic to the present, share a large number of other derived features (Figure 9.26), but the key feature linking all tetanurans is that the back half of the tail is stiffened by interlocking zygapophyses, fore-and-aft projections from the neural arches (Figure 9.27). We have seen that the tail is important counterbalance in theropod architecture, and it is no surprise that, through tetanuran evolution, there is a marked tendency to decrease the flexibility of the tail (except at its base).
Figure 9.26. Cladogram of Theropoda. Derived characters include: at 1, modification of the neural spines and transverse processes of the vertebrae, fusion ofthe sacral ribs with the ilium, ventral and lateral flaring of the crest above the acetabulum on the ilium, modification of the knee joint, and fusion between the upper ankle bones; at 2, low ridge demarcating the maxillary antorbital fossa, spine table on axis, reduced rodlike axial spinous process, prominent acromion on the scapula, loss of digit IV phalanges, metacarpal II nearly twice the length of metacarpal I, reduced femoral trochanteric shelf, prominent extensor groove on femur, fibular condyle on proximal tibia strongly offset from cnemial crest, broadly triangular metatarsal I attached to distal part of metatarsal II.
Figure 9.27. Zygapophyses in tetanurans. Note how these processes extend across the adjacent vertebrae both anteriorly and posteriorly, hindering flexibility.
- as well as an important clade of tetanurans known as Avetheropoda.
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