When bipedal archosaurs become aquatic and croc-like

It happened twice.
Some bipedal theropods and bipedal theropod-like archosaurs evolved to become quadrupedal crocs and quadrupedal croc-like theropods.

Most famously and most recently,
Spinosaurus (Fig. 1), long suspected as being a fish-eater, was reconstructed with shorter than expected hind limbs, thus forsaking any ability to walk on its hind limbs alone. Some workers think this chimaera reconstruction is bogus. Others are more accepting. Spinosaurus was derived from bipedal theropods like Suchomimus and Sinocalliopteryx. Deinocheirus was giant frill back bipedal theropod related to Spinosaurus.

Figure 1. Derived from bipedal sisters, giant Spinosaurus had such short hind limbs that it could no longer rise to a bipedal configuration. Not only did it have a croc-like head, it had something approaching a croc-like post-crania (sans the sail, of course).

Figure 1. Derived from bipedal sisters, giant Spinosaurus had such short hind limbs that it could no longer rise to a bipedal configuration. Not only did it have a croc-like head, it had something approaching a croc-like post-crania (sans the sail, of course).

Basal bipedal crocs evolved to become extant quadrupedal crocs
Basal crocs were bipeds (Fig. 2), only later shortening the hind limbs to become quadrupedal, like the theropod dinosaur, Spinosaurus (Fig. 1).

Figure 2. The evolution of extant crocs from primitive bipeds and transitional quadrupeds. One branch led to Protosuchus, the other, along with several side branches, to extant species.

Figure 2. The evolution of extant crocs from primitive bipeds and transitional quadrupeds. One branch led to Protosuchus, the other, along with several side branches, to extant species. Interesting to see that Sphenosuchus has taller dorsal spines than either predecessors or successors.

It’s of mild interest
to note that the transitional croc, Sphenosuchushad taller dorsal spines than either more primitive or more derived taxa (Fig. 2).

It’s of even milder interest
to note the quadrupedal poposaur, Lotosaurus, is derived from a sister to the bipedal poposaur, Poposaurus.

Figure 1. Lotosaurus, a finback poposaur.

Figure 3. Lotosaurus, a finback poposaur.

Contra this pattern,
the finback Arizonasaurus is a likely biped (based on its deep pelvis) derived from quadrupedal, shallow-pelvis, basal rauisuchians, like Vjushkovia. Even so, the closest relatives of Arizonasaurus include croc-like Yarasuchus and Qianosuchus both of whom have semi-tall spines.

Figure 1. Arizonasaurus configured as a biped. The depth of the pubis suggests a similar length for the femur and tibia. The gracile pectoral girdle suggests a gracile forelimb. The long deep tail is based on the related Yarasuchus.

Figure 4. Arizonasaurus configured as a biped. The depth of the pubis suggests a similar length for the femur and tibia. The gracile pectoral girdle suggests a gracile forelimb. The long deep tail is based on the related Yarasuchus.

A relative of Arizonasaurus
by analogy, not homology, is Qianosuchus (Fig. 5). It shares many traits with Spinosaurus, sans the frill.

Figure 4. Qianosuchus shares quite a few traits with Spinosaurus, sans the frill. Qianosuchus has similarly-sized limbs.

Figure 5. Previously unnoticed, the derived rauischian, Qianosuchus, shares many traits with Spinosaurus, sans the frill. Qianosuchus has similarly-sized limbs and a similar long rostrum and neck.

References
Ibrahim N et al. 2014. Semiaquatic adaptations in a giant predatory dinosaur. Science 345 (6204): 1613–6.

 

 

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