Abstract
The number of precaudal vertebrae in all extant crocodylians is remarkably conservative, with nine cervicals, 15 dorsals and two sacrals, a pattern present also in their closest extinct relatives. The consistent vertebral count indicates a tight control of axial patterning by Hox genes during development. Here we report on a deviation from this pattern based on an associated skeleton of the giant caimanine Purussaurus, a member of crown Crocodylia, and several other specimens from the Neogene of the northern neotropics. P. mirandai is the first crown-crocodylian to have three sacrals, two true sacral vertebrae and one non-pathological and functional dorsosacral, to articulate with the ilium (pelvis). The giant body size of this caiman relates to locomotory and postural changes. The iliosacral configuration, a more vertically oriented pectoral girdle, and low torsion of the femoral head relative to the condyles are hypothesized specializations for more upright limb orientation or weight support.
Highlights
The Neogene crocodylian fauna of northern South America is remarkable in terms of species richness, levels of species sympatry, and ecomorphological specialization (e.g., Riff et al, 2010; Sanchez-Villagra and Aguilera, 2006; Scheyer et al, 2013; Scheyer and Delfino, 2016)
We report here on the discovery of an exceptional skeleton of Purussaurus mirandai and several other remains from the late Miocene in Venezuela
Based on comparison with extant crocodylian skeletons (e.g., Crocodylus niloticus, Caiman yacare; Figure 4D) and the shapes of the sacral ribs and associated attachment sites on the ilia, we show that the last dorsal vertebra has been sacralised into a dorsosacral in P. mirandai
Summary
The Neogene crocodylian fauna of northern South America is remarkable in terms of species richness, levels of species sympatry, and ecomorphological specialization (e.g., Riff et al, 2010; Sanchez-Villagra and Aguilera, 2006; Scheyer et al, 2013; Scheyer and Delfino, 2016). Among a plethora of taxa, Purussaurus was an exceptionally large caiman (Alligatoroidea) that lived in the northern neotropics of South America in the middle and late Miocene Archosauria are represented today by 10,000 + species of birds (crown Aves), but less than 30 species of crocodylians (crown Crocodylia). As sister taxa, both groups have undergone profound changes in their body plans throughout ca.
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