Placerias (meaning 'broad body') is an extinct genus of dicynodonts that lived during the Carnian and Norian stages of the Triassic period (230–215 million years ago). Placerias belongs to a clade of dicynodonts called Kannemeyeriiformes, which was the last known group of dicynodonts before the taxon became extinct at the end of the Triassic.
Description
thumb|left|P. hesternus compared to a human
Placerias was one of the largest herbivores in the Late Triassic, weighing up to . The largest skull found had a length of .
Placerias had a powerful neck, strong legs, and barrel-shaped body with possible ecological and evolutionary parallels with the modern hippopotamus, spending much of its time during the wet season wallowing in the water and chewing at bankside vegetation.
Placerias used its beak to slice through thick branches and roots with two tusk-like flanges that could be used for defence and for intra-specific display. These so-called caniniforms were not true tusks derived from teeth, like in other dicynodonts, but were instead extensions of the skull merely mimicking tusks, likely covered in horn or beak tissue, a trait unique to the Stahleckeriidae family. The genus exhibits two morphs, one with short caniniforms and one with long caniniforms. This condition is inferred to be sexual dimorphism, with the longer-tusked individuals presumably being males. The only decently preserved Placerias skull, MNA V8464, was found in the same general area in 1991. Fragmentary Placerias fossils are also known from New Mexico Sedimentological features of the site indicate a low-energy depositional environment, possibly flood-plain or overbank. Bones are associated mostly with mudstones and a layer that contains numerous carbonate nodules. and Pentasaurus proved to be neither Cretaceous nor a dicynodont; it proved to be a specimen of a diprotodontid marsupial that probably dates to the Pliocene or Pleistocene.
Palaeobiology
Growth
P. hesternus outer cortical primary bone is generally zonal fibrolamellar in orientation, with a parallel-fibred peripheral layer. This suggests that P. hesternus experienced rapid osteogenesis punctuated by intervals of slower growth. Also, an external fundamental system has been described from a very large P. hesternus tibia, suggesting that this feature developed following the attainment of maximum size and cessation of growth.
See also
- List of therapsids
- Ischigualastia