LYSTROSAURUS (long) (was Re: Mammal - Like - Reptiles)

[Pieter.Depuydt@rug.ac.be]

The following contains no information about dinosaurs, but about one 
of the icons of vertebrate paleontology, the well-loved Lystrosaurus.
Since there have been some questions about this animal, I thought it 
would be appropriate to mention some recent studies which 
questioned some well-established ideas and gave new insights in 
the ecology of  Lystrosaurus and the end-Permian extinction 
event.

The generic name Lystrosaurus was proposed by Cope in 1870, although 
the holotype skull was described more than ten years earlier by Owen.
Lystrosaurus remains have been found in abundant numbers in the South 
African Karoo, on Antarctica, India, China, and Russia, thus 
providing one of the strongest arguments for Wegener's theory of 
continental drift, as Jonatan Woolf already mentioned.
More than 20 species have been described in the course of the years, 
but taxonomic revisions (by Cosgriff, by King and Grine) have 
steadily reduced the number of valid species to 6 and maybe even to 
2.
The classical view of Lystrosaurus, and the way it has been 
reconstructed in countless popular books, is that of a 
hippopotamus-like amphibious or even fully aquatic animal; the 
pecular skull morphology with the long downturned snout and the 
highly placed nasal openings have traditionally been interpreted 
as adaptations to feeding below the water table. However, King and 
Cluver have argued that the skull and jaw features more likely were 
modifications to increase the orthal bite force (opposite to the more 
longitudinal jaw movement in earlier dicynodonts) and hence were 
adaptations to a tougher and more fibrous diet, which is also 
illustrated by the presence of some polished surface on the tusks of 
some specimens. The sacrum was well developed and the hind limbs 
operated in a more semi-erect fashion, also pointing towards a more 
terrestrial way of living. And finally, sedimentologic study and 
taphonomic study of  the Lystrosaurus environment  by R. Smith of 
the South African Museum has revealed that Lystrosaurus lived and 
died on a rather dry floodplain (in contrast with the wet floodplain 
habitat of the earlier Dicynodon fauna).
Lystrosaurus traditionally has been used as a stratigraphic marker, a 
guide fossil which identifies the strata in which it occurs as 
earliest Triassic. However, a Lystrosaurus specimen has recently been 
identified by King in a fossil assemblage which is otherwise 
typically Latest Permian, coming from the Madumabisa sandstone of 
Zambia. In the South African Karoo, a transitional zone has been 
identified between typically Latest Permian and Early Triassic 
sediments, in which Lystrosaurus occurs together with the typical 
Permian dicynodont Dicynodon. A similar transitional assemblage has 
been identified in China, where small Lystrosaurus specimens occur 
together with the Permian dicynodont Jimusuaria.
Based on all this, a picture emerges of a gradually changing 
environment in the Karoo in the Latest Permian and Earliest Triassic, 
with aridification of the climate, and the Permian wetland horsetail  
habitat, with its Dicynodon fauna, giving slowly way to a more dry 
floodplain, dominated by Lystrosaurus, well adapted to drought and 
feeding on a more sparse and resistant vegetation.

references:

R.M.H. Smith 1995: Changing fluvial environments across the 
Permian-Triassic boundary in the Karoo Basin, South Africa and 
possible causes of tetrapod extinctions.
Palaeogeography, Palaeoclimatology, Palaeoecology 117, 81-104.

G.M. King 1992: Species longevity and generic divesity in dicynodont 
mammal-like reptiles.
Palaeogeography, Palaeoclimatology, Palaeoecology 102, 321-332.

G.M. King 1991: The aquatic Lystrosaurus: a palaeontological myth.
Historical Biology 4, 285-321.

G.M. King 1997: The dicynodont Lystrosaurus from the Upper Permian of 
Zambia: evolutionary and stratigraphic implications.
Palaeontology 40 (1), 149-156.

Pieter Depuydt