Australia: The Land Where Time Began

A biography of the Australian continent 

Riversleigh Wombats

Four species of wombats are presently living, 3 of which are hairy-nosed species, Lasiorhynus latifrons, L. kefftii, and L. barnardi. The 4th is the Common Wombat, Vombatus ursinus. In the Pleistocene there were a number of species that ranged in size from the size of a small dog to that of a cow (Phascolonus gigas). This  latter species had huge chisel-like upper incisors. There were also many living in the Pliocene, the main differences among tem was the size and shape of their incisors.

One stands out as being different from all the others, Warendja wakefieldi from the Late Pleistocene. This species appears to have been a survivor from an early stage of radiation of the wombats. When grasslands spread as the continent continued the long process of drying the marsupials gave rise to 2 main types that took advantage of the increasingly common habitat type. These are the wombats and the kangaroos. When any animal specialises in grass-eating it needs to evolve some method of avoiding the wearing out of its teeth prematurely because grass has small silica particles in the leaves that are very abrasive when ground by the molars, rapidly wearing them down.

These 2 groups responded to the new and increasing food source in different ways. The kangaroos responded by changing the sequence of eruption of their teeth and molar progression, as their teeth wore down they were replaced by new teeth moving to the part of the mouth that was used most heavily for the processing of the grass. The wombats responded differently, they evolved high-crowned teeth that were eventually rootless, that is, they continued growing throughout the life of the animal. Wombats never have their teeth worn down by years of chewing the tough grass.

Rhizophascolonus crowcrofti is the oldest known fossil wombat. It was found in the Middle Miocene deposits in the Kutjamparu Local Fauna in central Australia. The species, known only from a premolar and 2 molars, was identified as a wombat. The teeth have a form intermediate between the hypothetical ancestral wombat with low-crowned teeth with roots and the Pliocene and Quaternary forms whose teeth were high-crowned and rootless. This particular form had high-crowned wombat-like teeth with closed roots on the 3 known teeth, indicating it was at an early stage in the evolution of teeth that were required to process a tough type of food, such as the grasses with their high silica content.

At Riversleigh a species of wombat has been found in deposits from the end of the Middle Miocene that appears to be very similar to R. crowcrofti. A species has been found in the Encore Site on the GAG Plateau at Riversleigh that appears to be a primitive species of Warendja. At this site it was very abundant and there were 2 other animal types that seem to be present only here, a very large species of the genus Ekaltadeta, a carnivorous kangaroo, and a very large Hypposiderid bat. This assemblage of animals has suggested that the site may actually be of Late Miocene age. The Warendja-like had continuously growing teeth that were obviously adapted for feeding on abrasive plant material, probably grass.

Included among the wombats at Riversleigh are many that are much more primitive than Rhizophascolonus or Warendja. One of these primitive forms from the Early Miocene, Boid Site East Local Fauna, was about the size of a Brushtail Possum. It had the skull and dental formula of a wombat, but with rooted teeth that were relatively low-crowned. At Upper Site an animal of a similar size to that of modern wombats, but with rooted teeth, the crowns of which were much more complex than any previously known wombats. This animal is known from isolated teeth.

Sources & Further reading

  • Michael Archer, Suzanne J. Hand & Henk Godthelp, Australia's Lost World: Riversleigh, world heritage Site, Reed New Holland
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 25/02/2011

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading