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Riversleigh provides a rare view of what animals existed in Australian
rainforests between about 12 million and 25 million years ago, when the
predecessors of the modern-day fauna, along with some now lost marsupial
groups which left no descendants, resided in a much warmer and wetter Australia.
The teeming life of Riversleigh's rainforests supplied abundant prey for
meat eaters, which included a number of thylacine species as well as early
dasyurids.
The Dasyuridae are a diverse group which includes specialized carnivores
such as the Tasmanian devil (Sarcophilus harrisii), and the quolls
(Dasyurus spp.), as well as smaller predators and insectivores such
as the Brush-tailed Phascogale (Phascogale
tapoatafa), dunnarts (Sminthopsis spp.) and tiny
Planigale
species which weigh less than 10 grams. |
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A
life reconstruction of the prothylacynine borhyaenid sparassodont
Prothylacynus
patigonicus from the Early Miocene of South America (illustration by
Michael R. Long). It was somewhat similar to the thylacine in form,
but not ancestral to it.
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It wasn't until the early 1980s that the thylacine's position within Australian
marsupial taxonomy became truly clear. Morphologically, the thylacine
bears a striking resemblance to the borhyaenids, an extinct family of large,
wolf-like metatherian predators from South America. However, in 1982,
a comparative study of blood proteins called albumins indicated that the
thylacine is in fact a close relative of the dasyurids, and suggested that
it had branched off from the main dasyurid line of evolution approximately
12 million years ago. |
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A decade later, analysis of DNA recovered from a preserved thylacine skin
provided an even stronger confirmation of this evolutionary affinity.
The borhyaenid link was simply another deceptive case of evolutionary convergence:
similar lifestyles create similar physical forms. However, the albumin
study also misled evolutionary biologists by underestimating how long ago
the thylacinids and dasyurids had diverged from an early proto-dasyurid
predecessor. The recent discoveries of a number of fossil
thylacinid species and at least one ancestral dasyurid at Riversleigh
from approximately 22 million years ago suggest a minimum divergence time
of 25 million years, and possibly even as far back as 30 to 40 million
years ago. |
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Among marsupials, the evolutionary relationship between thylacines and
dasyurids such as the Tasmanian devil, quolls and phascogales is no closer
than that of the wolf to its placental relations within the order Carnivora
- bears, pandas and raccoons. Man's primate ancestors diverged from
monkeys some 25-30 million years ago.
The wide genetic gap that exists between the thylacine and modern dasyurids
may possibly be one of the most significant obstacles to cloning a thylacine. |
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The
skull of Borhyaena, a carnivorous sparassodont metatherian from
the Miocene Santa Cruz Formation of Patagonia, Argentina.
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Should the final hurdle
of cross-species mitochondrial recognition be cleared, then the carrying
of the thylacine embryo would not pose a significant problem as either
the Tasmanian devil or Tiger quoll would technically be able to carry the
mammary foetus full term. Marsupial young are born at a very early
stage in their development, and the only foreseeable problem with either
mother would be pouch space, as the thylacine pup would be larger than
either of their natural young as it matures. However, as with orphaned
kangaroo joeys, artificial pouches could be utilised as a substitute in
the later stages of pouch development. |
