PLEASE NOTE: THIS PROJECT IS NOT SCIENTIFIC. IT IS A HOBBY.
"I was looking for information on an old mammal and found this lot. What is this
project?"
It's got lots of information on old mammals. For a short bit of background information, see
here.
Metatheria refers to our cousins, the marsupials and the earlier ancestors. The oldest
known representative is 125 million years old and from China. At least most Mesozoic
members discovered so far come from the northern hemisphere; North America, Asia and
possibly Europe, (although South America and Madagascar may have some Upper Cretaceous
material). Some general comments on marsupials are in the first directory. There are many
more reliable sources than myself around, though.
Administrative note
A study by Case, Goin & Woodburne (2004) came to hand, and this allowed a review of the
structure of the metatherian directories. Rearrangements were at least generally in line
with that study. It's the most convenient scheme presently available to me, but that may
change at some point. Two new genera were involved; Nortedelphys and
Hatcheritherium. Both are from the Upper Cretaceous of North America, and may be
visited in the "Polydolopimorphia" & Ameridelphia enclosure.
In this scheme, the supercohort of Marsupialia is divided into three cohorts and an order of
uncertain affinities. The cohorts are: Alphadelphia, Ameridelphia and Australodelphia. The
order is "Polydolopimorphia". Alphadelphians (aka boreometatherians) are predominantly
Cretaceous as is at least one ameridelphian. The third cohort is beyond the scope of this
project. The attractively named "polydolopimorphians" presently include three Cretaceous
genera.
Polydollies?
However, their allocation of several North American Cretaceous critters to Polydolopimorphia
hasn't exactly met with universal agreement. Indeed, the resultant barrages from different
paleontological ack-ack brigades appear to have blown that suggestion out of the sky. In
short, the dental qualifications proposed for membership of the order were found to be
unconvincing and, as Nortedelphys and Co don't seem to be genuine members,
I've resorted to adding quotation marks to that ordinal name in the context of these
directories. There is such an order but, strictly speaking, it doesn't appear to have any
Cretaceous members. I'm still broadly following Case et al's bookkeeping until a better
option becomes available, or the means turn up to relocate individual genera.
Alphadelphia is something of an artificial construction rather than a natural taxon. It
doesn't include one ancestor and all of its descendants. It's paraphyletic. The ancestry
of the two other cohorts is presumably within it, (p.226).
Case et al (2004), "South American" Marsupials from the Late Cretaceous of North
America and the Origin of Marsupial Cohorts, Journal of Mammalian Evolution, 11(3-4),
p.223-255.
The following is based upon my reading of Cifelli & Davis, 2003.
The Therian Splits
Although metatherians and
eutherians employ significantly different modes of reproduction, both lineages are
closely related. They're sister lines and share ancestry. Quite when their most recent
common ancestor lived is the subject of on-going study, but the earliest representatives of
each hail from the lower Cretaceous of Liaoning, China, and seem to date back about 125
million years.
With the exception of the relatively recent, North American arrival, the Virginia opossum,
living marsupials are restricted to Australia and South
America, (p.1899). This is in stark contrast to the fossil record of the Upper Cretaceous;
North America and Eurasia, with perhaps one or two genera in South America.
"The balance of paleontological and morphological data suggest that the last common
ancestor of metatherians and eutherians was Laurasian..., although there is no universal
agreement on this point...", (reference numbers omitted). The date probably lies during
the Lower Cretaceous, seeing as that's when the first mammals with 'modern' tribosphenic
molars are known from the northern hemisphere, though new
finds could of course tell a differing story. Still, in terms of what's presently available,
the therian split seems to have occurred between 145 and 125
million years ago; certainly no later.
As Sinodelphys is
basal as well as early, this may indicate a Eurasian cradle for Metatheria. However,
no North American Cretaceous mammals older than about 110 million years have yet been
found, (p.1900). When some turn up the picture might change, Be that as it may, the
dominant therians in the Cretaceous of Eurasia were eutherians. In North America these were
absent to rare faunal elements, and only became noticeably diverse during the final ten
million years of the time. Given that both earliest know representatives presumably had
relatively similar lifestyles, (small clamberers with a taste for insects), the reasons
for this geographical contrast are unclear.
The age of the metatherian-eutherian split has also been estimated by molecular studies,
but results have been conflicting. Dates of between about 160 to 190 million years have
been suggested. If correct, the gaps in the fossil record would be enormous. One of these
studies put the most recent common
monotreme-therian ancestor deep in the Triassic,
(236-238 million years ago), which appears extremely improbable and vastly predates any
known mammal according to the broadest of definitions.
Cifelli RL & Davis BM (2003), Marsupial Origins, Science, 302, p.1899-1900.
