Re: Triassic mammal-like reptiles?

[K and T Dykes <ktdykes@arcor.de>]

Hi Rodlox,

<<What about the mammal-like-reptiles ?  *curious*>>

'Mammal-like reptiles' have more or less been abolished in English, although
similar phrases continue in other languages.  They've become non-mammalian
synapsids.  Synapsids and sauropsids (including reptiles) are now usually
considered to be sister lineages with ancestry among amniotes.  The fossil
record presently has both lines starting in the Upper Carboniferous, and
whether sauropsids or synapsids evolved first is unclear.

Three synapsid lineages are known to have survived the Permian-Triassic mass
extinction(s); dicynodonts, therocephalians and cynodonts.  Dicynodont
herbivores became widespread and diverse, but more or less disappear during
the beginnings of the Upper Triassic (Carnian).  Bizarrely, there are
fragments of skull apparently from the Lower Cretaceous of Australia, which
have been sensibly interpreted as dicynodont.  If the time and the diagnosis
are correct (and they seem to be), that is astonishing.  There's no sign of
the things anywhere else beyond the Carnian.  The most important Triassic
groups were lystrosaurids and kannemeyeriids.  The former tended to be
piggish-sized while the latter could be up to around five metres in length.

Therocephalians were smallish carnivores, rare and went out of production
relatively quickly.  They don't seem to have survived beyond the Middle
Triassic.

More exciting are the cynodonts, as we're still going strong.  Among the
earliest Triassic forms is /Thrinaxodon/; a pleasingly mammal-like,
fox-sized killer of lystrosaurids (there wasn't much choice in the first
couple of million years of the Triassic, as the genus /Lystrosaurus/
typically supplies around 90% of terrestrial vertebrate fossils, and that's
everywhere).  However, more derived eucynodonts ('true dog teeth') began
turning up relatively quickly; /Cynognathus/ and its kith and kin.  This
gang included both herbivores and carnivores.  The main plant eaters were:
diademodontids, trirachodontids (Lower-Middle Triassic), traversodontids
(Middle-Upper Triassic) and tritylodontids (Upper Triassic-Lower
Cretaceous -and I do really mean Cretaceous, /Xenocretosuchus/).  It's
possible the tritys are descended from the travies, but now seems
improbable.  Much research suggests they had similar specialisations for
similar diets.  I should mention /Cynognathus/ was an up to wolf-sized,
globetrotting predator of the Lower-Middle Triassic, otherwise it might bite
me.  It happens to be the most basal of the eucynodonts.

Other ferocious eucynodonts also emerged, with much interesting information
presently coming from South America.  The Middle Triassic of Brazil and
Argentina was a great place to be attacked by chiniquodontids.  Some
representatives were quite large; skull lengths of up to 30cm.  The latter
stages saw the arrival of a much smaller, insect-baiter: /Probainognathus/.
The skulls get up to around 6cm, so this is something like rat-sized.  The
lower jaw bone (dentary) was getting suspiciously close to an upper bone on
the skull called the squamosal.  The original description claimed there was
a joint between those two bones, but this was shown to be an error.  There's
a secondary jaw joint but its built from different elements.  Nevertheless,
those bones happen to be unusually close.

The systematics of Upper Triassic forms are presently somewhat hazy.
Sometimes, the word 'Tritheledonta' is used as a generalised term for a
variety of small insect killers, all of which are probainognathians of one
sort or another, but they aren't necessarily close relatives.  Prime among
the Upper Triassic brigade is a disreputable family called 'Dromatheriidae'.
These are poorly known isolated teeth and fragments of jaw.  Although
fragmentary, some of this stuff is suspiciously and very mammal-like.  Some
postcanine teeth are unusually complex and incipiently double-rooted.
Dromatheriids were joined relatively late in the Triassic by an increasingly
disreputable family called 'Tritheledontidae'.  I've added inverted commas
to that concept myself.  Many professional researchers treat it in the
literature as a valid family, although doubts are growing.  The mammalian
jaw joint happens to be present in at least some trithes: /Pachygenelus/ and
(perhaps but disputed) /Diarthrognathus/.  These critters were still
munching insects in the Lower Jurassic, but they're unknown from later days.
Basal mammals had similar tastes, and appear to have outeaten them.

Brief summary: The relevant later animals differ to the earlier ones, which
is as would be expected, and many funny names are involved.

I'm not very visually minded, so I don't know much about images.  There are
probably some links on my directories, but I'm not entirely sure where or
what.  Shockingly, graphics aren't high on my list of priorities (and the
visual arts' community may now be in a fury (-[
Cheers
Trevor

Triassic Eucynodonts
http://home.arcor.de/ktdykes/trias.htm
The Mesozoic - more than just the dinosaur