|
|
Eucynodontia (non-mammalian), an overview:
Cynodonts, ('dog teeth') include the most mammal-like non-mammalian therapsids
('mammal-like reptiles'). The most derived of these are grouped together with mammals within
Eucynodontia, ('true dog teeth'), which literally includes true dogs. Its membership was and is
made up of both meat-eaters and plant-eaters. The chronological range extends from at least
the Lower Triassic until the present day. This overview is concentrated on the proto-mammals,
which are known from the Lower Triassic until the Lower Cretaceous.
Amongst the first and most basal of the eucynodonts was Cynognathus. This was a wolf-
sized predator, which had pretty much a worldwide distribution. About 90% of its lower jaw
was accounted for by a single, tooth-bearing bone called the dentary. Its teeth were
differentiated, which enabled them to perform several functions; tearing and chewing. A
crocodile tears at its prey, but it can't chew. It's an effective hunter, but a wasteful and
messy eater. The ear of Cynognathus contained a solitary small bone for hearing, (the stapes).
The jaw was attached to the skull by a joint called the articular-quadrate. The significance
of these features and what happened next, is illustrated in a bit more detail below, (see
Transition).
Cynognathus is the only known representative of a family called Cynognathidae. It's on a
directory with several closely related families of herbivores. Along with Traversodontidae and
possibly Tritylodontidae, (below), these groups make up Cynognathia.
Taxon: CYNOGNATHIA
Cynognathia
Gomphodonts and tritylodontids
The most basal representatives are found within a family called Diademodontidae. Most fossils
come from the Lower Triassic of South Africa. Other reports stem from Asia and perhaps
Antarctica.
Family: Diademodontidae
Somewhat more derived are the trirachodontids of Africa, Asia, Russia and possibly North
America. Some were contemporaries of the diademodontids and the lineage seems to have
survived until the Middle Triassic. Trirachodon lived communally in warrens. This is known
from several fossilized burrows preserved in South Africa, along with their inhabitants.
Family: Trirachodontidae
The most diverse of the Triassic gomphodonts are the members of Traversodontidae. This
family emerged during the Lower Triassic and continued until the end of that age. The original
representatives were small, though later types reached lengths of 50cm or so. The most recent
known remains come from near the end of the European Triassic. These are teeth from shrew-
sized animals. Fossils have been found in all continents, (excepting for Australasia and
Antarctica), though the best remains are from the lower Upper Triassic strata of Argentina and
Brazil, which seems to have been the heyday of the traversodontids.
Family: Traversodontidae
Triassic Gomphodonts: Diademodontidae, Traversodontidae and Trirachodontidae
Taxon: PROBAINOGANATHIA
Basal Probainognathia
Probainognathidae
Unclear affinities
Probainognathia
Chiniquodontoidea
Chiniquodontids are best known from the lower part of the Upper Triassic of South America.
Some more enigmatic material, (mainly teeth), has been recovered from Mid - Upper Triassic
European strata. A Middle Triassic genus from Africa, Aleodon, has also been referred to this
family, though this is seen as questionable by others. They ranged in size from tiny Gaumia
(should it be a chiniquodontid) to the doggish-big Belesodon, (which has recently been referred
to Chiniquodon).
Family: Chiniquodontidae
Dromatheriidae is a possible family based mainly on teeny teeth from the Upper Triassic of
Europe and North America, (and perhaps India). Although remains are sparse, these fossils are
very mammal-like. A hypothesized ancestor of mammals could convincingly have been
equipped with choppers like this, and some of this material may (or may not) represent our
ancestors. It would require better fossils to test the validity of that possibility. It could be an
informal grade rather than a natural taxon.
Unclear affinities
Triassic Cynodonts: Chiniquodontoidea
Family: Dromatheriidae
Another proposed family within Chiniquodontoidea is Therioherpetidae from the Upper Triassic
of South America, and perhaps Europe (Meurthodon). The status of this family has been
differently interpreted by various researchers. Therioherpeton has alternatively been referred
to Dromatheriidae. Another possibility is at least in part affinities with Tritheledontidae. This is
a matter which will require further finds and study. The South American critters were mousey
or ratty-sized, whilst Meurthodon was even smaller.
Family: "Therioherpetidae"
The only non-mammalian, meat-eating chiniquodontoids
known to have survived beyond the Triassic were extremely mammal-like members of
Tritheledonta. These were small insectivores of up to 20cm in length, and their lifestyle
was presumably extremely similar to that of the first mammals. This may well explain their
disappearance. Remains are known from the Middle Triassic of South America, and the Lower
Jurassic of South Africa and North America. This 'group' may contain members of a number
of lineages.
Taxon: Tritheledonta
Possibly descended from the traversodontids is a family known as
Tritylodontidae. It's fairly often assumed that
non-mammalian cynodonts went extinct at the end of the Triassic. This is incorrect, as
demonstrated by the members of Tritylodontidae, (and the insect-eating
trithelodontans above). Tritys were generally larger,
(up to about a metre in length) and survived for longer; until at least the Lower
Cretaceous. Where preserved, the anatomy suggests burrowing animals, and suitably sized
fossilized burrows have been found at one location in Colorado, along with tracks and
anatomical remains. A post-Cretaceous representative has some limited support,
(Chronoperates), but this is more
generally seen as some kind of mammal or other. Tritylodontids were mammal-like in the
extreme, and were usually classed as such until the 1920s. However, their anatomy
maintained significant 'reptilian' features, especially in the jaw and the ears.
The earliest fossils recovered to date stem from the Upper Triassic of Argentina. Most
representatives are known from the Lower Jurassic, when this family had a more or less
worldwide distribution. (One genus, Oligokyphus, has been found in Europe, China and North
America. Fragmentary tritylodontid remains have also been recovered from Antarctica.) The
most recent undisputed material comes from Siberia and Japan. The demise of the
tritylodontids may be connected with the rise of
multituberculate mammals, especially with regards to the northern hemisphere.
Presently, I'm treating tritys as herbivorous probainognathians.
Family: Tritylodontidae
Jurassic Cynodonts: Tritylodontidae and Tritheledontidae
Possible non-mammals
Transition:
In extant mammals, (but not all Mesozoic representatives), the dentary is the only lower
jaw bone; where present, the teeth are strongly differentiated; the inner ear has three
bones for processing sound (incus, malleus and stapes); the jaw joint is the
articular-quadrate, (it's at least overwhelmingly dominant amongst the basal members).
All these features (and more) can be seen in transition in the fossil record of the Triassic and
Jurassic. The non-mammalian eucynodonts became progressively more mammal-like, and the
anatomical distinctions between the more derived forms and the earliest mammals, are best
described as matters of degree.
Amongst the Triassic proto-mammals: The dentary became ever more dominant, until the
lower jaw bones were tiny. The teeth grew in complexity and efficiency. The mammalian
jaw-cranium joint (dentary-squamosal) grew up alongside of, and eventually, (in mammals),
entirely replaced the non-mammalian one. Whilst the inner ear still worked with only one
small bone, other important structural changes were underway; eg. the
cochlear canal appeared (as shown by Yunnanodon).
|