Eucynodontia
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Eucynodontia ("true dog teeth") is a grouping of animals that includes both mammals, such as dogs, and mammal-like non-mammalian Therapsids ("mammal-like reptiles") such as Cynodonts ("dog teeth"). Its membership was and is made up of both carnivores and herbivores. The chronological range extends from at least the Lower Triassic, possibly the Upper Permian, until the present day. This overview is concentrated on the proto-mammals, which are known from the Lower Triassic until the Lower Cretaceous, both divisions of the Mesozoic era ("the age of the Dinosaurs").
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[edit] Cynognathidae
Among the first and most basal of the eucynodonts was Cynognathus. This wolf-sized predator had a nearly 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, including 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.
Cynognathus is the only known representative of a family called Cynognathidae. However, various further derived relatives are also known.
- Family: Cynognathidae
- Cynognathus
- Family: Probainognathidae
- Probainognathus
- Lepagia (perhaps)
- Unclear affinities
[edit] Chiniquodontoidea
Further non-mammalian eucynodonts are known, though the structure followed here is probably in need of review. The main lineage of meat-eaters is the superfamily of Chiniquodontoidea. Some schemes divide this into two families, a further pair of possible families and some odd bits and pieces.
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 Chiniquodon.
- Family: Chiniquodontidae
- Chiniquodon
- Aleodon
- Belesodon
- Gaumia
- Probelesodon
Dromatheriidae is a possible family based mainly on teeny teeth from the Upper Triassic of Europe, North America, and possibly India. Although remains are sparse, these fossils are very mammal-like. A hypothesized ancestor of mammals could convincingly have been equipped with teeth like this, and some of this material may or may not represent human ancestors. It would require better fossils to test the validity of that possibility. It could be an informal grade rather than a natural taxon.
- Family: Dromatheriidae
- Dromatherium
- Microconodon
- Pseudotriconodon
- Tricuspes
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 members were mouse or rat-sized, while Meurthodon was even smaller.
- Family: Therioherpetidae
Includes: Therioherpeton, Charruodon, Meurthodon
- Unclear affinities
Includes: Eoraetia, Kunminia
The only chiniquodontoids known to have survived beyond the Triassic were the extremely mammal-like members of Tritheledontidae, (also known as Trithelodontidae). These were small insectivores of up to 20 cm 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 Upper Triassic of South America, and the Lower Jurassic of South Africa and North America.
- Family: Tritheledontidae
Includes: Tritheledon, Diarthrognathus, Pachygenelus, Pattsia, and Riograndia
[edit] Gomphodonts and Tritylodonts
Traditionally, the herbivoreous counterparts of the chiniquodontoids were grouped together under the term gomphodonts, ("peg teeth"). Their chronological range extended from the Lower Triassic until the Lower Cretaceous, and remains have been found on every continent in the world, with the exception of Australasia. They’re also referred to as Tritylodontoidea. It’s very likely that this arrangement is more a matter of convenience than systematics. However, as it is a convenient structure, it will be followed here.
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
Includes: Diademodon, Hazhenia (?), Ordosiodon (?), and Titanogomphodon
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
Includes: Trirachodon, Cricodon, Neotrirachodon, and Sinognathus
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 traversodonts.
- Family: Traversodontidae
Includes: Traversodon, Andescynodon, Arctotraversodon, Boreogomphodon, Colbertosaurus, Dadadon, Exaeretodon, Gomphodontosuchus, Habayia, Ischignathus, Luangwa, Massetognathus, Maubeugia, Menadon, Microscalenodon, Pascualgnathus, Plinthogomphodon, Rosieria, Rusconiodon, Scalenodon, Scalenodontoides, and Theropsodon
Possibly descended from the traversodonts 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 already demonstrated by members of Tritheledontidae (the insectivores mentioned above). Tritylodonts were their plant-eating counterparts. They were generally larger (up to about 50 cm in length) and survived 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. Tritylodonts 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 tritylodont remains have also been recovered from Antarctica.) The most recent undisputed material comes from Siberia and Japan. The demise of the tritylodonts may be connected with the rise of multituberculate mammals, especially with regards to the Northern Hemisphere.
- Family: Tritylodontidae
Includes: Tritylodon, Bienotherium, Bienotheroides, Bocatherium, Dianzhongia, Dinnebitodon, Kayentatherium, Lufengia, Oligokyphus, Stereognathus, Xenocretosuchus, Yunnanodon
[edit] Possible non-mammals
A rather loose definition of Mammalia could start with creatures of roughly the anatomical grade of Morganucodon. There are a few genera dealt with here as mammals, which should possibly or probably be labelled as non-mammals. This is certainly the case for Sinoconodon. Its dental replacement and growth habits were not mammalian. It probably also applies for Adelobasileus and Gondwanadon, which are both from earliest Upper Triassic rock, (Carnian). It could also be the case for the members of Haramiyida. However, other than for one exception, haramiyids are presently known only from tiny teeth. Until more substantial remains turn up, the affinities of haramiyids is a matter beyond resolution.
[edit] Transitional fossils
In general, non-mammalian, terrestrial vertebrates, (land-living critters with backbones), have a less dominant dentary in the lower jaw than the aforementioned Cynognathus and, when equipped with gnashers, have only one kind of tooth. However, as with Cynognathus, the inner ear contains a single small bone and the jaw joint is the same.
In mammals, (with the exceptions of the most basal 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 among the basal representatives).
All these features as well as others 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.
Among the Triassic proto-mammals, the tooth-bearing 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. While the inner ear still worked with only one small bone, other important structural changes were underway; eg. the cochlear canal appeared (eg. Yunnanodon).
[edit] References
- Much of this article is derived with permission from MESOZOIC EUCYNODONTS
