The third great therapsid suborder, Theriodontia ("beast tooth", referring to the mammal-like teeth) constitute the higher or more mamml-like of the Therapsida (protomammals). They include both carnivores and herbivores, and various lineages correlated to late Cenozoic wolves, weasels, otters, rodents, and shrews. Most diverse and successful during the late Permain and early Triassic, they were increasingly overwelmed by the archosaurian radiation of the later Triassic. During the latest Triassic (late Norian to Rhaetic) at least one line of Eutheriodonts evolved into mammlas. Even so, one family, the rodent-like Tritylodontids continued right into the late Jurassic (Oxfordian).
The Theriodonts evolved a feeding mechanism much more successful than that of the Dinocephalia. The quadrate bone in the jaw was reduced in size. When the jaws are widely opened (drawing, above) the articular bone (art in the diagram) toghtly grips the head of the small quadrate (q). The lower incisors pass entirely behind the upper incisors when the jaws are closed. The development of versatile jaw movements and a more precise type of dental occlusion also played a critical role in preadapting the elements of the theriodont articular complex for becoming the ossicles of the mammalian middle ear.
The small quadrate may have been loosely attached to the skull, probably by a fibrous pad in life; set in a squamosal recess (B) in the diagram above; the upper end occupies a hollow in the anterior face of the squamosal which also buttressed it from behind. ) so that, Hopson suggests, it was probably capable of a small amount of fore-aft movement at its lower end. Nevertheless the joint between the quadrate and articular was a tight one, with the articular firmly gripping the quadrate condyle, allowing only a hinge (and some transversal sliding) movement at the jaw joint, but no fore-aft sliding.
The reduced and movable quadrate was a key theriodont adaptation. In large carnivores with very long canines, such as the gorgonopsians as some therocephalians, a wide gape was necessary, and so a secure attachment of the lower jaw is required. This same adaptation is also found in mammalian carnivores. The theriodont quadrate also allowed the anterior (front) surfaces of the lower teeth to contact and shear against the posterior (rear) surfaces of the upper teeth as the jaws are closed. These teeth, which may be serrated in gorgonopsian and other groups, could then function as very effective cutting instruments for dismembering a carcass or biting out chunks of flesh from it. In advanced herbivorous cynodonts, like the tritylodonts, the ability to move the lower jaws in a fore-aft direction was ex- ploited for the purpose of cutting up plant material by the cheek dentition.
The theriodonts fall into three distinct groups, which can be considered infraorders or (if Theriodonta itself is considered an order rather than as is usually the case, a suborder) suborders: the Gorgonopsia; the Therocephalia, and the Cynodontia. A fourth group, the Bauriamorpha, are now generally included under the Therocephalia.
The Therocephalia are at least as ancient as the Gorgonopsians, which they resemble, and in fact outlasted, making it through to the early Triassic period. The earlier therocephalians were in many respects as primitive as the gorgonopsians, but they did show certain advances such as the enlargement of the temporal opening (for a lighter skull and mopre efficent muscle attachment) and the reduction of the phalanges (fingers and toes) to the mammalian formula. The later therocephalians included the advanced bauriamorphs, which carried some theriodont characters to a high degree of specialization. For instance, in Bauria there was no bar of bone separating the orbit from the temporal opening; a condition typical for pritnitive mammals. These and other advanced features led to the long-held opinion, now rejected, that the Ictidosaurs and even some early mammals arose from a bauriamorph stem. Rather, the situation seems to be that mammalian characteristics evolved in parallel among a number of different groups, and this makes the therapsid phylogeny a particularily tricky subject. Although the current cladistic picture seems to offer a tidy arrangement, I wouldn't be at all surprised if the actual Therapsid and Theriodont family tree turns out to be very different!
The fragmentary nature of some of these fossils, and thr enthusiasm of early workers in the field to give each new scrap of bone a different species, genus, or even family designation, has led to a number of false taxa. It is not surprising that several therocephalian groups, like the Lycosuchidae and the Scaleposauridae, have turned out to be artificial, the former based on skulls that retain an extra set of canines (i.e. when the animal died one pair wa sin the process of replacing the other), and the latter based on mostly juvenile charcteristics.
The cynodonts, or 'dog teeth', were the most successful and one of the most diverse groups of therapsids, constituting a latest Permian and Triassic evolutionary radiation including such forms as large carnivorous cynognathids, equally large herbivorous traversodonts, and small and extremely mammal-like tritylodontids and ictidosaurs. The Ictidosaurs (Trithelodontids) are almost certainly close to the direct ancestry of of the class Mammalia (note - the above diagram shows mammals evolving from an unknown early triassic cynodont stem; perhaps a form like Thrinaxodon. This is almost certainly in error, for Cynognathus and later forms were more mammal-like. Recent cladograms relate teh first mammals more closley to late Triassic cynodonts). The extremely mammal-like structure of cynodonts has been known for nearly one hundred years, but only within recent years have we learned enough about them and about the very early mammals to say with confidence that all mammals are indeed descended from a single group of cynodonts
Even the earliest cynodonts - the Procynosuchidae of the Late Permian, already show many advanced mammalian chatacteristics, such as a reduced number of bones in the lower jaw, a secondary bony palate and a complex pattern or the crowns of their cheek teeth. It is likely that Cynodonts were at least partially if not completely warm-blooded, covered with hair, which would have insulated them and helped to maintain a high body temperature.
By early Triassic times, cynodonts had diverged into large predaceous carnivores such as Cynognathus and moderate large omnivorous and herbivorous types such as Trirachodon and Diademodon. the Middle Triassic saw a major radiation of herbivorous forms included in the family Traversodontidae. From this family evolved he highly specialized and extremely mammal-like Tritylodontidae of the late Triassic to Middle Jurassic, the "rodents" of the early Mesozoic and culmination of the herbivorous cynodont radiation. At the same time, the descendents of Cynognathus evolved into medium-sized to small carnivorous and insectivorous forms. It is interesting that as the archosaurian reptiles were becoming larger, the cynodonts became smaller, perhaps in the end nocturnal. The hot arid Triassic conditions favoured the ectothermic reptilian metabolism of the archosaurs over the warm-blooded mammalian organisation of the cynodonts. (In his The Dinosaur Heresies, Bob Bakker has clamed that even the early thecodontian archosaurs like Erythosuchus were warm-blooded, and out-competed the cynodonts for this reason, but this posittion is almost never held nowdays)
In the end, the small advanced cynodonts and their mammalian descendents became nocturnal, depebnding on hearing and smell and leaving the day to the visual-orientated archosaurs (thecodonts and dinosaurs). Cynodont and early mammalian brains were larger than sauropsid (reptilian) brains not because they were more intelligent, but because of the enlarged olfactory and auditory bulbs. The small cynodonts and Mesozoic mammals owned the cool night, or lived in trees, the large thecodonts and dinosaurs ruled the day and the ground. It was to be some 150 million years before a combination of environmental stress and cometary or asteroid impact brought about the end of the dinosaurs and the other great reptiles, and allowed the mammals to emerge and take control of the Earth.
|some Links and References|
Carroll, R. L. Vertebrate Paleontology and evolution. -W. H. Freeman and company, New York, 1988
Edwin H. Colbert, Evolution of the Vertebrates, 2nd edition, 1969, John Wiley & Sons
Illustrated Encyclopaedia of Dinosaurs and Prehistoric Animals, Barry Cox, R.J.G.Savage, Brian Gardiner, Dougal Dixon, illustration by Steve Kirk)
James A. Hopson, "The Origin and Adaptive Radiation of Mammal-Like Reptiles and Non-Therian Mammals, Annals of the New York Academy of Sciences 167:199-216, 1969
James A. Hopson and Herbert R. Barghusen, "An Analysis of Therapsid Relationships", in The Ecology and Biology of Mammal-Like Reptiles ed. by Nocholas Hotton III, Paul D. MacLean, Jan J. Roth and E. Carol Roth, Smithsonian Institute Press, Washington and London, 1986, pp.83-106
Palaeos Page (incorporates some of this material, plus a lot of additional material)