Athena Review Vol. 5, no. 1
Records of Life: Fossils as Original Sources
23. Early Mammals: Mammaliformes
The Middle Triassic saw a major radiation of herbivorous cynodont forms included in the family Traversodontidae. From them evolved the highly specialized and extremely mammal-like Tritylodontidae of the Late Triassic to Middle Jurassic, with rodent-like jaws and dentition, representing the final stage of the plant-eating cynodont radiation.
Tritylodontidae
Tritylodontidae means "three knob teeth". The members of this family were all small to medium-sized advanced synapsids with combined specialized structures for herbivorous eating. The first Tritylodont (named Tritylodon) was found in South Africa in upper Jurassic rocks over a century ago (Romer 1966) . It was first thought to be one of the earliest mammals. They are now classified as the closest relatives to the mammals supported by their specialized dentition as well as other skull features including a high, flat, crested jaw, large zygomatic arches, and a well developed secondary palate. Their shoulder girdle and forelimb structures are suggestive of digging animals.
Oligokyphus ("small curved animal") was a cmall cynodont belonging to the herbivorous Tritylodontidae family. It lived during the late Triassic to early Jurassic p
eriods (210-170 mya). Oligokyphus is placed in the subgroup Probainognathia. Originally considered to be an early mammal, based on its retention of the reptilian feature of a vestigial joint between the quadrate bone and the squamosal bone in the skull, it is now classified as a Mammaliamorph or near-mammal.Fig.1: Oligokyphus skeleton (after Romer 1966 fg. 291).
Oligokyphus, about 50 cm. in length, had a long slim body (fig.1) somewhat resembling that of a weasel. It lacked canine teeth, and had large, rodent-like incisors, separated by a wide gap, or diastema, from the cheek teeth (fig.2). This diastema is common among the Tritylodontiade, occurring also in other Early Jurassic forms including the related, larger Kayentatherium (fig.3), and the Chinese cynodont Bienotherium (fig.4). Oligokyphus fossils have also been found in the UK, Germany, China, and Antarctica..
Fig.2: Oligokyphus skull (after Romer 1966 fg. 289).
The lower cheek teeth of Oligokyphus (comparable to pre-molars) have two rows with three cusps in each row.These cusps enabled a well-fitting bite for shredding dense, fibrous plant material or for eating nuts and seeds, The lower jaw of Oligokyphus moved back and forth when the mouth was shut so that the food could be chopped up.
A related taxon with a similar shaped skull and diastema is Kayentatherium ("Kayenta beast") from northern Arizona which lived during the Early Jurassic. It is one of two tritylodonts from the Kayenta Formation of northern Arizona, first collected in the 1950s, and later recovered from 1977 -1982 (Jenkins). It was a robust animal about 1 m in length with a large head and stout backbone.
Fig.3: Kayentatherium wellesei skull (American Museum of Natural History).
Bienotherium (fig.4) is a Tritylodont cynodont from the Early Jurassic of China, with a temporal range in the Hettangian–Sinemurian stages. Bienotherium was first described by Young in 1940 as the species B. yuannanese. It was the largest tritylodont from the Lufeng Formation in China
Fig.4: Bienotherium skull (after Romer 1966 fg. 290).
Bienotherium had four incisors, no canines, and molar-like cheek teeth, for chewing tough plant material. Its dentition is notable for having a long gap or diastema between the incisors and cheek teeth. This diastema, common in Tritylodonts, foreshadows parallel adaptations in later Mesozoic and Tertiary insectivores, multituberculates, and modern rodents.
Diarthrognathus ("two joint jaw") was a synapsid carnivore found in South Africa, dating from the Early Jurassic period (201-189 mya). First described as D. broomi in 1958 by A.W. Crompton, Diarthrognathus (fig.5) was a small carnivore, slightly smaller than Thrinaxodon, which was under 50 centimetres long..
Fig.5: Diarthrognathus skull (after Romer 1966 fg. 292).
Diarthrognathus, as a carnivore, possessed canine teeth and lacked the diastema seem in the herbivore Tritylodontids. Its significance as a transitional figure between synapsids and early mammals lies in its dual jaw joint structure similar to both mammals and synapsids. Its primitive jaw joint is located between the quadrate and articular bones, and its derived, mammalian jaw joint is located between the squamosal and dentary bones.The articular and quadrate bones evolved to become two of the middle-ear bones in mammals. The transition exemplified by Diarthrognathus. This "twin-jointed jaw" can also be seen in other late cynodonts, as well as in
early mammaliforms.
Lower jaw evolution from reptiles to mammals
Fig.6 shows the difference between the synapsid and mammalian jaw joints.
As the chief diagnostic feature used in distinguishing early mammals from non-mammal synapsids,
it is linked to the evolution of the middle ear ossicles
(stapes, malleus, and incus) from the reptilian jaw bones originally
part of the jaw joint, The evolution of the lower jaw from reptile to
mammal involves the reuse of bony elements in the reptilian jaw joint
into elements of the middle ear. In non-mammalian synapsids, the jaw is composed of four bony elements. It is called a quadro-articular jaw because the joint is between the articular and quadrate bones. In therapsids (advanced synapsids including mammals), the jaw is simplified into an articulation between the dentary and the squamous part of the temporal bone, and hence referred to as a dentary-squamosal jaw.
Fig.6: Comparison of A) Synapsid and B) Mammal lower jaws
In mammals, the quadrate bone evolves and diminishes to form the incus, one of the tiny ossicles of the mammalian ear. Similarly, the articular bone evolves to form the malleus. The squamosal bone migrates and lengthens to become a new point of articulation with the lower jaw (at the dentary bone). In many mammals, including humans, the squamosal fuses with the periotic bone and the auditory bulla to form the temporal bone, then referred to as the squama temporalis.
Evolution of the Mammalian ear
Mammals are the most specialized vertebrates in their hearing, based on their complex middle and inner ear structures, whose development occured in several steps during the evolution from non-mammalian cynodonts to early mammals (Luo 2001).
The modification of reptilian jaw bones into mammalian middle ear bones or ossicles is summarized above. Some of the developmental stages reflecting evolution of the middle ear can also be seen in embryonic growth. As fig 7 shows, a diagnostic feature of reptilian jaws called Meckel's groove, which contains Meckel's cartilage, appears in mammals only during early weeks of embryonic growth around the development of the ear region.
Fig.7: Middle ear ossicles, and Meckel`s groove in stem mammals (after Luo 1994).
An attenutated version of Meckel's groove is also present in the adult forms of the early, Jurassic mammals named morganuconodonts, as a primitive feature extending forward from the postdentary trough (see top example in figure). These (the postdentary trough, and Meckel's groove) are among the most diagnostic landmarks of the lower jaw bones, which enable the accurate classification of fossils as either early mammals, mammaliformes (called stem mammals), cynodonts, or other non-mammals.
The bony structure around the inner ear also differs between mammals and non-mammalian cynodonts (Luo, 2001). The inner ear of cynodonts is enclosed by several bones in the basicranium, including the prootic and opisthotic bones as well as the exoccipital and basioccipital. In early mammaliformes, by contrast, the bony housing of the inner ear is formed exclusively by the petrosal bone, comprised of the fused prootic and opisthothic bone of the cynodont.
References:
Bloch, J. I., K. D. Rose, P. D. Gingerich, 1998 J. Mammal. 79, p. 804
Crompton, A.W. and Zhe-Xi Luo 1993
Crompton, A.W. and A. L. Sun 1985 Cranial structure and relationships of the Liassic mammal Sinoconodon. Zoological Journal of Linnean Society 85, pp. 99-119.
Fairman (1999)
Hu et al. (1997)
Kielan-Jaworowska, Z; Luo, Z-X; Cifelli, RL (2004). Mammals from the Age of Dinosaurs. Columbia University Press. Chapter 4.
Lucas, S.G. and A. P. Hunt. 1990. "The oldest mammal." New Mexico Journal of Science 30:41-4
Lucas, S.G. and Z. Luo, 1993. "Adelobasileus from the Upper Triassic of West Texas: The Oldest Mammal." Journal of Vertebrate Paleontology, Vol. 13(3) pp. 309-334
Luo, Zhe-Xi 1994
Luo, Zhe-Xi 2001 The inner ear and its bony housing in Tritylodonts and implications for evolution of the mammalian ear. Bull. Mus. Compl Zool. 156(1): 81-97.
Luo, Zhe-Xi, A.W. Crompton,. and Ai-Lin Sun 2001. Science 292, pp. 1535-1540
Luo, Zhe-Xi; Kielan-Jaworowska, Z; Cifelli, RL (2002). "In quest for a phylogeny of Mesozoic mammals". Acta Palaeontologica Polonica 47 (1), pp.1–78
McKenna and Bell 1997
Palaios
Patterson, B., and E. C. Olson, 1961. A Triconodontid Mammal from the Triassic of Yunnan: Internat. Colloq. On the Evolution of Mammals. Kon. Vlaamse Acad. Wetensch. Lett. Sch. Kunsten Belgie, Brussells, part 1, pp. 129-191.
Qiang et al. (1999)
Rougier, Wible, and Hopson 1996
Rowe 1988
Rowe 1993
Other fossil species of Sinoconodon include 1) Sinoconodon parringtoni (Young, 1982). The holotype (IVPP V7203, V4726) is a skull, found in the Lower Lufeng Series of Yunnan, China, dating, like S. rigneyi and other Sinoconodon fossils, from the Sinemurian stage of the Early Jurassic period (193 mya). 2) Sinoconodon changchiawaensis (Young, 1982), synonomous with Lufengoconodon changchiawaensis (Young, 1982). From the Lower Lufeng Series in Yunnan; its holotype (IVPP 4727) is a skull. 3) Sinoconodon yangi (Zhang & Cui, 1983). Also from the Lower Lufeng Series in Yunnan. Holotype: a skull. (A.W. Crompton and A. L. Sun. 1985
Rowe and Gauthier 1992
Wang et al. (2001).
Wible and Hopson 1993
Wikipedia
Young, C-C. 1982. Two primitive mammals from Lufeng, Yunnan. Selected Works of Yang Zhungjian. Science Press, Beijing, pp. 21-25.
Zhang, F. and G. Cui, 1983. New material and new understanding of Sinoconodon. Vertebrata Palasialica, 21, pp. 32-41
Glossary