CMNH Vertebrate Paleontology

This study is focused on several skull structures that underwent fundamental changes through the transition from nonmammalian cynodonts to mammals (“mammaliforms” of Rowe, 1988). The main purposes of the research are to explore new morphological data in early fossil mammals and the advanced cynodont relatives of mammals, to test the phylogenetic hypotheses of nonmammalian cynodonts and early mammals, and to elucidate the pattern of phylogenetic transformation in derived structures in the mammal skull.

Evolutionary Origins of the Mammalian Middle and Inner Ears
Homology of the mammalian middle-ear ossicles with accessory jaw bones on the mandible of other vertebrates is a classic example of comparative anatomy and vertebrate evolution. Previously it was not clear how the accessory mandibular (postdentary) bones of cynodonts (close relatives to mammals) could migrate from the mandible to the cranium in mammals. Luo and Crompton (1994) show that the phylogenetic transformations of several features of the quadrate (incus) made it possible for the postdentary bones to establish new sites of attachment to the cranium. Through the cynodont–mammal transition, the simple changes in the quadrate and its surrounding structures have allowed a fundamental reorganization of the basicranium while a presumably functional tympanum was maintained in the mandible. New data from Luo and Crompton (1994) support Allin and Hopson’s (1992) hypothesis on the postdentary origin of the mammalian tympanic membrane.

The mammalian petrosal (periotic) promontorium is an important apomorphy absent in nonmammalian cynodonts. It is crucial to the mammalian hearing function because it houses the cochlea, the main auditory organ. Analysis of the primitive mammals from the Late Triassic (Adelobasileus) and the Early Jurassic (Sinoconodon) shows that the mammalian promontorium was formed by the expansion of the petrosal displacing the basisphenoid and the basioccipital (not by fusion of the petrosal and other bones). Serial sections and computer 3-D reconstructions of Sinoconodon reveal that its entire inner ear is enclosed by the petrosal (instead of several bones as in nonmammalian cynodonts). The cochlea extends into only one-third of the promontorium. This mosaic of a fully-formed promontorium and a short cochlea suggests that the formation of the promontorium preceded the elongation of the cochlea in phylogenetic evolution. The expansion of the petrosal is a crucial change in the cranium through cynodont–mammal transition, resulting in an enlarged bony cochlear housing (the promontorium) and a single petrosal enclosing the entire inner ear. This change in ossification of the auditory capsule would provide much better acoustic insulation of the inner ear. The formation of the promontorium may be a necessary precondition for further transformation of the cochlea. (Luo et al., 1995.)

Testing Relationship Hypotheses of Early Mammals
New anatomical data from the cynodonts and early mammals have made it possible to test several competing phylogenetic hypotheses. Crompton and Luo (1993; Luo, 1994) argue that Sinoconodon is the sister taxon to all other mammals within a monophyletic Mammalia (Mammaliformes of Rowe, 1988, 1993). Parsimony analysis by Luo (1994) of these new data and the data from published literature (Hopson and Barghusen, 1986; Rowe, 1988; Wible, 1991) shows that both tritylodonts and tritheledonts are strong candidates for the sister taxon to the Mammalia, but these two alternative hypotheses are supported by very different anatomical characters. The tritylodont–mammal hypothesis is supported mostly by the palatal and orbital apomorphies, while the tritheledont–mammal hypothesis has more support from the apomorphies of the craniomandibular joint and the mandible. Apomorphies of the dentition, the basicranium, and the braincase are about evenly split for both hypotheses. Both sister-taxon hypotheses (tritylodonts vs. tritheledonts) invoke numerous convergences, although the tritheledont–mammal hypothesis has a few more apomorphies (Luo, 1994). These alternative phylogenetic schemes would result in very different scenarios of functional evolution of early mammals, and more morphological studies of new fossils are necessary to resolve this controversy.

Allin, E. F., and J. A. Hopson. 1992. Evolution of the auditory system in Synapsida (“mammal-like reptiles” and primitive mammals) as seen in the fossil record. Pp. 587–614 in The Evolutionary Biology of Hearing (D. B. Webster, R. R. Fay, and A. N. Popper, eds.). Springer–Verlag, New York.

Crompton, A. W., and Z. Luo. 1993. The relationships of the Liassic mammals Sinocodon, Morganucodon oehleri and Dinnetherium. Pp. 30–44 in Mammal Phylogeny (F. S. Szalay, M. J. Novacek, and M. C. McKenna, eds.). Springer–Verlag, New York.

Hopson, J. A., and H. R. Barghusen. 1986. An analysis of therapsid relationships. Pp. 83–106 in The Ecology and Biology of Mammal-like Reptiles (N. Hotton III, P. D. MacLean, J. J. Roth, and E. C. Roth, eds.). Smithsonian Institution Press, Washington, D. C.

Luo Z. 1994. Sister taxon relationships of mammals and the transformations of the diagnostic mammalian characters. Pp. 98–128 in In the Shadow of Dinosaurs—Early Mesozoic Tetrapods (N. C. Fraser and H. D. Sues, eds.). Cambridge University Press, Cambridge, United Kingdom.

Luo, Z., and A. W. Crompton. 1994. Transformations of the quadrate (incus) through the transition from non-mammalian cynodonts to mammals. Journal of Vertebrate Paleontology 14:341–121.

Luo, Z., A. W. Crompton, and S. G. Lucas. 1995. Evolutionary origins of the mammalian promontorium and cochlea. Journal of Vertebrate Paleontology 15:113–121.

Rowe, T. 1988. Definition, diagnosis and origin of Mammalia. Journal of Vertebrate Paleontology 8:241–264.

Rowe, T. 1993. Phylogenetic systematics and the early history of mammals. Pp. 129–145 in Mammal Phylogeny (F. S. Szalay, M. J. Novacek, and M. C. McKenna, eds.). Springer–Verlag, New York.