The Origin of the Idea of the Mammal-Like Reptile

The Origin of the Idea of the Mammal-likeReptile RICHARD P. AULIE mammalian evolution. As fossil reptiles which more nearly approximatedthe mammalian condition continued to be discovered,he suggested they would be found to have larger and larger dentary bones, with a corresponding * Conclusion of a three-part article. In earlier parts the diminution of the other osseous components of the lower Downloaded from http://online.ucpress.edu/abt/article-pdf/37/1/21/32569/4445038.pdf by guest on 26 September 2021 author recounted the discovery of the Karroo fossils in jaw. Remarkably, he even attempted to predict the kind South Africa and the controversy surrounding them of articulation in the jaw-joint between mandible and (ABT 36[81:476)and traced the growth of scientific un- cranium that would characterize an intermediate fossil derstanding of the reptile-mammal transition (ABT 36[91:545).He discusses, here, the implicationsfor zoology at the transition between reptile and mammal (Seeley and paleontology today and summarizes with comments 1889: 291, 292). Moreover, he noted, an improved second- on the "model"aspects of the controversyand its resolu- ary palate would be found, which, by separating the nasal tion. References cited in earlier parts are included here, from the mouth cavities, would in mammals increase the as are the author's acknowledgments. ease of breathing and masticating. And by proposing a division of the Karroo strata into stratigraphical zones, he drew attention to the possibility of arranging the fossil Ill. The Mammal-like Reptiles reptiles into chronological sequences (Seeley 1892: 311- 314). These were the kind of problems that emerged from Owen's studies, problems that Owen perhaps only im- perfectly understood. Applications of Evolutionary Theory In 1895, Seeley described several species of a cynodont which, as the most advanced of any then known, seemed to corroboratewhat he had foreseen (Seeley 1895a). Calling FOLLOWING THE PUBLICATION of Darwin's this genus Cynognathus, meaning dog-like jaw, he found Origin of Species, in 1859,biologists were eager to ap- that the dentary comprised almost the entire mandible, ply the theory of evolution to the paleontological with the other bones crowded at the back. Moreover,be- record.In the case of the Karrooreptiles, this was a cause the dentary was very near the squamosal bone of gradual trend which lagged behind other applica- the cranium, the joint rather closely resembled the mam- tions, such as the promptevolutionary interpretation malian condition (fig. 18). Besides having a secondary of the fossil horses. But Owen's pioneer work, his palate, Cynognathus had two occipital condyles, the all- speculations of 1876 and 1880, and the supportgiven important trait, that, as in mammals, provides articula- his views by Cope, inevitably fostered an increasing tion with the backbone. These typical cynodonts, the most confidencethat at least some mammals were derived mammalian-like of the theriodonts, were represented from reptiles rather than from amphibians, and that hitherto mainly by Owen's Galesaurus (and Thrinaxodon). the theory of evolutionwould offer a fruitfulline of in- quiry. As this realizationgrew, the predictivevalue of evolutionarytheory quickly became apparent. Emerging Concensus Predictive Value New trends in research, the continued accumulation of fossil evidence - much of it circumstantial - and reflection the discussions Even before a concensus had been achieved concerning on Owen's diagnostic methods enlivened of the of mammals during the closing years of the mammalian origins, Seeley in 1889 already foresaw im- origin 19th century. At the 4th International Congressof Zoology portant problems that have, in fact, since been the subject in 1898, paleontologists had in mind a rather clear con- of paleontological research. Three of these have to do with cranial morphology and are central to the question of ception of a long-limbed, quadrupedal, terrestrial, and aquatic reptile, with pronounced-often astounding- mammalian features, which flourished during the Permian The author is in the department of natural science at Loyola University, 820 N. Michigan Ave., Chicago60611. A biographical and Triassic periods. note (with photo) appeared in the first part of this article (ABT But questions remained (Osborn 1898; Osborn et al. 36[8]:476). 1899). Did mammals come from amphibians or reptiles? 21 The.Fr. 'Pro. Syaw4> /11d.,~ _ ~~~~~~~~IX_ _ Fig. 18. Cynognathus,figured by Seeley, about 30 cm long. The lateral view has a striking resemblance to a dog; at least Seeley thought so, judging from the name he coined. The mammalian features of this advanced theriodont give us a good idea of the life it led during the lower Triassic. The teeth are specialized into incisors, fangs, or canines, and cusped molars. Notice the sagittal crest, rising just posterior to the eye orbit, for attachment of Downloaded from http://online.ucpress.edu/abt/article-pdf/37/1/21/32569/4445038.pdf by guest on 26 September 2021 strong temporal muscles. Cutting, piercing, and chewing of food were assisted by the well-developed secondary palate (unseen here). Unseen also are two occipital condyles, extremely important, in mammals, where the skull swung on the vertebralcolumn. Undoubtedly this creature was very predaceous, an active and powerfulcarnivore. (Seeley 1895a: 72.) If the latter, which ones? Were these Karroo reptiles ancestors of monotremes only, or of all the mammals? Although Huxley's view still persisted, Seeley's summary Fig. 19. RobertBroom, South African vertebrate paleontologist, of the conflicting evidence made plain that the questions physician, and student of mammalian evolution. Born, Paisley, could not yet be settled. After ten years' hard labor, Seeley Scotland, Nov. 30, 1866; died, Pretoria, South Africa, April 6, 1951. Educated in medicine at the University of Glasgow, he was puzzled. Perhaps those mammalian traits he had con- emigrated in 1897 to South Africa, following his recognition of firmed in the Karroo reptiles were due only to parallelism. the importanceof the Karrooreptiles for the study of mammalian He suggested that a "common parent" for mammals and evolution. He taught geology and zoology in 1903-10 at what is anomodonts (including the theriodonts) might be found in now Stellenbosch University, near Cape Town, and from 1934 rocks even older than the Permian (Seeley, in Osborn, served as curator of paleontology at EransvaalMuseum in Pre- toria. He is most famous for his contributions to physical anthro- H., et al. 1899: 68-70). pology by his studies of prehistoric man in Africa, and for his Meanwhile, the rapidly increasing supply of fossil speci- discovery in Pleistocene deposits of the Australopithecines, mens had made reforms in nomenclature imperative. In Plesianthropus transvaalensis at Sterkfontein in 1936, Paran- 1903, Henry Fairfield Osborn (1857-1935), vertebrate thropus robustus at Kromdrai in 1938, and Paranthropus cras- paleontologist at the American Museum of Natural His- sidens at Swartkrans in 1948. But most extensive were his studies, beginning in 1897, of the mammal-like reptiles. He made tory in New York City, drew attention to the temporal important visits to the U. S. in 1909, 1913-14, and 1928, and he fenestrae which Owen had noticed in 1845 in Dicynodon. came for lectures in 1937 and 1949. Among other honors, Broom Accordingly, Osborn classified the Karroo reptiles as was awarded the Daniel Giraud Elliot Gold Medal of the U. S. "synapsids" because they have in common a single zygo- National Academy of Sciences in 1950 in honorof his work on the matic arch (cheekbone) bordering this region, which thus Australopithecinae.Among other books (see references), he wrote The Origin of Man - Was It Accident or Design? (1933) and Find- separates them from the "diapsids" (two-arched), which, ing the Missing Link (1950). With firm, religious convictions, Osborn thought, contained all other reptiles. And he was Broomsought to interpret evolution as an expression of a cosmic, perhaps the first, the same year, to use the term "mammal- divine plan. Courtesy of the American Museum of Natural His- like reptile" (Osborn, H., 1903: 453). tory, New York, where he posed with Endothiodon uniseries At least with respect to interpreting the Karroo reptiles, (Owen 1879b) for this picture during his visit in 1913-14. the break with idealistic morphology was now practically complete. The mammal-like reptiles, as the term implies, mens available was limited and the literature manageable, were henceforth explained somehow by the theory of Seeley could master all that was then known. But when evolution: homologies suggested a common ancestry, at the turn of the century the mammal-like reptiles were rather than empirical expressions of a transcendental placed in an evolutionary context, the flood-gates seemed eidos (Idea). As a unifying principle, the theory of evolu- to open. During the next three decades, newly-discovered tion since 1900 has given meaning and coherence to an fossils of primitive, specialized, and advanced forms flowed otherwise bewildering mass of observations. The 19th- steadily from the Karroo beds. Paleontologists of several century work, represented by Bain's initial discovery, nationalities took up highly theoretical and recondite Owen's foundation studies during a 42-year span, and problems, many of them extensions of those Seeley had Seeley's elaborate memoirs, were for the most part de- foreseen. The result was for some years an almost ex- scriptive and exploratory. Because the number of speci- ponential increase of the literature. 22 THE AMERICAN BIOLOGY TEACHER,JANUARY 1975 The Leadership of Robert Broom specialists, such as David Meredith Seares Watson (1866- 1973), to the subject (fig. 19). Following his graduation from the University of Glas- The study of the Karroo reptiles was an international gow medical school in 1889, Broom went to Australia, endeavor from the start-when Bain sent his first collec- where he hoped the most primitive living mammals would tion to the Geological Society of London.

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