Early Mammalian Evolution Research Articles

A Jurassic mammal from South America.

The Jurassic period is an important stage in early mammalian evolution, as it saw the first diversification of this group, leading to the stem lineages of monotremes and modern therian mammals. However, the fossil record of Jurassic mammals is extremely poor, particularly in the southern continents. Jurassic mammals from Gondwanaland are so far only known from Tanzania and Madagascar, and from trackway evidence from Argentina. Here we report a Jurassic mammal represented by a dentary, which is the first, to our knowledge, from South America. The tiny fossil from the Middle to Late Jurassic of Patagonia is a representative of the recently termed Australosphenida, a group of mammals from Gondwanaland that evolved tribosphenic molars convergently to the Northern Hemisphere Tribosphenida, and probably gave rise to the monotremes. Together with other mammalian evidence from the Southern Hemisphere, the discovery of this new mammal indicates that the Australosphenida had diversified and were widespread in Gondwanaland well before the end of the Jurassic, and that mammalian faunas from the Southern Hemisphere already showed a marked distinction from their northern counterparts by the Middle to Late Jurassic.

A heuristic approach of maximum likelihood method for inferring phylogenetic tree and an application to the mammalian SOX-3 origin of the testis-determining gene SRY

Applying the tree bisection and reconnection (TBR) algorithm, we have developed a heuristic method (maximum likelihood (ML)-TBR) for inferring the ML tree based on tree topology search. For initial trees from which iterative processes start in ML-TBR, two cases were considered: one is 100 neighbor-joining (NJ) trees based on the bootstrap resampling and the other is 100 randomly generated trees. The same ML tree was obtained in both cases. All different iterative processes started from 100 independent initial trees ultimately converged on one optimum tree with the largest log-likelihood value, suggesting that a limited number of initial trees will be quite enough in ML-TBR. This also suggests that the optimum tree corresponds to the global optimum in tree topology space and thus probably coincides with the ML tree inferred by intact ML analysis. This method has been applied to the inference of phylogenetic tree of the SOX family members. The mammalian testis-determining gene SRY is believed to have evolved from SOX-3, a member of the SOX family, based on several lines of evidence, including their sequence similarity, the location of SOX-3 on the X chromosome and some aspects of their expression. This model should be supported directly from the phylogenetic tree of the SOX family, but no evidence has been provided to date. A recently published NJ tree shows implausibly remote origin of SRY, suggesting that a more sophisticated method is required for understanding this problem. The ML tree inferred by the present method showed that the SRYs of marsupial and placental mammals form a monophyletic cluster which had diverged from the mammalian SOX-3 in the early evolution of mammals.

Evidence for an early appearance of modern post-switch isotypes in mammalian evolution; cloning of IgE, IgG and IgA from the marsupial Monodelphis domestica.

In birds, reptiles and amphibians the IgY isotype exhibits the functional characteristics of both of IgG and IgE. Hence, the gene for IgY most likely duplicated some time during early mammalian evolution and formed the ancestor of present day IgG and IgE. To address the question of when IgY duplicated and formed two functionally distinct isotypes, and to study when IgG and IgA lost their second constant domains, we have examined the Ig expression in a non-placental mammal, the marsupial Monodelphis domestica (grey short-tailed opossum). Screening of an opossum spleen cDNA library revealed the presence of all three isotypes in marsupials. cDNA clones encoding the entire constant regions of opossum IgE (epsilon chain), IgG (gamma chain) and IgA (alpha chain) were isolated, and their nucleotide sequences were determined. A comparative analysis of the amino acid sequences for IgY, IgA, IgE and IgG from various animal species showed that opossum IgE, IgG and IgA on the phylogenetic tree form branches clearly separated from their eutherian counterparts. However, they still conform to the general structure found in eutherian IgE, IgG and IgA. Our findings indicate that all the major evolutionary changes in the Ig isotype repertoire, and in basic Ig structure that have occurred since the evolutionary separation of mammals from the early reptile lineages, occurred prior to the evolutionary separation of marsupials and placental mammals.

The dorsal thalamus of jawed vertebrates: a comparative viewpoint.

In anamniotes, the dorsal thalamus comprises: (1) a caudal division, the collothalamus, which receives its predominant input from the midbrain roof and projects ipsilaterally to the telencephalon, predominantly to the striatum, and (2) a rostral division, the lemnothalamus, which predominantly receives a direct retinal (lemniscal) input and projects bilaterally to the telencephalon, predominantly to the pallium. In amniotes, collothalamic nuclei relay visual, auditory, and somatosensory-multisensory inputs from the midbrain roof to the ipsilateral telencephalon, terminating in both striatum and pallium. For example, the collothalamic visual nuclei consist of the LP-pulvinar complex in mammals and nucleus rotundus in diapsid reptiles, birds, and turtles. Among amniotes, the latter nuclei are homologous to each other as discrete nuclei, as are the collothalamic auditory and collothalamic somatosensory-multisensory nuclei. Lemnothalamic nuclei (and nuclear groups) in amniotes predominantly (and/or plesiomorphically) receive lemniscal inputs; some project to the telencephalon bilaterally, and most, in contrast to collothalamic nuclei, do not project to the striatum. In mammals, the lemnothalamic nuclei include most of those in the anterior, medial, intralaminar, and ventral nuclear groups and the dorsal lateral geniculate nucleus. In diapsid reptiles, they include the dorsomedial and dorsolateral anterior nuclei and the dorsal lateral optic nucleus; comparable nuclei are present in birds and turtles, with birds additionally having a discrete somatosensory lemniscal relay nucleus. These lemnothalamic nuclei in each amniote radiation are homologous as a field to the lemnothalamus (i.e., nucleus anterior) in anamniotes. Both divisions of the dorsal thalamus were elaborated to some degree in the common ancestral amniote stock. A further major elaboration of the lemnothalamus characterized the ancestral stock of mammals and may have been one of the key events in early mammalian evolution. Birds have independently, to a lesser degree, elaborated the lemnothalamus.

Mesozoic Mammals from Arizona: New Evidence on Mammalian Evolution

Knowledge of early mammalian evolution has been based on Old World Late Triassic-Early Jurassic faunas. The discovery of mammalian fossils of approximately equivalent age in the Kayenta Formation of northeastern Arizona gives evidence of greater diversity than known previously. A new taxon documents the development of an angular region of the jaw as a neomorphic process, and represents an intermediate stage in the origin of mammalian jaw musculature.

Towards an understanding of the molecular mechanisms regulating gene expression during diploidization in phylogenetically polyploid lower vertebrates.

Polyploidization and regional gene duplication have occurred frequently during vertebrate evolution, providing the genetic material necessary for creating evolutionary novelties. Mammals, including man, can be regarded as diploid species with a polyploid history of evolution. Polyploidization steps during the phylogeny of mammals probably took place in the genomes of amphibian- or fish-like mammalian ancestors. The polyploid status has subsequently been shaped by the process of diploidization, leading to genomes that are polyploid with respect to the amount of genetic material and the number of gene copies, and diploid with respect to the level of gene expression and chromosomal characteristics. Phylogenetically tetraploid amphibian and teleost species together with their diploid close relatives can be used as a model system to study the effect of polyploidization and the mechanisms of diploidization of a parallel event during early mammalian evolution. Experimental evidence permits the assumption that the diploidization of gene expression in tetraploid cyprinid fish may be functionally correlated with structural modifications of the ribosomal components, RNA and protein. These findings are discussed in the light of reduced protein synthesis in diploidized tetraploid species and a mechanism to explain diploidization during mammalian evolution.

Vertebrate paleontology: new approaches and new insights

In the past fifteen years, the biological interpretation of vertebrate fossils has been markedly advanced by the application of experimental techniques in functional morphology, biomechanical and allometric modelling, more rigorously-derived empirical correlations between form and function in living species, and studies of modern community structure and of the taphonomic processes by which fossil assemblages are formed. Traditional paleontological methods of phylogenetic analysis are under attack by the advocates of cladistics; the resulting debate is leading to increased rigor in paleontological systematics. Paleobiogeographical studies of vertebrates has become an extremely active area of research due to the need to reanalyze past distribution patterns in the light of plate tectonics.New information has extended the fossil record of vertebrates back to the Late Cambrian, but little more is known about the nature of the earliest vertebrates or of the pre-vertebrate ancestors. Much has been learned of the fishes antecedent to tetrapods but little of the earliest tetrapods. The early reptiles are now well known, but the nature of their amphibian ancestors is uncertain. Restudy ofArchaeopteryxhas suggested a dinosaurian ancestry for birds. The origin and early evolution of mammals is the subject of extensive research as a result of greatly augmented recent collecting. The most active and innovative research is on the biology of dinosaurs, especially on their physiology. Among Cenozoic mammals, the fossil record of primates has been greatly expanded and knowledge of primate history, from the basal prosimians of the Paleocene to early man, is increasing rapidly.

Books-Received

Books-Received Get access Ecological Genetics: The Interface.— Peter F. Brussard (ed.). 1978. Springer-Verlag, New York, vii + 247 pp. $22.80.—Twelve chapters on the evolution of ecological parameters, coadaptarion, enzyme variation, hypoxia, sexual selection and genetics of Hawaiian and other Drosophila, speciation in field crickets, snail genetics, plant demography and genetics, and population differentiation, reproduced from typescript. Ecological Genetics: The Interface.— Peter F. Brussard (ed.). 1978. Springer-Verlag, New York, vii + 247 pp. $22.80.—Twelve chapters on the evolution of ecological parameters, coadaptarion, enzyme variation, hypoxia, sexual selection and genetics of Hawaiian and other Drosophila, speciation in field crickets, snail genetics, plant demography and genetics, and population differentiation, reproduced from typescript. Life Cycles: An Evolutionary Approach to the Physiology of Reproduction, Development and Ageing.— Peter Calow. 1978. Chapman and Hall, London, distributed by Halsted Press, John Wiley & Sons, New York, ix + 164 pp. $14.95.—A wideranging synthetic treatment of prokaryotes, protists, plants, and animals. Animal Learning: Survey and Analysis.— M. E. Bitterman, V. M. LoLordo, J. Bruce Overmier, Michael E. Rashotte 1979. Plenum Press, New York, xi + 510 pp. $42.50.—Covers conditioning, reward training, and allied topics. Fine Structure of Parasitic Protozoa: An Atlas of Micrographs, Drawings and Diagrams.— E. Scholtyseck 1979. Springer-Verlag, New York, vii + 206 pp. $39.80.—Illustrations of the ultrastructure of representative non-ciliate parasites. Dispersal Centres of Sphingidae (Lepidoptera) in the Neotropical Region.— Harald Schreiber. 1978. Biogeographica, Vol. 10; W. Junk, The Hague and Boston. 195 pp.—An argument for the existence of 18 “dispersal centres” in the Neotropics, based on sphingid distribution, of which “the majority” are congruent with those of Miiller based on terrestrial vertebrates. American Spiders.— Willis J. Gertsch. Second edition, 1979. Van Nostrand Reinhold, New York, xiii + 274 pp. $24.95.—A revised edition (after thirty years) of this classic popular account of the natural history of the North American spider fauna; with numerous new color plates. Behavioral Biology of Aplysia: A Contribution to the Comparative Study of Opisthobranch Molluscs. — Eric R. Kandel. 1979. W. H. Freeman and Co., San Francisco, xiii + 463 pp. No price supplied. —Survey of the sensory structure and function, development, and behavior of marine snails. Contrasts in Behavior.— Ernst S. Reese Frederick J. Lighter (eds.). 1979. John Wiley and Sons, New York, xi + 406 pp. $27.50.—Twelve chapters on comparative aspects on the reproductive, communicative, regulatory, feeding, and social behavior of terrestrial and aquatic vertebrates. Why the Dinosaurs Became Extinct.— J. L. Cloudsley-Thompson. 1978. Meadowfield Press, Shildon, England DL4 2QZ. 86 pp. $8.40.—Stories for swapping on a wintry evening. Turtles: Perspectives and Research.— Marion Harless and Henry Morlock (eds.). 1979. John Wiley and Sons, New York, xiv + 695 pp. $45.00.—A general survey of turtle biology, with a single, brief, and less than up-to-date chapter on “Taxonomy, Evolution, and Zoogeography.” Das evolutive Plateau Saugetier: Origin and Early Evolution of Mammals.— D. Starck, H. F. Moeller, W. Maier, and G. Hahn 1978. Sonderbände des Naturwissenschaftlichen Vereins in Hamburg, No. 3; Verlag Paul Parey, Hamburg. 95 pp. 26 marks.—Proceedings of the “22. Phylogenetisches Symposion” (1977) with chapters by the four authors, respectively, on monotremes, marsupials, the tribosphenic molar, and multituberculates. Sirenian Evolution in the North Pacific Ocean.— Daryl P. Domning 1978. Univ. of California Publ. in Geol. Sci., Vol. 118; University of California Press, Berkeley. 187 pp. $9.00.—A monographic study pertaining to material published in SZ 25:352-362. Kangaroo Rats: Intraspecific Variation in Dipodomys spectabilis Merriam and Dipodomys deserti Stephens.— Iyad A. Nader. 1978. Illinois Biological Monographs 49, University of Illinois Press, Urbana. 116 pp. $12.50.—Study of 14 skull measurements, six external measurements, and color in seven and four subspecies, respectively. Sensory Systems of Primates.— Charles R. Noback (ed.). 1978. Plenum Press, New York, xiii + 207 pp. $25.00.—Another volume in the “Advances in Primatology” series. Extra-ocular Photosensitivity.— N. Millott 1978. Meadowfield Press, Shildon, England DL4 2QZ. 52 pp. $8.40.—Popularized survey of the structural, biochemical, and physiological basis of light receptors other than eyes. Analysis of Ecological Systems.— David J. Horn, Rodger D. Mitchell, Gordon R. Stairs (eds.). 1979. Ohio State University Press, Columbus, ix + 312 pp. $27.50.—Chapters of evolutionary interest include “Fitness, Survival, and Optimality” by R. C. Lewontin, “Darwinian Analyses: The New Natural History” by Rodger D. Mitchell and Mary B. Williams, “The Ecology of Colonizing Species” by D. C. Embree, “Niche Segregation in Desert Lizards” by Erik R. Pianka, Raymond B. Huey, and Lawrence R. Lawlor, and “Development of Theory in Insect-Plant Interactions” by Lawrence E. Gilbert. Systematic Biology, Volume 28, Issue 3, September 1979, Pages 412–413, https://doi.org/10.1093/sysbio/28.3.412 Published: 01 September 1979

The Behavior of the Moonrat, Echinosorex gymnurus (Erinaceidae) and the Pentail Shrew, Ptilocercus lowi (Tupaiidae) with Comments on the Behavior of other Insectivora

AbstractObservations in captivity of 9 Ptilocercus and 12 Echinosorex are described. Behavioral data include information on predation, nesting, defense, locomotion and communication (including physical details of vocalizations). Inferences concerning early mammalian evolution are discussed.

CYNODONT POSTCRANIAL ANATOMY AND THE "PROTOTHERIAN" LEVEL OF MAMMALIAN ORGANIZATION.

The origin of mammals from synapsid reptiles occurred during the Late Triassic. Although evolutionary events both preceding and following that time were important to the ultimate development of living mammals, certain Late Triassic forms possess such an even balance of reptilian and mammalian characters that they are truly transitional between the two classes. It should be emphasized that this transition comprised a biological series of events, and that these are meaningful only when interpreted in biological terms. However, the usual techniques of direct observation and experimentation on living animals are unavailable for these unique biological events. Instead, investigations of mammal origins must utilize indirect means, viz., functional interpretations of transitional forms and comparative studies of primitive living mammals. This paper uses both approaches to evaluate the question of the kind of biological organization that characterized the transitional forms on or near the reptilemammal boundary. Some authors (e.g., Winge, 1941; Kermack, 1963; MacIntyre, 1967) have implied or overtly claimed that Mesozoic mammals were advanced little toward a mammalian level of organization, and that it was only in late Mesozoic or early Tertiary times that taxonomic Mammalia underwent the transition from a basically reptilian to a mammalian level of biological organization. Bakker (1968) claimed that advanced mammal-like reptiles and Jurassic mammals were no more mammalian in posture and locomotion than modern lizards. New evidence bearing on the origin of mammalian characters is here Dresented through (1) a functional interpretation of the postcranial anatomy of certain advanced mammal-like reptiles and (2) through the phylogenetic implications of monotreme biology. Cynodonts, a diverse group of PermoTriassic therapsids (mammal-like reptiles), possess a suite of cranial features indicative of a probable ancestral relationship to mammals. Among these features are ethmoturbinals, double occipital condyles, secondary palate (cf. Brink, 1956), jaw structure (Barghusen, 1968), and dentition (Crompton and Jenkins, 1968). Previously, the origin of mammals was obscured by an apparent hiatus in the paleontological record between the last major therapsid faunas of the early Triassic and the earliest Mesozoic mammalian faunas of the middle Jurassic. Recently, however, the remains of Late Triassic mammals have been recovered in relative abundance (Kermack, 1967a; Parrington, 1967); this new evidence now provides a more substantial basis for deductions on the derivation and initial radiation of Mesozoic mammalian orders. Crompton and Jenkins (1968) concluded that cynodonts are the most likely ancestors of mammals on the basis of similarities in dentition; the same conclusion was reached by Barghusen (1968) on evidence of the jaw apparatus. With a few exceptions (Gregory and Camp, 1918; Romer, 1922; Simpson, 1928, 1929; Brink, 1956; Parrington, 1961), students of early mammalian evolution have generally neglected the postcranial skeleton-primarily because postcranial material of Mesozoic mammals is relatively scarce or, when available, is