Cenozoic Mammalian Evolution


With the demise of the dinosaurs and other groups at the end of the Cretaceous, the mammals underwent an adaptive radiation. One of the current controversies is between molecular systematists who claim that molecular clocks indicate that many mammalian orders originated in the later Mesozoic and the paleontologists who insist that the fossil record of the time is complete enough that they would not be missing the diversity claimed by the molecular types. A portion of the difference may be in that molecular evidence may be picking up differences before they are morphologically apparent; differences at the base of two recently separated phylogenetic lines are on the order of those seen between species, even though eventually the lines are very different. However, the sudden increase in the number of known taxa in the early Cenozoic suggests that this cannot be the whole story. One suggestion (by paleontologists) is that the molecular clock was not constant during the time of rapid radiation.

Also possibly important to mammalian evolution in the later Mesozoic and in the Cenozoic were changes in the relationships of the continents. About 225 million years ago, the major land masses were gathered together, forming the continent Pangaea. As Pangaea broke apart, isolation among the separating plates became greater and greater. The result eventually was independent evolutionary trends between organisms on the various land masses.

The brief survey here cannot begin to do justice to the Cenozoic evolution of mammals, but a few highlights and things of possible interest are noted below.

A number of placental mammals whose relationships are obscure are known from the Paleocene, including insectivore-like forms apparently adapted to feeding on insects and other small invertebrates, feeders on aquatic invertebrates and/or fish, and a number of herbivores. With release of size constraints following the Cretaceous die-off, larger forms appeared early. Tillodonts and taeniodonts grew to bear and pig sizes, apparently feeding on tough plant material that probably included roots and tubers. A number of other medium-sized herbivores show inconclusive relationships. A large group, often termed condylarths, seems to have consisted of perhaps five or six lineages near the base of ungulate radiation. Various lineages are hypothesized as ancestral to horses or artiodactyls. Some uintatheres of the late Paleocene to middle Eocene grew as large as a rhinoceros.

Carnivorous types also radiated into a number of size niches. The largest were ungulates known as mesonychids; the largest known, from the late Eocene of Mongolia, has a skull some 830 mm in length and 560 mm in width—the largest of any known terrestrial carnivore. The mesonychids have long been considered as likely ancestors of whales (based on tooth morphology), but more recent evidence indicates that whale ancestry almost certainly lies with the Artiodactyla.

Other carnivorous types include the creodonts, early Cenozoic forms ranging roughly from weasel to bear size. True carnivores have a carnassial pair of teeth consisting of P4/m1; creodonts have different pairs of upper and lower teeth involved, such as M1/m2. The creodonts may be polyphyletic.

According to Benton (2005), some 15 lines of placental mammals radiated widely during the 9 million years of the Paleocene. In addition, several other orders appeared, apparently for the first time. These included the Soricomorpha, Carnivora, Chiroptera, Dermoptera, Primates, Perissodactyla, and Rodentia. The other modern orders evolved during the 19-million-year Eocene. Benton (citing Stucky and McKenna 1993) gives figures on diversity as follows: 21 families in the latest Cretaceous; 37 families in the early Paleocene; 86 in late Paleocene; and 111 in early Eocene. In terms of genera, (Gingerich 1984) ca. 40 in the late Cretaceous and early Paleocene, about 120 in the late Paleocene, and 200 in early Eocene. Gingerich also reported that well-preserved late Cretaceous mammalian faunas contain 20-30 species, whereas those in the mid Paleocene typically have 50-60.

The true Carnivora appeared in early Paleocene times in the form of the extinct family Viverravidae (suborder Feliformia). Several other extinct families of the suborder appeared relatively early, with the living family Felidae showing up by the early Oligocene. The first Caniformia appeared during the late Paleocene (Miacidae), with the living family Canidae found in the middle Eocene of North America and the Mustelidae in the Oligocene.

Rodents appeared in the Paleocene. The most versatile group of rodents, the suborder Myomorpha, with about 1569 living species, was present by the early Eocene, but figuratively exploded evolutionarily during the Miocene. The myomorph family Muridae currently has some 730 species, all of which you will be expected to be able to identify in lab (just kidding).

The evolution of horses has been covered in hundreds of publications and won't be considered here other than to point to MacFadden's (1992) book as a good overview. The artiodactyls appear to be recognizable by early Eocene times, starting out as approximately rabbit-sized animals, undergoing major radiations in the late Eocene to early Oligocene and, among the ruminants, early Miocene. There are more than 70 living genera of artiodactyls.

Whales can be recognized as early as the early Eocene, and middle Eocene skeletons are beautifully intermediate between terrestrial and aquatic animals, and able to function in both habitats (after a fashion). By the late Eocene, fully aquatic, large whales were present, though with greatly reduced hind limbs.

Proboscideans arose in Africa and had much of their evolution in that continent. The earliest probable members of the taxon appeared in the late Paleocene.


Last Update: 11 Jan 2008

Centennial Museum and Department of Biological Sciences, The University of Texas at El Paso