ABSTRACT
Continuity is important for tracing evolutionary lineages through geological time. Modern Odontoceti and Mysticeti can be traced backward in time to Eocene Archaeoceti, and before them to mesonychian Condylarthra. Within this shared continuum, the origin of Archaeoceti and the origin of Cetacea is marked by the first indication of a derived evolutionary transition-in-grade from terrestrial to aquatic life characteristic of later cetaceans. Archaeocetes are known from many fossil localities in Eocene marginal marine and shallow marine strata on six continents. These range in age from Priabonian (late Eocene; ca. 36 Ma) through late Ypresian (late early Eocene; ca. 49.5 Ma), a 13.5 m.y. time range, and they are widely distributed in North America (18 sites), Europe (5 sites), Asia (8 sites), Africa (8 sites), Australia (New Zealand; 2 sites), and Antarctica (1 site). Forty-two sites can be considered statistically-independent records.
With this information and a model sampling distribution of potential fossils, we can compare different hypothesized times of origin of archaeocetes by calculating relative likelihoods for each. The model sampling distribution reflects changing outcrop area of sedimentary rocks through geological time and changing numbers of archaeocetes during their diversification. The maximum-likelihood time of origin of archaeocetes is given by the age of the earliest fossil, which defines the beginning of the temporal range and requires no hypothesized extension. Likelihood ratios of 0.5 and 0.05 have associated probabilities less than or equal to 0.5 and 0.05, respectively, representing confidence limits equal to or greater than 50% and 95%. A critical likelihood l = 0.05 defines the maximum extension of range we can reasonably expect to find, and from this we estimate the time of origin of Archaeoceti to have been at or after about 51.6 Ma—within the early Eocene.
Fifty-six independent records of Mesonychidae and Hapalodectidae on the three northern continents range in age from Rupelian (early Oligocene; ca. 33 Ma) to Torrejonian (middle Paleocene; ca. 63 Ma). We estimate the time of origin of mesonychian condylarths to have been at or after about 66.7 Ma (virtually at the Cretaceous-Paleocene boundary). Artiodactyla, the extant sister-group of whales, has a fossil record extending to the beginning of the Eocene, with an arctocyonian ancestry extending into the latest Cretaceous. The fossil record as now known indicates evolutionary divergence of Mesonychia + Cetacea from Arctocyonia + Artiodactyla in the early Paleocene or, reasonably, at the very end of the Cretaceous.
Philip D. Gingerich, Department of Geological Sciences and Museum of Paleontology, The University of Michigan, Ann Arbor, Michigan 48109-1079, U.S.A.
Mark D. Uhen, Cranbrook Institute of Science, Bloomfield Hills, Michigan 48303-0801, U.S.A.
Keywords: Stratigraphic range, Cetacea, Mesonychia, Artiodactyla, Eocene
Copyright: Paleontological
Society, 1 August 1998
Submission: 30 April 1998, Acceptance: 7 July 1998
http://palaeo-electronica.org/1998_2/ging_uhen/issue2.htm