J Paleont 78 1 2004 pp 39 44 Copyright q 2004 The Paleontological Society 0022 3360 04 0078 39 03 00 MOLECULAR CLOCK DIVERGENCE ESTIMATES AND THE FOSSIL RECORD OF CETARTIODACTYLA JESSICA M THEODOR Department of Geology Illinois State Museum Springfield 62703 jtheodor museum state il us ABSTRACT Molecular clock estimates of divergence times for artiodactyls and whales vary widely in their agreement with the fossil record Recent estimates indicate that the divergence of whales from artiodactyls occurred 60 Ma a date which compares well with the first appearances of fossil whales around 53 5 Ma and artiodactyls at 55 Ma Other estimates imply significant gaps in the fossil record A date of 65 Ma for the divergence of Suidae and Ruminantia predates the appearance of Ruminantia by over 10 million years and an estimate of 58 Ma for the divergence of Suidae from Cetacea implies a gap of over 20 million years Further although a molecular clock estimate has not been reported the hypothesis that hippos are the closest living relatives of the whales implies a potential ghost lineage for hippos of over 40 million years There are only two living species of hippos and their fossil record is sparse while cetaceans and other artiodactyls are speciose and have rich fossil records A 40 million year gap in the fossil record of hippos could be explained by several possibilities inadequate biogeographic sampling taphonomic biases or undifferentiated primitive morphology Similarly a number of possible problems may exist in the molecular data rate variation in the genes sampled the low numbers of genes examined and insufficient age calibrations In addition there are potential problems in molecular phylogeny estimation such as long branch attraction and inappropriate taxonomic sampling Additional estimates of divergence times among living taxa should provide a broader framework for comparison with the fossil record and provide information to help identify which of these factors are causing conflict 1998 The hippopotamid cetacean relationship is especially difficult to reconcile given that whales appear in the early Eocene and the oldest hippopotamid fossil is known from the mid Miocene Behrensmeyer et al 2002 15 6 15 8 Ma a gap of 40 million years This gap is somewhat lessened to about 10 million years if the fossil anthracotheres are closely related to the hippopotamids Colbert 1935 as they have a long fossil record going back to the middle Eocene However the morphological evidence linking anthracotheres and hippos is not strong and it has been suggested that hippopotamids may be derived from other lineages including the tayassuids Pickford 1983 and the cebochoerids Pearson 1927 Such a relationship would mean that whales diverged genetically from hippopotamids or their close relatives before the first appearance of whale fossils and hence over 53 million years ago This study was undertaken to examine additional molecular clock estimates of divergence times within the Artiodactyla and Cetacea and compare those divergence times with the fossil record to better understand the causes of conflict between the molecular data and the fossil record INTRODUCTION phylogenetic work on the relationships of placental mammals show strong support for close relationship between whales Cetacea and artiodactyls with numerous studies nesting Cetacea deep within a paraphyletic Artiodactyla in most cases sister taxon to the Hippopotamidae Gatesy 1997 1998 Gatesy et al 1996 1999 Graur and Higgins 1994 Milinkovitch et al 1998 M A Nikaido et al 1999 New fossil discoveries of postcranial material of early whales show they share what had been thought to be a key synapomorphy of artiodactyls the double trochleated astragalus Gingerich et al 2001 Thewissen et al 2001 Although these finds provide evidence supporting the close relationship of whales and artiodactyls there is as yet no fossil evidence that supports a sister taxon relationship between the whales and the family Hippopotamidae However if correct this hypothesis has radical implications for the fossil record of cetaceans and artiodactyls and the history of divergences within the Cetartiodactyla because it implies that there are several ghost lineages of considerable length either yet to be found in fossil deposits or lurking in museum drawers and undetectable by morphological methods In order to evaluate that possibility molecular clock estimates of divergence times could point to lineages in need of further study but the reliability of such estimates for Cetartiodactyla is unknown Therefore molecular clock estimates for more nodes within Cetartiodactyla must be calculated and compared with the known fossil record The fossil record for Artiodactyla and Cetacea accords well with published molecular clock estimates for the divergence of the two groups the oldest known whale Himalayacetus is early Eocene in age 53 5 Ma Bajpai and Gingerich 1998 and the oldest artiodactyl Diacodexis known from the earliest Eocene 55 Ma Gingerich 1989 are both slightly younger than the molecular clock estimate of 60 Ma Arnason and Gullberg 1996 The most recent molecular clock estimate of the divergence of odontocete and mysticete whales around 34 35 Ma also agrees well with the fossil record M Nikaido et al 2001 However estimates of divergence times for other subgroups are scarce and some estimates of suid divergence from ruminants predate the oldest records of either order Kumar and Hedges R ECENT MOLECULAR METHODS The data used include the cytochrome b cytb mitochondrial gene sequence data 1 143 base pairs 63 taxa and the kappa casein kcas nuclear gene sequence data 679 base pairs 33 taxa from the WHIPPO 2 data matrix Gatesy et al 1999 Each data set was tested for clocklike behavior using relative rate tests Fitch 1976 Sarich and Wilson 1967 Tajima 1993 and likelihood ratio tests Felsenstein 1981 Huelsenbeck and Crandall 1997 Huelsenbeck and Rannala 1997 D L Swofford et al 1996 The substitution models selected for the likelihood ratio tests were selected using hierarchical likelihood ratio tests to select an appropriate model as implemented in Modeltest 3 06 Posada and Crandall 1998 The likelihood ratio tests were performed on three trees using PAUP the best likelihood tree calculated using the selected substitution model the pruned shortest topology and the shortest parsimony tree constrained for monophyletic Artiodactyla Gatesy 1999 Likelihood ratio tests were calculated for the full
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