Elephantidae phylogeny: morphological versus molecular results

Abstract

T radition ally, m orph ologically based phylogenetic relationships within the fam ily E lep hantidae (m ostly Loxodanta, E lephas, and M am m uthus) depicted E leph as and Mainrnuthus as closely related tax a w ith Loxodanta as a sister-group to this clade. U n til recen tly, m olecular stu dies w ere unable to resolve relationsh ips am ong the w oolly m am m oth (.Mammuthus), A sian (E lephas), and A frican (Loxodanta) elephants, or indicated a ph enetic pa iring o f Loxodanta and M am m uthus w ith E leph as as a sister-group to this grade. In this study we provide further m orphological evidence for the traditional hypothesis and data from align ed DNA sequences o f the m ito chondrial gene cytochrom e b in support o f the m onophyletic M am m u th us-E leph as clade. In both d a ta sets, m orp h ologica l and m olecu la r, the extin ct A m erica n m astodon (M am m ut) w as em p loyed as an outgroup. The m olecular results dem onstrate the im portance o f using a closely related taxon as an ou tgrou p for resolving phylogenies o f highly derived species. Tests on the im portance o f the num bers o f outgroups and w hich ou tgrou p m ay be better for testing phylogenetic relationships reveal that the closer the outgroup to the ingroup, the more corroborative the results w ill be, E volu tionary rates calculated from the m orphological characters indicate that, a m on g the three genera studied, Loxodanta is the slow est evolving taxon, follow ed by E lephas, and M am m uthus. DNA sequences indicate sim ilar rate differences a m on g the three taxa. M orphological data also corroborate the cla ssica l hypothesis that the fam ily Stegodon tidae is m onophyletic and its m em bers {S tegolophodon and S tegod on) do not grou p w ithin E lephantidae. A com parison am ong m am m oths reveals that m any o f the sk u ll ch a ra cters a re in terlin k ed and m ay be con sidered as one, or as a suite o f ch aracters, eg antero-posterior com pression o f skull. An im portant trend has been observed -the m ost prim itive m am m oths had few er num bers o f plates per given tooth and low er lam ellar frequencies. A sim plified possible m am m oth ancestry and radiation is provided.