Phylogenetic Relationships of Mesozoic Turtles

Abstract

Presented herein is the first morphological analysis of turtle relationships to examine the monophyly of many turtle groups by using only single species as terminals and by integrating a large number of primitive fossil taxa. The data matrix consists of 136 osteological parsimony informative characters with 169 derived character states for 45 fossil and 22 living species of the clade Testudinata. The results corroborate the monophyly of a large number of previously hypothesized clades, but refute the accepted hypothesis regarding the basal split of living turtles. In particular, the primitive turtles Proterochersis robusta, Kayentachelys aprix, Mongolochelys efremovi, Meiolania platyceps, and Kallokibotion bajazidi are removed from their current position as crown turtles and placed along the phylogenetic stem of this clade. The age of the turtle crown is thereby adjusted from the Late Triassic to the Late Jurassic, which is relevant to testing molecular clock hypotheses. This revised topology has important implications for the evolution of several character complexes, because it implies that the common ancestor of all living turtles must have had a partially braced brain case and a primitive trochlear mechanism. Other noteworthy conclusions include the tentative exclusion of protostegids from Chelonioidea, the placement of Platysternon megacephalum outside of Chelydridae, and the tentative interpretation of Sandownia harrisi as a basal eucryptodire.

Jaw musculature during the dawn of turtle evolution

“…Compared to the remainder of the skull, no crucial evolutionary changes occurred in the arrangement of the posterior lower jaw bones of taxa on the stem linage of Testudines (see Joyce 2007). This could, on the one hand, speak in favour of the hypothesis that related muscle characters have been conserved since the first occurrence of Testudinata.…”

“…The discussion of the present study on the phylogenetic signal of the jaw musculature (see above) should not be applied as a general feature to other vertebrate taxa and muscles, as, when compared to other vertebrate groups, turtles show a high degree of cranial diversity, which evolved relatively recently (e.g. Zdansky 1923Zdansky -1925Gaffney 1979;Joyce 2007;Werneburg 2012). Among other vertebrate taxa, muscle anatomy contains much phylogenetic signal (e.g.…”

“…I refrained from simply combining my muscle data set with published data sets on cranial bone characters (e.g. Gaffney et al 1991;Joyce 2007). Regarding the details defined for soft tissue anatomy herein, comparably detailed bone characters, such as the ratios of bony contacts in the lower jaw or small tuberosities and "rippled" areas, which may be correlated to attachment sites (e.g.…”

“…Their morphological data supported a long-necked and a short-necked clade of chelid pleurodires. Other morphological works differ in the arrangement of Chelydridae, Platysternidae, and Chelonioidea (marine turtles) (Brinkman and Wu 1999;Joyce 2007;. Since the late 1990s, genetic studies have consistently resulted in a stable chelid phylogeny consisting of distinct South American and Australasian clades (Shaffer et al 2007;Krenz et al 2005;Parham et al 2006; Thomson and Shaffer 2010).…”

“…Gaffney 1975;Gaffney et al 1991;Joyce 2007;Lyson et al 2010; Thomson and Shaffer 2010;Sterli 2010), only a few other character complexes have been used to reconstruct turtle phylogeny. I studied the jaw musculature as being a potential new source of phylogenetic information.…”