Why coelacanths are not ‘living fossils’

Abstract

A series of recent studies on extant coelacanths has emphasised the slow rate of molecular and morphological evolution in these species. These studies were based on the assumption that a coelacanth is a 'living fossil' that has shown little morphological change since the Devonian, and they proposed a causal link between low molecular evolutionary rate and morphological stasis. Here, we have examined the available molecular and morphological data and show that: (i) low intra-specific molecular diversity does not imply low mutation rate, (ii) studies not showing low substitution rates in coelacanth are often neglected, (iii) the morphological stability of coelacanths is not supported by paleontological evidence. We recall that intra-species levels of molecular diversity, inter-species genome divergence rates and morphological divergence rates are under different constraints and they are not necessarily correlated. Finally, we emphasise that concepts such as 'living fossil', 'basal lineage', or 'primitive extant species' do not make sense from a tree-thinking perspective.

Ancestor–descendant relationships in evolution: origin of the extant pygmy right whale, Caperea marginata

“…Interestingly, if †Miocaperea were reconsidered as in the genus Caperea, this long-term morphological stasis between †Miocaperea-Caperea may represent stabilomorphism [32]. Of note, here we consider that living Caperea may be descended from the fossil species †Miocaperea, but Caperea should not be considered as living fossil [33]. Recent debate on Caperea phylogeny [23,34,35] reflects different character selections and interpretations of homology, as results here also indicate (see electronic supplementary material).…”

What Is the Meaning of Extreme Phylogenetic Diversity? The Case of Phylogenetic Relict Species

“…Actually, none of the classic relicts has ever been found similar to early fossil relatives after closer investigation, therefore refuting the idea of a generalized evolutionary stasis. For example, the venom in Platypus is not archaic but totally original, neither squamate nor mammal-type (O'Brien 2008 ), the coelacanth fi sh is originally modern in its reproduction mode, being ovoviviparous (Casane and Laurenti 2013 ). The term "panchronic" (e.g., Janvier 2007 ) has also been used in this way with the same wrong assumption that relict taxa did not evolve.…”

The salmon, the lungfish (or the coelacanth) and the cow: a revival?

“…Apart from the fact that low intraspecific molecular diversity does not necessarily imply in a low mutation rate (cf. Casane & Laurenti, 2013), the issue is that coelacanths along with lungfishes are the few fish-like survivors of the Sarcopterygii, a Devonian lineage (over 410 million years old) that diversified into more than 32,700 species of living tetrapods (cf. Schipper et al, 2008;AmphibiaWeb, 2013;Clements et al, 2013;Uetz, 2013) but that possessed, also, a stunning diversity of extinct fish-like forms (Figure 1).…”

“…The recent discovery of the tuna-like †Rebellatrix divaricerca from the Lower Triassic of British Columbia, which bears a streamlined body and a deeply forked caudal fin, suggestive of a fastswimming and active lifestyle (Wendruff & Wilson, 2012), also undermines the ingrained notion that coelacanths are morphologically static over their roughly 410 million years of existence (cf. Friedman & Coates, 2006;Wendruff & Wilson, 2012;Casane & Laurenti, 2013). Even within the crown clade Latimeroidei, the differences between head shape, dentition, structure of swim bladder (lung), and several other anatomical structures, as well as in paleohabitats, are striking, such as those exhibited by the Lower Cretaceous marine †Axelrodichthys (e.g.…”

“…Maisey, 1986Maisey, , 1991 and the Triassic freshwater †Chinlea (cf. Elliott, 1987;Dutel et al, 2012;Casane & Laurenti, 2013).…”