Molecules, morphology, and ecology indicate a recent, amphibious ancestry for echidnas

Abstract

The semiaquatic platypus and terrestrial echidnas (spiny anteaters) are the only living egg-laying mammals (monotremes). The fossil record has provided few clues as to their origins and the evolution of their ecological specializations; however, recent reassignment of the Early Cretaceous Teinolophos and Steropodon to the platypus lineage implies that platypuses and echidnas diverged >112.5 million years ago, reinforcing the notion of monotremes as living fossils. This placement is based primarily on characters related to a single feature, the enlarged mandibular canal, which supplies blood vessels and dense electrosensory receptors to the platypus bill. Our reevaluation of the morphological data instead groups platypus and echidnas to the exclusion of Teinolophos and Steropodon and suggests that an enlarged mandibular canal is ancestral for monotremes (partly reversed in echidnas, in association with general mandibular reduction). A multigene evaluation of the echidna-platypus divergence using both a relaxed molecular clock and direct fossil calibrations reveals a recent split of 19 -48 million years ago. Platypus-like monotremes (Monotrematum) predate this divergence, indicating that echidnas had aquatically foraging ancestors that reinvaded terrestrial ecosystems. This ecological shift and the associated radiation of echidnas represent a recent expansion of niche space despite potential competition from marsupials. Monotremes might have survived the invasion of marsupials into Australasia by exploiting ecological niches in which marsupials are restricted by their reproductive mode. Morphology, ecology, and molecular biology together indicate that Teinolophos and Steropodon are basal monotremes rather than platypus relatives, and that living monotremes are a relatively recent radiation.calibration ͉ molecular dating ͉ Monotremata ͉ niche ͉ phylogeny

Functional characterization of the rod visual pigment of the echidna (Tachyglossus aculeatus), a basal mammal

“…They are endemic to the Australia-New Guinea shelf (Dawson et al, 1979;Rismiller, 1999), with the short-beaked echidna being the most widely distributed extant monotreme and found both in Australia and in New Guinea (Griffiths, 1989;Nicol & Andersen, 2006). The origin of Monotremata presumably occurred sometime in the Late Triassic/Early Jurassic, a date supported by fossil as well as molecular data (Luo et al, 2002;Woodburne et al, 2003;Phillips et al, 2009). Based on fossil evidence, the divergence of modern platypus and echidna lineages occurred in the Early Cretaceous (Rowe et al, 2008).…”

Did sex chromosome turnover promote divergence of the major mammal groups?

“…Likewise, one of the platypus XY elements is represented by an autosome in echidnas. This implies that the three translocations in common occurred before platypus and echidna diverged, probably about 50 MYA 56, 57, but one further translocation occurred independently in each of the two lineages at or after divergence, and involved different autosomes.…”

The upper dentition and relationships of the enigmatic Australian Cretaceous mammal Kollikodon ritchiei

“…Revisions to the matrix were made on the basis of corrections and criticisms by Woodburne et al (2003), Rougier et al (2007), Rowe et al (2008) and Phillips et al (2009). The final matrix included 104 taxa and 438 characters.…”

“…Some authors, however, have questioned the inclusion of monotremes within Australosphenida (Pascual et al, 2002;Rich et al, 2002;Rowe et al, 2008;Woodburne, 2003). Archer et al (1985) described Steropodon galmani as a plesiomorphic, toothed monotreme in the monotypic family Steropodontidae, an assignation that has been widely accepted (Kielan-Jaworowska et al, 1987;Musser, 2006;Phillips et al, 2009). Inclusion of Teinolophos trusleri within Monotremata is also uncontested, despite some debate over placement within the stem or crown group (Phillips et al, 2009;Rich et al, 2001b;Rowe et al, 2008).…”

“…Archer et al (1985) described Steropodon galmani as a plesiomorphic, toothed monotreme in the monotypic family Steropodontidae, an assignation that has been widely accepted (Kielan-Jaworowska et al, 1987;Musser, 2006;Phillips et al, 2009). Inclusion of Teinolophos trusleri within Monotremata is also uncontested, despite some debate over placement within the stem or crown group (Phillips et al, 2009;Rich et al, 2001b;Rowe et al, 2008). Flannery et al, (1995) described Kollikodon ritchiei as a possible monotreme, placing it in its own monotypic family, the Kollikodontidae.…”