Dual origin of tribosphenic mammals

Abstract

Marsupials, placentals and their close therian relatives possess complex (tribosphenic) molars that are capable of versatile occlusal functions. This functional complex is widely thought to be a key to the early diversification and evolutionary success of extant therians and their close relatives (tribosphenidans). Long thought to have arisen on northern continents, tribosphenic mammals have recently been reported from southern landmasses. The great age and advanced morphology of these new mammals has led to the alternative suggestion of a Gondwanan origin for the group. Implicit in both biogeographic hypotheses is the assumption that tribosphenic molars evolved only once in mammalian evolutionary history. Phylogenetic and morphometric analyses including these newly discovered taxa suggest a different interpretation: that mammals with tribosphenic molars are not monophyletic. Tribosphenic molars evolved independently in two ancient (holotherian) mammalian groups with different geographic distributions during the Jurassic/Early Cretaceous: an australosphenidan clade endemic to Gondwanan landmasses, survived by extant monotremes; and a boreosphenidan clade of Laurasian continents, including extant marsupials, placentals and their relatives.

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

“…Furthermore, all of the aforementioned trees united the Chinese ''primitive triconodont'' Hadrocodium with monotremes, contrary to the majority opinion (5,6,24,27,48) that monotreme affinities lie with the Gondwanan clade, Australosphenida. The original incarnation of the present morphological data sets (27) Backbone constraints were used to define these alternatives for morphol439 and morphol441, both with the remainder of the tree topology unconstrained or with monotremes constrained to group within Australosphenida (ЉAustra-losphenida-constrainedЉ).…”

“…One of these species is the semiaquatic, invertebrate feeding platypus (Ornithorhynchus anatinus) of eastern and southern Australia; the others are the terrestrial echidnas (Tachyglossidae), the short-beaked echidna or spiny anteater, Tachyglossus aculeatus of Australia and New Guinea, and three species of New Guinean long-beaked echidnas (Zaglossus bruijni, Z. attenboroughi, and Z. bartoni), which feed on worms and arthropod larvae. Fossil monotremes, such as Teinolophos trusleri and Steropodon galmani (2), along with their putative relatives, the insectivore-like ausktribosphenids (3)(4)(5)(6), make up the bulk of the known Australian Cretaceous mammal fauna. Known monotreme diversity then contracts to only platypus-like taxa, subsequent to the arrival of marsupials from South America via Antarctica Ϸ71-54.6 million years ago (Ma) (7,8).…”

The Influence of Foraging Mode and Arid Adaptation on the Basal Metabolic Rates of Burrowing Mammals

“…Monotremata diverged from other therian lineages during the Early Jurassic (Luo et al 2001). The Metatheria/Eutheria split was dated according to Kumar and Hedges (1998).…”

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

“…Kollikodon ritchiei was added to a revised version of the character matrix of Luo et al (2011), which is in itself a modified version of earlier matrices (Luo et al, 2001(Luo et al, , 2002(Luo et al, , 2007bLuo & Wible, 2005). 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).…”

“…These include: from the Aptian Flat Rocks locality in Victoria, the ausktribosphenids Ausktribosphenos nyktos (Rich et al, 1997) and Bishops whitmorei (Rich et al, 2001a), the monotreme Teinolophos trusleri (Rich et al, 1999(Rich et al, , 2001b, and the multituberculate Corriebaatar marywaltersae (Rich et al, 2009); from the Albian Dinosaur Cove locality in Victoria, the partial humerus of (the possible monotreme) Kryoryctes cadburyi (Pridmore et al, 2005); and from Lightning Ridge in New South Wales a very large, mammal-like isolated tooth (Clemens et al, 2003). Most recent phylogenetic analyses have placed ausktribosphenids and monotremes within a larger Gondwanan radiation termed Australosphenida (Luo et al, 2001(Luo et al, , 2002(Luo et al, , 2007aMartin and Rauhut, 2005;Rougier et al, 2011;Wood and Rougier, 2005), together with the early Jurassic (Toarcian) (Cúneo et al, 2013) South American Asfaltomylos (Rauhut et al, 2002) and Henosferus (Rougier et al, 2007), and the middle Jurassic (Bathonian) Ambondro mahabo (Flynn et al, 1999) from Madagascar. 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).…”