Meeting Abstract

22.1  Sunday, Jan. 4  Monotreme postcranial ontogeny and the evolution of mammalian skeletal development WEISBECKER, Vera*; SANCHEZ-VILLAGRA, Marcelo R; Cambridge University; Universitaet Zuerich vwei07@esc.cam.ac.uk

Mammalian postcranial diversity and development is often discussed with view to a potential relationship of mammalian reproduction with postcranial ontogeny. However, data on monotremes, representing the earliest diverging extant mammalian clade, are lacking. Using micro-CT, I collected the first data on monotreme postcranial ossification to assess postcranial ontogeny in the earliest mammals, and to relate marsupial ossification patterns to the ancestral therian condition. Event-pair coded ossification sequences from 5 platypus and 5 echidna juveniles were added to previously obtained data of 27 therians and 3 sauropsids, and analysed with the Parsimov program and phylogenetic analysis using PAUP. The latter yields monophyletic monotremes within monophyletic Marsupialia, retrieving no other currently accepted clades. Monotremes are virtually unique in 14 characters, and 6 are unique in echidnas; cervical and lumbar vertebrae and the coracoid ossify like in non-mammalian amniotes. Parsimov analysis indicates delayed ossification of scapula and stylopods relative to axial and zeugopod elements in monotremes. Echidnas show a unique ossification shift of the lower spine with respect to stylopods and scapula. Parsimov results confirm that marsupial ossification patterns are derived compared to the therian ancestor. The derived ossification patterns in monotremes, which are not assignable to either those of marsupials or placentals, agree with recent suggestions that monotremes display a mosaic of traits. This, and the differences between echidnas and platypuses, also prevent determination of ancestral mammalian ossification patterns. The late ossification of both stylopods in monotremes points is congruent with hypotheses of modularity between mammalian limbs.