New information about the skull and dentary of the Miocene platypusObdurodon dicksoni, and a discussion of ornithorhynchid relationships

Abstract

A reconstruction of the skull, dentary and dentition of the middle Miocene ornithorhynchid Obdurodon dicksoni has been made possible by acquisition of nearly complete cranial and dental material. Access to new anatomical work on the living platypus, Ornithorhynchus anatinus, and the present comparative study of the cranial foramina of Ob. dicksoni and Or. anatinus have provided new insights into the evolution of the ornithorhynchid skull. The hypertrophied bill in Ob. dicksoni is seen here as possibly apomorphic, although evidence from ontogenetic studies of Or. anatinus suggests that the basic form of the bill in Ob. dicksoni (where the rostral crura meet at the midline) may be ancestral to the form of the bill in Or. anatinus (where the rostral crura meet at the midline in the embryonic platypus but diverge in the adult). Di¡erences in the relative positions of cranial structures, and in the relationships of certain cranial foramina, indicate that the cranium may have become secondarily shortened in Or. anatinus, possibly evolving from a more elongate skull type such as that of Ob. dicksoni. The plesiomorphic dentary of Ob. dicksoni, with well-developed coronoid and angular processes, contrasts with the dentary of Or. anatinus, in which the processes are almost vestigial, as well as with the dentary of the late Oligocene, congeneric Ob. insignis, in which the angular process appears to be reduced (the coronoid process is missing). In this regard the dentary of Ob. insignis seems to be morphologically closer to Or. anatinus than is the dentary of the younger Ob. dicksoni. Phylogenetic conclusions di¡er from previous analyses in viewing the northern Australian Ob. dicksoni as possibly derived in possessing a hypertrophied bill and dorsoventrally £attened skull and dentary, perhaps being a specialized branch of the Obdurodon line rather than ancestral to species of Ornithorhynchus. The presence of functional teeth and the robust, £attened skull and dentary in Ob. dicksoni argue for di¡erences in diet and lifestyle between this extinct ornithorhynchid and the living Ornithorhynchus.

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

“…Each of these traits would be highly anomalous if derived directly from a more generalized terrestrial insectivore morphotype (typical of basal mammals). The embryologic presence in echidnas of the marginal cartilage that contours the bill of platypuses (58) similarly suggests that a bill (rather than a beak or snout) is ancestral for crown monotremes. Other features of echidnas also suggest a substantial, relatively recent ecological shift.…”

“…A number of aspects of echidna biology are consistent with an origin from a platypus-like ancestor with such traits as aquadynamic streamlining (58), dorsally projecting hind limbs acting as rudders (59), and locomotion founded on hypertrophied humeral long-axis rotation, which provides a very efficient swimming stroke (60). In echidnas, traits that are potentially homologous with these are dorso-ventral compression, reversed hind-foot posture, and ''front-wheel drive'' locomotion based on humeral long-axis rotation.…”

The origin and evolution of mammals

“…Even the beautiful, complete skull found in 1985. Musser and Archer 1998) added nothing useful to the debate on monotreme relationships, for the teeth (Fig. 5.21(b)) are very similar to the juvenile teeth of Ornithorhynchus, and equally difficult to compare with those of other taxa.…”

“…The greatest mystery of all concerning mammalian evolution stretches back for 200 years: the question of what exactly the monotreme mammals are, and how they relate phylogenetically to therians (Musser and Archer 1998). The three living genera of monotremes, Ornithorhynchus (platypus), Tachyglossus (short-beaked echidna), and Zaglossus (long-beaked echidna) of Australasia constitute a monophyletic group supported by several characters such as the low, broad skull, reduced dentition, secondary sprawling gait, and hindlimb spur.…”

Four mammal fossil calibrations: balancing competing palaeontological and molecular considerations

“…A partial, edentulous dentary (QMF18977) and several cheek teeth (QM F18978, QMF30249, QMF30716 and QMF30717) are slightly older, providing the calibration minimum bound. The slightly older cheek teeth closely match the holotype skull for size and "insertion" and are near-identical to other dental material from the same site as the skull, thus justifying conspecificity (Archer et al, 1993;Musser and Archer, 1998). Phylogenetic Justification.…”