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The oldest platypus and its bearing on divergence timing of the platypus and echidna clades - PubMed

Timothy Rowe  1 ,

The oldest platypus and its bearing on divergence timing of the platypus and echidna clades

Timothy Rowe et al. Proc Natl Acad Sci U S A. .

Abstract

Monotremes have left a poor fossil record, and paleontology has been virtually mute during two decades of discussion about molecular clock estimates of the timing of divergence between the platypus and echidna clades. We describe evidence from high-resolution x-ray computed tomography indicating that Teinolophos, an Early Cretaceous fossil from Australia's Flat Rocks locality (121-112.5 Ma), lies within the crown clade Monotremata, as a basal platypus. Strict molecular clock estimates of the divergence between platypus and echidnas range from 17 to 80 Ma, but Teinolophos suggests that the two monotreme clades were already distinct in the Early Cretaceous, and that their divergence may predate even the oldest strict molecular estimates by at least 50%. We generated relaxed molecular clock models using three different data sets, but only one yielded a date overlapping with the age of Teinolophos. Morphology suggests that Teinolophos is a platypus in both phylogenetic and ecological aspects, and tends to contradict the popular view of rapid Cenozoic monotreme diversification. Whereas the monotreme fossil record is still sparse and open to interpretation, the new data are consistent with much slower ecological, morphological, and taxonomic diversification rates for monotremes than in their sister taxon, the therian mammals. This alternative view of a deep geological history for monotremes suggests that rate heterogeneities may have affected mammalian evolution in such a way as to defeat strict molecular clock models and to challenge even relaxed molecular clock models when applied to mammalian history at a deep temporal scale.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Ornithorhynchus anatinus, in lateral (A), dorsal (C), and posterior (E) views, compared with its extinct relative Obdurodon dicksoni (lacking mandible) in lateral (B), dorsal (D), and posterior (F) views. Volumetric reconstructions based on HRXCT, illustrated at same lengths (not to scale). Abbreviations: Den (dentary), Fm (foramen magnum); m1 (sclerified first lower molar); m2 (sclerified second lower molar); Mt (mandibular tubercle); P3 (upper third premolar); P4 (upper fourth premolar); V2 (foramina for maxillary nerve), and V3 (foramen for mandibular nerve).

Fig. 2.

Teinolophos trusleri (NMV P216575). Volumetric reconstruction of left dentary, from HRXCT, in medial (A) and lateral (B) views, and in selected coronal cross sections (slice thickness = 0.019 mm). Medial tubercle (Mt) and mandibular canal (V3) are labeled. Complete CT serial section stacks and volumetric animations are available at

http://digimorph.org/specimens/Teinolophos_trusleri/216575/

.

Fig. 3.

Teinolophos trusleri (NMV P216750). Volumetric reconstruction of right dentary, built from HRXCT, in medial (A) and increasingly oblique views (B and C). (D–J) Selected coronal cross sections (slice thickness = 0.012 mm) of this jaw, with slice sequence positions indicated by numbers and vertical white lines in A to show position of the slice plane. Abbreviations: Alv, tooth alveoli; V3, mandibular canal; Mt, Medial tubercle. Complete CT serial section stacks and volumetric animations are available at

http://digimorph.org/specimens/Teinolophos_trusleri/216750/

.

Fig. 4.

Comparative diameters of the mandibular canal (in red) shown in HRXCT coronal slice planes, with the location of the slice marked by a white line on three-dimensional reconstructions of complete skulls in lateral view. (A) Morganucodon sp. (IVPP 8685), Early Jurassic fossil from China. (B) Ornithorhynchus anatinus (AMNH 252512), juvenile specimen still in possession of its deciduous dentition. (C) Ornithorhynchus anatinus (AMNH 200255) adult specimen with keratinous adult dentition. (D) Zaglossus bruijni (AMNH 157072) adult specimen. (E) Didelphis virginiana (TMM M-2517) adult specimen. (F) Dasypus novemcinctus (TMM M-7417), adult specimen. The mandibular canal is labeled (V3). Images not to scale; scaled imagery and complete CT data sets for each are available at

www.DigiMorph.org

.

Fig. 5.

Cladogram showing relationships of Teinolophos to other mammals and their extinct relatives among Cynodontia (see

SI Text

for matrix and details of methodology).

References

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