Highly specialized mammalian skulls from the Late Cretaceous of South America

Journal name:
Nature
Volume:
479,
Pages:
98–102
Date published:
DOI:
doi:10.1038/nature10591
Received
Accepted
Published online

Dryolestoids are an extinct mammalian group belonging to the lineage leading to modern marsupials and placentals1, 2. Dryolestoids are known by teeth and jaws from the Jurassic period of North America and Europe2, 3, but they thrived in South America up to the end of the Mesozoic era and survived to the beginnings of the Cenozoic2, 4, 5, 6, 7. Isolated teeth and jaws from the latest Cretaceous of South America provide mounting evidence that, at least in western Gondwana, dryolestoids developed into strongly endemic groups by the Late Cretaceous4, 5, 6, 7, 8, 9. However, the lack of pre-Late Cretaceous dryolestoid remains made study of their origin and early diversification intractable. Here we describe the first mammalian remains from the early Late Cretaceous of South America, including two partial skulls and jaws of a derived dryolestoid showing dental and cranial features unknown among any other group of Mesozoic mammals, such as single-rooted molars preceded by double-rooted premolars, combined with a very long muzzle, exceedingly long canines and evidence of highly specialized masticatory musculature. On one hand, the new mammal shares derived features of dryolestoids1, 2, 3 with forms from the Jurassic of Laurasia, whereas on the other hand, it is very specialized and highlights the endemic, diverse dryolestoid fauna from the Cretaceous of South America. Our specimens include only the second mammalian skull known for the Cretaceous of Gondwana, bridging a previous 60-million-year gap in the fossil record, and document the whole cranial morphology of a dryolestoid, revealing an unsuspected morphological and ecological diversity for non-tribosphenic mammals.

At a glance

Figures

  1. Cronopio dentiacutus MPCA 454 type skull and MPCA 453 right lower jaw.
    Figure 1: Cronopio dentiacutus MPCA 454 type skull and MPCA 453 right lower jaw.

    ac, MPCA 454 skull in ventral (a) and dorsal views (b); labial view of the left upper dentition (c). d, e, Cronopio dentiacutus MPCA 453 right lower jaw in labial and lingual views (d) and detail of the left petrosal and basicranium (e).

  2. Cronopio dentiacutus MPCA 454 type skull and MPCA 453 skull and right lower jaw.
    Figure 2: Cronopio dentiacutus MPCA 454 type skull and MPCA 453 skull and right lower jaw.

    a, MPCA 454 skull in dorsal (left) right lateral (top right) and left lateral views (bottom right). b, Cronopio dentiacutus MPCA 453 left lateral view of the skull (top left), detail of the petrosal and basicranium (bottom left) and occlusal, labial and lingual views of the right lower jaw (right). Ap, angular process; Cp, coronoid process; Fc, fenestra cochleae; Fv, fenestra vestibuli; Frs, foramen for the ramus superior of the stapedial artery; Mp, masseteric process; VII, facial foramen, for hyomandibular branch of cranial nerve VII. All scale bars are 5mm, except for bottom-left panel of b, which is 2mm.

  3. Cronopio dentiacutus MPCA 454 holotype (upper teeth).
    Figure 3: Cronopio dentiacutus MPCA 454 holotype (upper teeth).

    ad, Upper teeth in occlusal (a, d) and labial view (b, c). Scale bar, 3mm.

  4. Reconstruction of the skull and jaws of Cronopio dentiacutus based on MPCA 453 and 454.
    Figure 4: Reconstruction of the skull and jaws of Cronopio dentiacutus based on MPCA 453 and 454.

    The only areas not preserved in the specimens described here are the lower canines and lower jaw incisors, the septomaxilla, the sagittal crest and part of the cranial vault in the skull. Scale bar, 5mm. Estimated skull length of MPCA 454 is 27 mm.

  5. Simplified phylogenetic tree of the major taxa included in the analysis.
    Figure 5: Simplified phylogenetic tree of the major taxa included in the analysis.

    The topology reflects a strict consensus tree of 8 equally most parsimonious trees (length 1,114) performed on a matrix including 304 characters and 57 taxa. The dryolestoids (green) show a basal dichotomy with most of the South America forms (dark green) forming a monophyletic group that includes as terminal taxa highly specialized relatively large-sized forms from the Late Cretaceous and Palaeocene. Unlike most other dryolestoids, these derived South American mammals were omnivorous/herbivorous with complex tooth-on-tooth occlusion and thick enamel, and group together under Mesungulatoidea (see Supplementary Information). Groebertherium from the latest Cretaceous of Argentina appears as a derived member of a northern group of dryolestoids, indicating a probable Jurassic origin from groups of Pangeic distribution for at least some Late Cretaceous South American dryolestoids. Letters at the nodes indicate high-level clades: D, Dryolestoidea; M, Mammalia; Me, Meridiolestida; T, Theria. See Supplementary Information for character list, data matrix and analysis protocol. Myr, million years.

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Author information

Affiliations

  1. Department of Anatomical Sciences and Neurobiology, University of Louisville, Louisville, Kentucky 40202, USA

    • Guillermo W. Rougier
  2. CEBBAD - Fundación de Historia Natural ‘Félix de Azara’, Universidad Maimónides, Hidalgo 775, Buenos Aires (1405), Argentina

    • Sebastián Apesteguía
  3. CONICET, Consejo Nacional de Investigaciones Científicas y Técnicas. Av. Rivadavia 1917, C1033AAJ, Buenos Aires, Argentina

    • Sebastián Apesteguía &
    • Leandro C. Gaetano
  4. Departamento de Cs. Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II, C1428EHA, Buenos Aires, Argentina

    • Leandro C. Gaetano

Contributions

G.W.R. wrote the manuscript with contributions from all authors, edited the figures, scored the matrix and performed the phylogenetic analysis; S.A. edited the manuscript and figures; L.C.G. edited the manuscript, figures, matrix and performed the phylogenetic analysis.

Competing financial interests

The authors declare no competing financial interests.

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Supplementary information

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  1. Supplementary Information (1.3M)

    The file contains Supplementary Text (Parts 1 and 2), Supplementary Figures 1-4 with legends and additional references.

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