Early origin of artiodactyls in South America

Bonaparte’s (1999) description of the Early Cretaceous sauropod Agustinia ligabuie was notable for its identification of nine bony fragments as representing dermal armour, which he classified into Types 1–4. Here are some examples:

Bonaparte 1999: figure 3. Agustinia ligabuei gen et sp. nov. Osteoderms. A, Type 1. B, Type 2. C, Type 3. Abbrev.: po, thick proximal ossification.

Consequently, Augustinia was for many years restored as uniquely spiky, a sort of “stegosaur sauropod”:

But Bellardini and Cerda (2017) showed that the so-called “osteoderms” are probably no such thing, but represent other, more normal, bony elements. Bonaparte’s types 1, 3 and 4 were all reinterpreted as complete or partial dorsal ribs, and type 2 as a portion of the iliac blade:

Bellardini and Cerda 2017: figure 6. Bony element type 2 (MCF-PVPH-110/08) of A. ligabuei, in medial (A1–2), and lateral views (B1–2) with interpretive line drawings. In this work we propose that MCF-PVPH-110/08 is a dorsal portion of the left iliac blade, centered above the acetabulum (C). Abbreviations: dmil dorsal margin of iliac blade, poap post-acetabular process, prap pre-acetabular process, sr sacral rib. Dashed lines indicate missing bone. Asterisks indicate the hypothetical position of morphological features.

However, careful re-examination of Bellardini and Cerda 2017: figure 6: part A1 shows clearly that the so-called type-2 osteoderm is in fact a sheep:

This is a significant finding, as the origin of Artiodactyla is generally held to have occurred in the early Eocene, with the earliest known representatives being from Europe, Asia, and North America (Rose 1996). The reidentification of the A. ligabuei type-2 osteoderm as a sheep pushes back the origin of artiodactyls by some 60 million years to the Aptian or Albian, and locates South America as the continent of origin.

References

• Bellardini, Flavio, and Ignacio A. Cerda. 2017. Bone histology sheds light on the nature of the “dermal armor” of the enigmatic sauropod dinosaur Agustinia ligabuei Bonaparte, 1999. The Science of Nature 104(1):1-13. doi:10.1007/s00114-016-1423-7
• Bonaparte, Jose F. 1999. An armoured sauropod from the Aptian of northern Patagonia, Argentina. pp. 1-12 in Y. Tomida, T. H. Rich and P. Vickers-Rich (eds.), Proceedings of the Second Gondwanan Dinosaur Symposium, Tokyo National Science Museum Monograph 15. Tokyo National Science Museum, Tokyo. x+296 pp.
• Rose, Kenneth D. 1996. On the origin of Artiodactyla. Proceedings of the National Academy of Sciences 93(4):1705-9. doi:10.1073/pnas.93.4.1705