Re-evaluating "Apatosaurus" minimus, a bizarre Morrison Formation sauropod with diplodocoid and macronarian features

25th September 2012

Mike Taylor1 & Mathew Wedel2

1University of Bristol
2Western University of Health Sciences

American Museum of Natural History 675 is a sauropod specimen consisting of a sacrum with fused ilia, two ischia and a partial pubis. It was initially described as belonging to Apatosaurus, and subsequently given its own species, Apatosaurus minimus. However, it is universally recognised that this assignment was incorrect: the specimen differs from Apatosaurus in many respects including proportional width of the sacrum, relative lowness of the neural spines, apparent absence of pneumatic foramina in the sacral centra, low and broadly flared ilia, and less expanded distal ends of ischium and pubis. Numerous other details preclude referral to other well-known Morrison sauropods.

The specimen shows a strange mix of diplodocoid characters (the tall and elaborately laminated neural spines), macronarian characters (the breadth of sacrum and flaring ilia), and characters unique to it (e.g. the ilium remains vertical as it projects laterally, and has a distinctive horizontal ridge above the pubic articulation).

We attempted to determine its affinities by phylogenetic analysis. In a diplodocoid-focussed analysis, "Apatosaurus" minimus was scored for 22 of 189 characters (12%), yielding 3 most parsimonious trees differing only in the arrangement of diplodocines. In all trees, "Apatosaurus" minimus was the most basal diplodocoid. However, in a more general sauropod analysis, scoring for 37 of 331 characters (11%) yielded 86 most parsimonious trees, of which 74 placed "Apatosaurus" minimus as a very derived titanosaur, sister to Saltasaurus. These conflicting results underline the paradoxical nature of the specimen. Whether it belongs to Diplodocoidea or Macronaria, it has features convergent with the other clade.


The abstracts volume for SVPCA 2012 can be downloaded.

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