Did sauropods really decline in the Early Cretaceous of North America?

21st April 2008

MATHEW J. WEDEL, University of California, Merced;
MICHAEL P. TAYLOR, University of Portsmouth, United Kingdom.

The latest Jurassic has traditionally been considered the golden age of sauropods, when well-known Morrison Formation genera such as Diplodocus, Apatosaurus, Camarasaurus, Brachiosaurus and Barosaurus, together with many less well-known genera, dominated the fauna of North America. By contrast, the record of sauropods in the Early Cretaceous of North America was until recently very sparse, with all elements referred to the poorly known macronarian Astrodon (= Pleurocoelus).

This has changed dramatically in the last decade, with the new sauropods Sonorasaurus, Cedarosaurus, Sauroposeidon, Venenosaurus and Paluxysaurus all having been described from the Early Cretaceous of North America, and with at least three more awaiting description in the near future.

So, did sauropods really decline in the Early Cretaceous of North America?

First, although Early Cretaceous sauropods rivaled their Morrison forebears in generic diversity, the former were spread across half a dozen formations and 20 million years, whereas the latter were much more concentrated in space and time.

Second, the phylogenetic content of the North American sauropod fauna shifted dramatically from Jurassic to Cretaceous: the Morrison is dominated by diplodocoids, and macronarians are poorly represented (although the basal macronarian Camarasaurus is abundant); the Early Cretaceous sauropod fauna consists entirely of macronarians, most of them basal titanosauriforms. To date, no definitive Cretaceous diplodocid is known from anywhere in the world.

Third, while sauropods are the most abundant dinosaurs in the Morrison, they are among the rarest in their respective faunas in the Early Cretaceous, with most genera known from single specimens.


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