Wednesday, April 25, 2012

The Spinosaurids Arrive in Asia

This week in the journal Naturwissenshaften, we have the advance online publication of research describing a new spinosaurid, which the authors, led by Ronan Allain, dub Ichthyovenator laosensis: the Laotian Fish Hunter.

Ichthyovenator is described from portions of the spine and pelvis. Unfortunately, it's headless, but based on the postcranial remains, the authors are confident that it belongs to the baryonychine tribe of spinosaurids, allied with Suchomimus and Baryonyx. It possessed elongated neural spines, as is common in the family. In Ichthyovenator, these created a "sinusoidal" shape; that is, it rose and fell like a sine wave when viewed in profile, with a dip at the base of the tail enclosed by two peaks. Concerning the sail's function, the authors favor species recognition and sexual display over a thermoregulatory use. When trying to sort out who's who, a spinosaur would identify potential rivals or mates by the shape of their sail.

Baryonyx profile
Baryonyx walkeri, a close relative of Ichthyovenator, illustrated by Mark Witton. Shared via Flickr.

Spinosaurids have been represented by notoriously scrappy material; the discovery of Baryonyx in England about thirty years ago shed more light on their anatomy (Baryonyx may not be as flashy as its bigger, more extravagantly ornamented cousins, but it's the one who delivers the goods). Now, Ichthyovenator is the first definitive spinosaurid from Asia, living during the Early Cretaceous, like most of its described relatives. Teeth found elsewhere hint to a few stragglers persisting into the Late Cretaceous, and Allain et al's phylogenetic analysis places the poorly understood late Cretaceous theropod Chilantaisaurus in the spinosaurids, making it the youngest member of the clade - as well as a second one from Asia.

Tantalizingly, the authors reappraise a single finger bone from the Late Jurassic Morrison formation of the US, previously thought to be from a Torvosaurus, a member of the closely related megalosaurids (also known as the villain in the second episode of Dinosaur Revolution). Allain et al maintain that the proportions of this ungual phalanx - the bone to which that big thumb claw attached - fit better within the baryonychines. They are one of the only theropod groups known to have such an outsized first digit (you'll see the spinosaurines reconstructed this way as well, but this is inferred from baryonychine material). The Allosauroid megaraptorids also possessed an enlarged "thumb," though their claw is so recurved they were first thought to come from the foot of a dromaeosaur. As usual, this hypothesis depends upon the serendipity of new fossil discoveries.

The reason the authors mention this single phalanx is because of its implications for the early geographical distribution of spinosaurids. They are known from Europe, Africa, South America, possibly Australia and now Asia. It's likely that in the middle-to-late Jurassic as Pangaea slowly splintered, early spinosaurids enjoyed a distribution over much of the globe. If new finds can sort out the owner of this finger, Allain writes that it "may not only be the first record of Spinosauridae from North America but also the oldest known spinosaurid specimen," showing that they had indeed lived across Pangaea, and giving crucial insights into their origins. It would just be nice to have something relatively complete. As productive as the Morrison has been over the years, it's amazing to think of what wonders it still hides.

I'm a sucker for the -venator suffix; it just sounds cool. But of course, without cranial material for this animal, it's being named by the spinosaurid fish-eating reputation alone. If a head of Ichthyovenator turns up, there's the possibility it may have had a different diet, which would be a pretty interesting kink in this kinky-sailed dinosaur's story.

Revised 4/26/12 to include the bits on megaraptorids and Australia. Thanks to Mark Robinson and Tony Martin for catching these goofs.

More on Ichthyovenator:
Theropoda
Domain of the C-Rex
Dinosaur Tracking

Also of interest: Baryonyx, illustrated by Jim Conaway, made an appearance in an old post in the Vintage Dinosaur Art series

4 comments:

  1. Regarding outsized first (hand) digits, the morphology might not be similar but don't Megaraptor and friends have very large recurved unguals on their "thumbs"?

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    1. Yes, of course... my brain not work so good. I'll have to revise that bit. Good catch!

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  2. "If a head of Ichthyovenator turns up, there's the possibility it may have had a different diet, which would be a pretty interesting kink in this kinky-sailed dinosaur's story."

    If the head turns out to be something more akin to a non-spinosaurid Ichthyovenator might as well not be a spinosaurid at all.

    But the large thumb claw of Torvosaurus depicted in many reconstructions is a case of misidentify then? Or this single finger bone doesn't weight to large thumb claw in Torvosaurus?

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  3. True, true.

    The Torvo thing is interesting, since such a claw is certainly not out of the question for the animal. Really, this seems like grasping for straws just to make spinosaurs more global. It's possible that there simply would not have been room for them in the Morrison (though that's just me going off the top of my head). This paper needs to have a response!

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