Saturday, August 14, 2010

Is a theropod-ornithischian group that crazy?

While recoding Holtz's (1994) theropod analysis, I've been using Heterodontosaurus as one of the main outgroup taxa, being a basal ornithischian known at that time.  And I've noticed just how many characters it shares with basal avepods that aren't found in basal sauropodomorphs.  For instance-

- Promaxillary recess and fenestra (Butler et al., 2008).
- Subnarial gap.
- Dorsoventrally expanded anterior dentary with enlarged tooth.
- At least five sacral vertebrae.
- Deep extensor pits on metacarpals.
- Manual phalanx III-3 longer than III-1 or III-2.
- Elongate preacetabular process.
- Fully open acetabulum.
- Well developed antitrochanter.
- Narrow pubic apron.
- Proximally projected anterior trochanter (in other ornithischians; it's fused in Heterodontosaurus)
- Fibular crest on tibia (also in Saturnalia; though seemingly absent in Pisanosaurus).
- Fused tibiotarsus in adult.
- Fused tarsometatarsus in adult.
- Metatarsal V reduced in size and without phalanges.
- Feathers? (in Tianyulong- they're not known from sauropodomorphs yet after all)

Also keep in mind that nearly all analyses have used the more derived Lesothosaurus as a proxy for Ornithischia, at least using its codings instead of Heterodontosaurus' when they differ.  Not that I'm saying Saurischia is wrong, I just think we can get so comfortable with an idea that we stop looking at alternatives.

9 comments:

  1. Deep extensor pits on metacarpals are also in Thecodontosaurus.

    Following similar considerations, I'm planning to remove from my large analysis the "derived ornithischians" OTU, based on Eocursor, Lesothosaurus and Heterodontosaurids, and I will enter these three taxa separately.

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  2. To answer the thread's title: Yes, if you mean to exclude Sauropodomorpha. Sheesh. Get with the lamestream cladistics, Mickey.

    A lot of the basal forms appear to be plastic, and this may have a lot to do with the apparent rampant omnivory in the base of all three Dinosauria groups: Theropoda had "leaf-shaped" teeth and ectaxonal manus design, Sauropodomorpha had ziphodont teeth and recurved, trenchant manual claws with strong anterior adductors of the jaw, and Ornithischia had fanged, sabretoothed, or "ziphodont" forms with equally trenchant, recurved claws and robust grasping limbs and strong anterior jaw adductors. The basal condition seems to be very predatory, trending to herbivory, with a general omnivore gradient among all three clades.

    4 of your features seem to be very much related to predation and acquisition of food, while much of the limb features are indicative of cursoriality and can be interpretive of pursuit adaptations (although whether as prey or predators requires better ecological data to be used).

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  3. The deep extensor pits are technically in Asylosaurus, whose phylogenetic position has never been tested.

    I agree with your points of basal plasticity and features being ecologically-based and thus perhaps loosely correlated, but these cinsiderations hold no matter the phylogeny. If they make Ornithischia+Avepoda a weaker hypothesis, they make Avepoda itself a weaker hypothesis by the same amount.

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  4. >>Not that I'm saying Saurischia is wrong, I just think we can get so comfortable with an idea that we stop looking at alternatives. <<

    This is exactly why I continue to make noise and push the idea of there being no Triassic ornithischians. Pisanosaurus might be an ornithischian and Eocursor might be from Triassic rocks, but because there is still so much ambiguity regarding these cases I don't want people to get too content.

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  5. You can add to your list of theropod-like features of Heterodontosaurus deep ventral recesses on the basisphenoid.

    re. the deep extensor pits of Thecodontosaurus, they are present in all the metacarpals II and III from the Tytherington sample which is A) apparently monospecific, and B) IMO Thecodontosaurus. I doubt the validity of Asylosaurus.

    Hopefully the age of Eocursor will be sorted relatively soon - we now have potentially datable rocks from the lower Elliot.

    cheers

    Adam

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  6. Nifty regarding the deep basisphenoid recess. We really need a detailed description of Heterodontosaurus' skull.

    Well, remove the deep extensor pits from my list then. I admit I haven't worked my way through the Asylosaurus paper, so I don't know the specifics of Galton's reasoning. I'll be interested to read about the Tytherington material once its described.

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  7. Very interesting post. If basal ornithischians are the sister group to avepod theropods only, where would the non-avepod "theropods" likely go in relation to Sauropodomorpha and Avepoda+Ornithischia?

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  8. Tawa lacks some of the above characters (promaxillary fenestra, dorsoventrally expanded anterior dentary, five sacrals, long preacetabular process, fibular crest), has others (subnarial gap, basisphenoid recess, elongate phalanx III-3, fully open acetabulum), and has yet other theropod-like characters Heterodontosaurus lacks (cervical pleurocoels, manual digit V absent). So, good question. It'd require plugging all this into a matrix and doing some constraint trees.

    As for Guaibasaurus, herrerasaurids and Eoraptor, as my last post indicated, these are only ambiguously theropods anyway.

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  9. No, this is all crazy.

    Just kidding...well said Mickey!

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