Tuesday, January 17, 2012

It's amazing how young most theropod specimens are

Theropod analyses contain so many characters that are ontogenetically variable.  Not just the numerous fusion characters (e.g. premaxillae, parietals, dentaries, cervical ribs, pygostyle, scapulocoracoids, pelvis, ischia, tibiotarsi, tarsometatarsi), but also less obvious things.  Skull shape, orbit shape, maxillary fenestra position, postorbital orbital process presence, nasal rugosity, premaxillary tooth serration, pubic boot size, and more.  We know that if you try to code juveniles for these characters, you get the wrong topology.  Juvenile tyrannosaurines end up basally placed in that clade (Carr, 2005; near certainly Brusatte et al., 2009 for Alioramus); juvenile confuciusornithids end up basal to Pygostylia (Gao et al., 2008 for Zhongornis).  And yet, basically no one accounts for this when coding.  You might think that it shouldn't matter that much since comparatively few juvenile theropods are known, but I'm checking basically phylogeny-neutral fusions like braincase, neurocentral sutures and between sacral centra for my upcoming analysis and I'm surprised by just how few adult specimens we have.  Among basal coelurosaurs, for instance-

Tanycolagreus' holotype has visible and open dorsal sutures, free sacral centra and is 63% the size of another specimen.
Coelurus' holotype has visible cervical and some dorsal sutures, open sutures on other dorsals and on caudals, and free sacral centra.
Ornitholestes' holotype has an unfused braincase, visible presacral and some proximal caudal sutures.
Nedcolbertia has open proximal caudal sutures on even the largest specimen.
Nqwebasaurus has open dorsal and caudal sututes.
Santanaraptor has visible caudal sutures.
Mirischia has open and visible dorsal sutures, open sacral sutures and sutured sacral centra.
Compsognathus' largest specimen has open and visible cervical sutures, visible dorsal, sacral and caudal  sutures, and free and sutured sacral centra.  The holotype is 62% its size.
Huaxiagnathus has open caudal sutures.
Sinocalliopteryx has a completely unfused braincase, at least visible cervical sutures and visible dorsal sutures.
Juravenator has open sacral and caudal sutures and free sacral centra.
Scipionyx has everything free of course.
In fact, among sampled basal coelurosaurs (excluding tyrannosauroids and maniraptoriforms), only Aniksosaurus seems to be perhaps an adult based on obliterated sutures on a cervical and a caudal.

Similicaudipteryx's holotype is probably an adult based on obliterated presacral and sacral sutures and has a pygostyle.  Referred specimen STM22-6 is 64% its size (length proxied by femoral length) and also has one, but STM4-1 is 17% its size and doesn't.  Zhongornis is 26% the size of adult Confuciusornis and didn't either, but specimens are known only 64% the size of the largest adults and have pygostyles.  Did Sinosauropteryx have a pygostyle?  Of course not you'd say, the holotype has a series of sixty-four unfused caudal vertebrae (the two larger described specimens don't preserve the distal tail).  The largest specimen lacks fused neurocentral sutures on its proximal caudals, has sutured but unfused sacral centra, and did not fuse at least one sacral rib to its vertebra.  So it's immature and the holotype 64% its size.  Thus the holotype must be quite a bit less than 64% adult size which falls within the range of pygostyle uncertainty in Similicaudipteryx, and so would be better coded unknown.  Of course I consider it unlikely given its phylogenetic position, but codings shouldn't assume that.  Now remember that every ontogeny-related character has its own timing (that probably varied between taxa, but our sample sizes and description quality doesn't even let us go there), and you can see the problem. 

References- Carr, 2005. Phylogeny of Tyrannosauroidea (Dinosauria: Coelurosauria) with special reference to North American forms. Unpublished PhD dissertation. University of Toronto. 1170 pp.

Gao, Chiappe, Meng, O'Conner, Wang, Cheng and Liu, 2008. A new basal lineage of Early Cretaceous birds from China and its implications on the evolution of the avian tail. Palaeontology. 51(4), 775-791. 

Brusatte, Carr, Erickson, Bever and Norell, 2009. A long-snouted, multihorned tyrannosaurid from the Late Cretaceous of Mongolia. Proceedings of the National Academy of Sciences. 106(41), 17261-17266.


  1. One should qualify that skeletal and sexual maturity are not always coincidental. These animals may achieve a fully developed skeletal structure, and still be growing, while fusion occurs after adulthood (or sexual maturity) is actually attained. Of course, the size discrepancy of specimens matters.

  2. You're quite right about sexual maturity being a different matter. The largest Sinosauropteryx individual contains two eggs after all, but has the various skeletal indications of immaturity I noted as well as juvenile bone texture. Since sexual maturity is extremely difficult to determine in fossils (there's the nesting specimens and the specimens with internal eggs like this one and oviraptorid NMNS-VPDINO-2002-0901, but besides that...), it doesn't seem too useful to concentrate on.