Wednesday, 4 August 2010

Triceratops And Tribulations
Do a Google News search for Triceratops and you'll be faced with some worrying headlines. Screeching at you from the screen are "Triceratops never actually existed, scientists say", "Scientists: Triceratops May Not Have Existed‎", "Triceratops' status as a distinct species threatened" (note that I've italicised the genus name, because FSM knows that subtlety is lost on the majority of journalists). I'm wondering if the Daily Fail will be decrying this as Political Correctness Gone Mad, shrilly complaining about the further destruction of our childhoods...

Of course, as numerous bloggers have already commented, this is all bollocks. Nothing is happening to Triceratops.

The paper that has kicked it all off is this: "Torosaurus Marsh, 1891, is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae): synonymy through ontogeny", by John Scannella and John Horner (find it here). Before you even look at the paper itself, there are a couple of clues in the title, and you don't need a PhD in palaeontology to pick up on these clues.

The first is that the authors say "Torosaurus is Triceratops". While that could be open to some linquistic interpretation (perhaps more so for non-native English speakers), it is pretty clear (I checked with the Token Non-Scientific Adult, aka Paul) that this means Triceratops is the one that gets to stay.

The second is that Tricertatops was named first. In most fields, it's the first named/discovered/invented item that gets to take priority, and genus and species names are no exception. Naming animals is governed by the International Code of Zoological Nomenclature (ICZN), and if you're feeling a bit nerdy you can look up Article 23.1:
Statement of the Principle of Priority. The valid name of a taxon is the oldest available name applied to it, unless that name has been invalidated or another name is given precedence by any provision of the Code or by any ruling of the Commission.
So the taxon made up of all Triceratops specimens and all Torosaurus specimens is validly known as Triceratops, the oldest available name applied to it. It is this rule that means Brontosaurus is actually Apatosaurus, although in marginally different circumstances.

So what did Scannella and Horner actually do? They measured their specimens using ontogenetic markers - the development of the parietal fenestrae and the elongation of the squamosals. They also looked at osteohistological features, and found a sequence from Triceratops specimens with primary, spongy bone structure to Torosaurus specimens with secondary, compact bone structure. They conclude that this is a growth sequence, and looking at the photos in the paper I see no reason to doubt this.

As for squamosal elongation, they have a fascinating graph of squamosal length/width against squamosal length (and I hope they don't mind me showing this):

Significant overlap between Triceratops specimens and Torosaurus specimens (I presume the categories of "baby", "juvenile", "subadult" and "young adult" are obtained from osteohistology samples - must read this more fully...). I'd have loved to see some of these analysed using geometric morphometrics - in such analyses elongation is most commonly the largest component of variation, but if you start looking at the smaller components you can get some really interesting results.

This paper highlights a problem that Peter Dodson discussed 35 years ago - that there is a real problem with juveniles and adults being incorrectly classified as different species or genera. Cheneosaurus and Procheneosaurus are juvenile lambeosaurines (which is all reminding me to write up my geometric morphometric Ornithopoda paper...), for example. While, with the benefit of more specimens, better understanding of osteology, microscopic techniques and improved communication within the field, palaeontologists may not be splitting juveniles and adults now, there are undoubtedly question marks raised over some of the rapidly-described skeletons from the Cope-Marsh "Bone Wars" era, of which Brontosaurus-Apatosaurus and Torosaurus-Triceratops are two examples.

I imagine my students will have picked up on this over the summer holidays, so we'll be able to use it as a springboard to talking about classification, taxonomy and the all-important "How Science Works" aspect of the curriculum. Time for me to start practising drawing Triceratops and Torosaurus skulls freehand on an interactive whiteboard!

Scannella, J., & Horner, J. (2010). Torosaurus Marsh, 1891, is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae): synonymy through ontogeny Journal of Vertebrate Paleontology, 30 (4), 1157-1168 DOI: 10.1080/02724634.2010.483632
Dodson, P. (1975). Taxonomic Implications of Relative Growth in Lambeosaurine Hadrosaurs Systematic Zoology, 24 (1) DOI: 10.2307/2412696

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