Tuatara tuesday – its not a dinosaur, OK?

From today I’ll be starting a semi-regular series of posts about my favourite reptile and #1 study organism, the tuatara.  I want to start by clearing up a misconception that I see repeated time and time again, that tuatara are “New Zealand’s living dinosaur”.

Tuatara are an entirely different lineage of reptiles from the dinosaurs.  Here’s a simplified phylogenetic tree of reptiles to illustrate:

Phylogeny of reptiles, based on that of Hugall et al. 2007 with additional information from the Tree of life (http://tolweb.org/Dinosauria/14883). Dinosaurs, including the lineage that evolved into birds, are in blue.

The closest living relatives of the dinosaurs are the crocodilians (alligators and crocodiles), and birds.  In fact, it is birds that are the “living dinosaurs”, as they evolved from the Theropod dinosaurs, the lineage that includes tyrannosaurs, Velociraptors and Archaeopteryx.  Crocodilians, dinosaurs and birds are collectively known as Archosaurs.

Tuatara are in their own Order, Rhynchocephalia, which is entirely separate from the Archosaurs.  The closest relatives of the Rhynchocephalids are the squamates (lizards and snakes), but they are not particularly close relatives at all, having diverged early in reptilian evolution, around 250 million years ago.  The tuatara is the only remaining species of Rhynchocephalid living today, but back in the time of the dinosaurs (around 65-230 million years ago), Rhynchocephalids were everywhere.  Numerous different species of fossil Rynchocephalid have been found across Europe, Africa and the Americas.   However, Rhynchocephalids appear to have died out everywhere except New Zealand around the same time the dinosaurs went extinct 65 million years ago.  Why they hung on in New Zealand is a mystery, but may have something to do with the apparent lack of competition from mammals.

Modern-day tuatara share a lot of morphological features with some of the earliest Rhynchocephalid fossils.  That, combined with the fact that they are the only living species from this lineage, has earned them the title of “living fossil”.  However, it is premature to say that tuatara are “unevolved” or “unchanged since the dinosaur era”.  For one thing, we don’t actually have any ancestral tuatara fossils  from New Zealand dating back to the dinosaur era for comparison (the earliest tuatara fossil dates to a measly 16 mya), and recent studies have shown that many aspects of their morphology and biology are derived, making them just as “evolved” as any other species (but that’s the subject for another post).

So, although tuatara are the last member of an ancient reptile lineage, and still retain some morphological characteristics of these early reptiles, they are a completely different type of reptile from dinosaurs.  Their ancestors lived alongside the dinosaurs, but so did the ancestors of modern-day turtles, crocodiles, lizards, and snakes.

References:

Hugall AF, Foster R, Lee MSY (2007) Calibration choice, rate smoothing, and the pattern of tetrapod diversification according to the long nuclear gene RAG-1. Systematic Biology 54: 543-563
Jones MEH, Tennyson AJD, Worthy JP, Evans SE, Worthy TH (2009) A sphenodontine (Rhynchocephalia) from the Miocene of New Zealand and palaeobiogeography of the tuatara (Sphenodon). Proc. R. Soc. London B 276: 1385-1390.
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3 Responses to Tuatara tuesday – its not a dinosaur, OK?

  1. Trevor Worthy says:

    Hillary – a good wee blog – long overdue. Chevron bones attach to the ventral side of caudal vertebra….not uncinate processes

  2. […] related to birds or reptiles. This tree illustrates the relationships. (borrowed from this blog: http://chickenoreggblog.wordpres…)Over the last two decades this idea has become very widely accepted among paleontologists thanks to […]

    • hilaryml says:

      I don’t think there is any question that birds are the closest living relatives of dinosaurs. At what point “warm-bloodedness” evolved on this lineage is still up for debate – I think there is some evidence that some dinosaurs were already warm-blooded.

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