Home > sauropodomorphs > Leonerasaurus and the path to sauropodhood

Leonerasaurus and the path to sauropodhood

<i>Leonerasaurus</i> a skeletal mount, note that the skull and many othe bones are modelled. Image from wikimedia commons

In the background, between the flurry of new ceratopsian papers and the normal constant rain of theropod research, there has been a marked uptick in the number of basal sauropodomorph (‘prosauropod’ if you want) papers over the last couple of years. Simply in terms of new taxa we’ve had Panphagia, Massospondylus kaalae, Aardonyx, Seitaad, Sarahsaurus, Chromogisaurus, Ignavusaurus, Xixiposaurus, Chuxiongsaurus as well as the admission of an old favourite, Eoraptor, to the ranks. And now we have one more; Leonerasaurus traquetrensis.

Its an interesting little beast, with some quite advanced sauropod-like characteristics packaged in quite a lithe, small body. One stand-out feature is the number of vertebrae joining the pelvis to the spine (sacral vertebrae). Most basal sauropodomorphs had three such vertebrae, whereas the large sauropods have at least four, usually five or or more. Leonerasaurus is like a sauropod with four vertebrae joining the pelvis. The extra vertebra has been recruited from the base of the tail. One might think that this extra bracing was related to weight support for increasing size as these sauropodomorphs edged closer to gigantism and ‘sauropodhood’. But not so, Leonerasaurus is rather small (bone histology confirms that the skeleton was of an adult), and a reconstruction of ancestral body sizes based on the authors’ own cladistic analysis shows that the aquisition of the extra sacral vertebra happened before the big spike in body size on the lineage leading to sauropods. In other words one more feature characteristic of later gigantic sauropods has been shown to have evolved long before they were giants.

Another aspect that is becoming a theme in early sauropodomorph evolution is rampant homoplasy (messy evolution that involves either convergent evolution of characteristics or reversal back to the ancestral state). Pol et al. find that Leonerasaurus is really quite derived actually slotting in between Aardonyx and Melanorosaurus on their cladogram. That makes sense in the light of the increased sacral count and a few other very ‘sauropody’ features, such as the spoon-shaped front teeth, but much less sense when you look at the primitive low neural spines of the dorsals and the slender metatarsal of the first toe of the foot. Hence the massive sauropod-like foot of Aardonyx (that bore the bulk of the weight on the inner digits) was not present in this guy. Did it reverse in Leonerasaurus or is it convergent in Aardonyx and sauropods? Another option is that  Leonerasaurus is simply in the wrong place. Indeed it does share some features with a more basal early sauropodomorph familiar to many: Anchisaurus. These include a long, spine-like process sticking out of the front of the ilium in the pelvis (it is short and triangular in other basal sauropodmorphs) and quite slender sacral ribs. So is Leonerasaurus a sister taxon of Anchisaurus that has picked up some sauropod-like characteristics convergently? That would have been my guess but its an hypothesis that the authors directly tested and no, trees that enforce such a relationship are significantly longer (=significantly worse explanation of the data) than those that place it up as the sister to what I call the quadrupedal clade (Melanorosaurus + Sauropoda). Of course who knows what future observations may bring?

I’m writing this  before the embargo breaks so I cannot provide a link to the paper. However if you want to see it just head on over to PloS ONE and you’ll find it.

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Categories: sauropodomorphs
  1. 220mya
    January 26, 2011 at 5:34 pm
  2. Brad McFeeters
    January 27, 2011 at 3:38 am

    According to the paper, an anchisaurid relationship for Leonerasaurus is “only moderately suboptimal, implying two extra steps when it is depicted as more basal than Aardonyx or Anchisaurus, or as the sister group of either of these taxa.” So your characterization of this hypothesis as “significantly worse” may be a little misleading to some readers, even if it is true in the technical sense of “significantly.”

    The position of the Late Triassic Isanosaurus among eusauropods is also quite interesting in this analysis, since most other evidence points to them radiating in the Middle Jurassic…

  3. January 27, 2011 at 8:55 am

    Hi Adam, great to see you blogging again.

    I’ve not read the paper as yet, but this extreme mix of derived and basal characters suggests to me that this is is what you might expect from a new and radiating clade. The sauropodomorphs were newly vegetarian (or getting that way) and getting bigger at a time when there were few, if any, big terrestrial veggies, and so could (potentially) evolve quite ‘freely’ leading to this big mix of characters. Whadda you reckon?

    • adam yates
      January 28, 2011 at 2:32 am

      Hi Dave
      Maybe, there appear to be some evolutionary shifts that involve lots of homoplasy while others (I’m thinking specifically of basal hardosauroids to saurolophids) that seem to be very clean stepwise accumulations of specialised traits with little homoplasy. I can’t really say I understand why this is.

  4. January 27, 2011 at 3:03 pm

    Very cool animal. Is there any case in dinosaurs where the addition of sacral vertebrae is actually connected to increasing size? In theropods, all the biggest taxa retain the plesiomorphic count of five while the ones with the most sacrals are all small- ceratosaurs (primitively), caenagnathoids, parvicursorines, pygostylians, etc.. In sauropods, titanosaurs add the extra vertebra but are generally smaller than average or so I thought.

    • adam yates
      January 28, 2011 at 2:37 am

      Bingo!, with the removal of sauropodomorphs, which until now really did look like they picked up their extra sacrals as they shifted towards big fat graviportalism, there aren’t any lineages that show a linkage between increasing size and increasing number of sacrals.
      Why does sacral number increase in so many dinosaur lineages? Because their ilia are getting relatively longer I guess. Greg paul relates this to increasing basal metabolic rate and thus higher activity levels (=bigger hind limb musculature). I prefer the idea that maybe there was a pervasive arms race lasting most of the Mesozoic resulting in faster predators and prey. How would one test this more rigorously I wonder?

      • January 28, 2011 at 6:52 am

        In this regard, see the abstract for the poster that Matt Wedel did with Sarah Pine at SVP in 2007:

        INCREASES IN SACRAL VERTEBRAE IN NON-AVIAN DINOSAURS: A PERVASIVE, HOMOPLASTIC, DRIVEN EVOLUTIONARY TREND

        PINE, Sarah, University of California, Berkeley, Berkeley, CA, USA; WEDEL, Mathew, Museum of Paleontology, Berkeley, CA, USA

        Increase in the number of sacral vertebrae is a pervasive evolutionary trend in non-avian dinosaurs. The primitive sacral count for Dinosauria is three. Increases in the number of sacrals occurred across Dinosauria as a clade, and also as repeated parallel events within clades. For example, the number of sacrals increased to five in basal theropods, and this count persisted along the ‘backbone’ of theropod phylogeny from the base of Neotheropoda to the base of Aves (including Archaeopteryx). However, increases to six or more sacrals occurred independently in ceratosaurs, alvarezsaurs, therizinosaurs, oviraptorosaurs, troodontids, and dromaeosaurs. The number of sacral vertebrae also increased independently in sauropodomorphs, thyreophorans, ornithopods, and ceratopsians (and in many subclades thereof). The increase in sacral vertebrae in Dinosauria passes all of the standard tests for a driven evolutionary trend (i.e., moving minimum, subclade, and ancestor-descendant tests). Further, it is homoplastic, with increases occurring independently in every major dinosaurian clade and in many of the subclades. Remarkably, the trend shows few or no reversals in non-avian dinosaurs; sacral count increases in all lineages but decreases are limited to a handful of singleton taxa, and even these reductions are doubtful (i.e., because of ontogenetic or preservational factors). The incorporation of large numbers of vertebrae into the synsacrum in the evolution of birds can now be seen as an instance of this pervasive, homoplastic, apparently irreversible, driven evolutionary trend.

  5. January 28, 2011 at 9:02 am

    Indeed, I’ve cited this abstract myself. The question remains: what’s driving this trend? I like the arms race idea.

    • January 28, 2011 at 9:12 am

      So far as I can see, it’s the only idea on the table.

  6. February 8, 2011 at 9:28 am

    I wonder what’s been done with ontogeny of the synsacrum in birds and in crocs. Does this trend also tie into something developmental?

  1. January 27, 2011 at 7:58 am
  2. February 8, 2011 at 9:23 am

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