Ok, here we are: welcome to the fifth and final part of that ‘month in dinosaurs (and pterosaurs)’ series (for previous parts: part I, part II, part III, part IV). This time, more pterosaurs…
In the previous article we looked at the new tiny pterosaur Ningchengopterus (a juvenile Eosipterus?), and at new ideas on nyctosaur crest function. However, another very interesting pterosaur paper appeared in January 2009: Dyke et al.’s (2009) look at morphological disparity in pterosaurs across time. For the record, ‘disparity’ refers to the degree of morphological variation seen within a clade. It is not the same as diversity: this term applies to phylogenetic richness and is not necessarily linked to disparity [adjacent picture, by Mark Witton and from here, shows the weird suspension-feeding ctenochasmatoid Pterodaustro].
Dyke et al.’s (2009) paper looks a several things. By analysing pterosaur distribution over time, they argue that the pterosaur fossil record is not strongly biased by a ‘lagerstätte effect’ as has been argued* and that future pterosaur finds may well come from any Mesozoic rock unit. By looking at discovery rates over time, the authors also report that the pterosaur discovery curve has not yet reached the plateau phase: that at which discovery rates have slowed and are levelling off. This indicates that loads more pterosaurs are yet to be discovered. However, discovery rates that are plotted on the basis of new species descriptions assume that the ‘new species’ really are just that, and the authors also note that the discovery curve data might be affected by the possibility that ‘taxonomists working in this area are dedicated splitters’ (p. 3). Are certain pterosaur workers ‘dedicated splitters’? This depends on who you ask, but do note that there are numerous in-fights among pterosaur workers as to which pterosaur taxa really are valid, and which are not. In fact the discovery curve might differ according to whose taxonomic system is followed [Fig. 1 from Dyke et al. (2009) below. I include it because - in (a) - you can clearly see the impressive cumulative rise of new discoveries made since 1812].
* This is the idea that lagerstätten – places of exceptional preservation – provide a disproportionately high amount of information on diversity, and that they effectively punctuate large periods where relatively little information is available.
It has often been suggested that pterosaur disparity was correlated with the rise of birds. Perhaps, as birds began moving into habitats previously occupied by pterosaurs, pterosaurs were forced to adapt or die. Unwin (2006) proposed that, during the Cretaceous, the niches left vacant as pterosaur species went extinct were not occupied by new pterosaur species, as they were before, but (opportunistically) by birds instead: ‘Ultimately, the effect of this process was to leave pterosaurs adapted to a relatively narrow range of specialist lifestyles’ (Unwin 2006, p. 264). Over time, this left pterosaurs as a whole vulnerable to extinction [adjacent image shows a selection of ctenochasmatoids. Pterodaustro and Cearadactylus, at the top, are Cretaceous and might be regarded as more specialised than the Jurassic forms Pterodactylus and Gnathosaurus, shown below].
Dyke et al. (2009) set out to test this model of waning pterosaur diversity. They report that Cretaceous pterosaurs are more morphologically disparate than older forms, not less so. This suggests, they conclude, that Cretaceous pterosaur evolution was not constrained by birds, and that new morphologies were appearing among pterosaurs during this time. For a full and excellent discussion of this research do see Al McGowan’s article over at Dave Hone’s Archosaur Musings. I swear, this is the last plug that Dave’s blog is getting for the time being [composite below shows anurognathid and nightjar, apparently similar in ecology].
I confess, however, that part of the logic escapes me here. Dyke et al. (2009) are saying that pterosaurs were not morphologically constrained by birds, and that pterosaur disparity peaked during the Early Cretaceous. This is all clear in their Fig. 5 (p. 7). They then use this data to argue that Unwin was wrong in claiming that pterosaurs were replaced by birds. However, the crux of Unwin’s argument is that the majority of pterosaur lineages were being replaced over time by birds, and that Cretaceous pterosaurs were becoming restricted ‘to a relatively narrow range of specialist lifestyles’ (Unwin 2006, p. 264). The assertion that pterosaurs became restricted to a narrow range of lifestyles is, based on the evidence we have, pretty evident given that the only latest Late Cretaceous pterosaurs were oceanic pteranodontians and the terrestrial stalking azhdarchids. Given that – in birds – those taxa at the edges of morphospace occupy specialised ecological niches (Dyke et al. (2009) cite Ricklefs (2005) in support of this)… haven’t Dyke et al. (2009) actually supported Unwin’s proposal that pterosaurs were becoming more specialised and more ‘peripheralised’ during the Cretaceous? In fact Dyke et al. (2009) state that ‘Early Cretaceous new [pterosaur] taxa appear mainly at the periphery of morphospace’ (p. 5). Your thoughts please!
There is actually a lot more to Dyke et al. (2009) that what I’ve discussed here, and I think it’s a very interesting paper that adds a lot to the debate about pterosaur diversity, disparity and evolution. Like all the other articles and discoveries discussed in this little series, it has provoked a lot of debate and argument.
And on that note I must move on. I have a few new papers coming out over the next few weeks that I’ll be discussing here, and – when and where possible – I’ll be adding a lot of other new stuff. Some very exciting papers have just appeared on such things as extant mammal discovery rates, Late Cretaceous theropod diversity (I’m thinking Hesperonychus), and pliosaur skull mechanics. I turned down a news interview for Channel 4′s More4 News today concerning the last two discoveries as I don’t feel up to travelling to London [UPDATE: Les Noè did the interview instead. Well done that Les!]. And, as many of you know, something very neat Mesozoic-wise is due to be announced this Thursday. Circumstances have meant that Tet Zoo has been pretty quiet over the past couple of weeks, but there’s not much I can do about that. I’ve actually made myself ill by working too hard.
Finally, several kind individuals provided various sorts of help that assisted in my production of this series of articles: many thanks to Richard Butler, Andrea Cau, Gareth Dyke, Andy Farke, Dave Hone and David Unwin.
Refs – -
Dyke, G. J. McGowan, A. J., Nudds, R. L. & Smith, D. 2009. The shape of pterosaur evolution: evidence from the fossil record. Journal of Evolutionary Biology doi:10.1111/j.1420-9101.2008.01682.x
Ricklefs, R. E. 2005. Small clades at the periphery of passerine morphological space. American Naturalist 165, 651-659.
Unwin, D. M. 2006. The Pterosaurs From Deep Time. Pi Press, New York.