Guest Post: Dinosaurs and the latitudinal biodiversity gradient

Today Phil Mannion returns to the Musings with a guest post on his recent paper on dinosaur diversity patterns and their relationships to latitude. Take it away Phil:

The presence of a latitudinal biodiversity gradient (LBG), whereby species richness is highest in the tropics and declines polewards, is a pervasive pattern affecting the majority of life on Earth today, and was recognised by early naturalists such as Charles Darwin and Alexander von Humboldt (whose foundation coincidentally partly supported the research outlined below). Despite its near ubiquity (on both land and in the sea), the causes of the gradient are less well established, with numerous hypotheses proposed over the last fifty or so years. Most of these have been refuted, leaving climate and the distribution of area as the two most likely causes. Understanding the cause and evolution of the gradient is vital to predicting biodiversity loss driven by present-day climate change and explaining geographical variation in biodiversity; as such the fossil record offers a unique perspective on this issue.

Previous work investigating the deep time LBG focused on marine invertebrates – these studies tended to find support for a modern type pattern throughout the Phanerozoic (approximately the last 530 million years). Little prior work has been carried out on terrestrial species, but the few studies to look at the deep time LBG on land found no evidence for a modern pattern until after the Eocene (approximately 30 mya).

Along with colleagues from the UK (Roger Benson, Paul Upchurch, Richard Butler and Paul Barrett) and USA (Matt Carrano), we looked at the LBG in Mesozoic dinosaurs (including birds). Using a number of different methods to mediate for sampling biases in the fossil record, we found no evidence for a modern type pattern at any point in the 160 million year evolutionary history of Mesozoic dinosaurs; instead we found dinosaur diversity to peak at palaeotemperate latitudes (30-60° North and South). The consistency of this result across analyses for different time slices indicates that this pattern is not controlled by climatic fluctuations – evidence suggests that the climatic gradient was “flatter” in the Mesozoic than today (i.e. there was less of a difference in temperature between tropical and temperate regions) – but was instead driven by the amount of available land area in latitudinal belts.

Residual dinosaur diversity after controlling for sampling, plotted against non-marine area (NM area) and palaeogeographical reconstructions for the Late Triassic (bottom), Jurassic (middle) and Cretaceous (top). From Mannion et al., 2012

Given that living birds conform to the modern day pattern, a significant change must have occurred at some point in the last 65 million years. Evidence from molecular phylogeny (and work on fossil insects) suggests that this change occurred at the end of the Eocene (34 mya), following the strengthening of the climatic gradient and an increase in seasonality. As such, there is no evidence for a modern type LBG on land before the last 30 million years.

Mannion, P. D., Benson, R. J. B., Upchurch, P., Butler, R. J., Carrano, M. T. and Barrett, P. M. 2012 (Published online). A temperate palaeodiversity peak in Mesozoic dinosaurs and evidence for Late Cretaceous geographical partitioning. Global Ecology and Biogeography, doi: 10.1111/j.1466-8238.2011.00735.x

Guest post: Dinosaurs dying off?

Today it is the turn of Graeme Lloyd to entertain us with tails of dinosaurs. You might know him from his blog This Life’s A Fiction, or as the lead author on an old paper of mine on dinosaur supertrees. Of course he should be best known for our most important contribution as PhD students where we instigated the arrival of Dr Leonard P. Annectens to the University of Bristol, Geology Department. Here Graeme, talks about his most recent work revisiting issues of dinosaur diversity and their potential decline before the famous KT extinction.

The Morphological Species Concept

As a palaeontologist the taxonomy and systematic work I do (and of course analyses built of these) is based around the morphological species concept. There are lots of species concepts out there (over a dozen as I recall) and these are there to take into account the vagaries of evolution and biology. Most people will be familiar with the idea that a species is a group of organisms that cannot successfully reproduce with another. This reproductive species concept is fine as things go, but it can’t work all the time. What about asexual lizards or bacteria? What about animals that can interbreed but don’t because they are separated by a barrier like a river? And testing this is not always feasible – should we be trying to interbreed all manner of seals just to make sure they are species? And of course, what exactly do you do with fossils?

And so enter the morphological species concept (or hereafter MSP to save me typing it out all the time). With no possible way of looking at interbreeding, genetics, geographical isolation or other issues with fossils we must work with what we have and that is the anatomy of the animal to hand. Thus the MSP uses differences in the anatomy of specimens for taxonomists to judge their membership of species.

This is not as daft as it may first sound. After all, most species can be separated in multiple ways – they have different genes, breed with only their own species, live in different places, behave differently and so on, but they also look different. Based on living animals the average person will be struck by the different colours or patterns of different species or major differences in size, but dig deeper and their skeletal anatomy will be different too. Different antelope species have different horns, different cats have bigger or smaller canines, lizards have very varying tail lengths and so on.

These are of course simple and dramatic examples, but it should be intuitively clear that species can be separated out by their anatomy and that using this as a basic for defining them (and indeed other taxa of higher ranks) is a perfectly acceptable way of doing it. Of course this still requires caution with intraspecific variation and the rest, but the MSC as an idea is perfectly workable and no more imperfect than any other. It does also have the benefit of being relatively easy to cross compare with all manner of species both living and dead, which isn’t true of many others.

Dinosaurian diversity

One thing that Linhenykus does demonstrate well is that fact that even very well studied and excavated areas can still yield new species. Obviously there are dinosaur localities that have had far greater attention than Bayan Mandahu, where this little guy heralds from, and show similar patterns of new discoveries and new taxa. Part of this of course is that old collections get reassessed and long buried specimens turn out to be something new and interesting but merely previously overlooked.

In this case though despite years of collecting at Bayan, by multiple long expeditions, often well manned and with lots of real specialists, we have clearly not got a complete breakdown of the dinosaurs there, and indeed there could be a lot more to find. Obviously we have great representatives of the more common material (not least Protoceratops) and there will likely be diminishing returns, but there is more to find. I’d not be surprised if there were another dozen more genera out there to turn up in the next few years and even more after that. The fact that even century plus old localities are turning up new and surprising taxa shows how far we have to go with dinosaur diversity and that it’s not just about finding new places to dig. The oldies are still goodies.

Guest post: sauropod diversity over time

Phil Mannion and his pet dino.

PhD student Phil Mannion guests this time out on the Musings. Phil works on sauropods and has just had a new paper published looking at their fossil record and diversity over time. Do they really reach a peak of diversity in the Jurassic, or is this just a bias from how complete the specimens are from that time? Read onto find out more: