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Re: Dinosaur paleodiversity peaked in temperate zones
Couple of comments...
But first, it's soap-box time... The entire myth of an equitable
Mesozoic world is a terrible fallacy... I swear it's a dogmatic
hold-over from Victorian notions of "terrible lizards". In fact, the
silly part? Look at today's tropics... even they are far from
OK... So, the article draws some plausible conclusions. As the
article discusses, a biodiversity theory often cited to explain
diversity differences between the tropics and temperate zones is
seasonality... Sure, the tropics have seasons (normally just wet vs
dry), but in temperate zones, it's a tad bit more extreme, with
aspects like prolonged freezing temps and other conditions that can
bring terrestrial ecology to a halt... It's a "discrete" ecology. The
later is important because that doesn't happen in the tropics, which
has a "continuum" of ecological processes (all of this falls into
something called "latitudinal gradients in biodiversity".. although
it's not the latitude that does it, but the environmental variables
correlated with those latitudes).
Now, that's the situation today. In the Mesozoic, seasonality extremes
in the temperate zones were not as extreme, so both tropic and
temperate zones probably had similar "continuous" dynamics in ecology.
There are also studies indicating reduced temperature variations and
latitudinal differences in elevational climatic zonation may increase
opportunities for geographical isolation, speciation and the
associated build-up of species diversity in the tropics relative to
temperate zones. Again, as seasonality wasn't as extreme in Mesozoic
temperate zones, coupled with indications that some type of species
partitioning/zonation was going on, especially in NA... having
biodiversity in temperate zones that was close to or higher than in
tropical zones makes sense... So, add this to the article's conclusion
about the importance of land area... Good food for thought, I think.
But... I believe a word of caution is needed. We don't have a
super-firm understanding of why biodiversity is as it is today. Even
the fundamentals are still being debated. In addition, when studying
ecological patterns, specifically, latitudinal trends in species
diversity, resolving/taking into account spatial scale is a
fundamental factor. In other words, what's regional and what's local
diversity? The number of species within a biome, or continent, or
climatic zone is regional diversity that's determined by regional
factors such as geology, climate, migration, and extinction. Local
diversity concerns the number of species within a patch of forest, a
river, a valley, etc. I think this has a huge implication for
determining fossil species diversities because of the obvious
preservational bias factors this invokes. So, I think one must be very
careful about resolving fossil diversity for these obvious reasons.
A couple of interesting "why biodiversity is different between
temperate and tropical zones"...
On Thu, Dec 22, 2011 at 12:58 PM, Ben Creisler <email@example.com> wrote:
> From: Ben Creisler
> A new online paper:
> Mannion, P. D., Benson, R. B. J., Upchurch, P., Butler, R. J., Carrano, M. T.
> and Barrett, P. M. (2011)
> A temperate palaeodiversity peak in Mesozoic dinosaurs and evidence for Late
> Cretaceous geographical partitioning.
> Global Ecology and Biogeography (advance online publication)
> doi: 10.1111/j.1466-8238.2011.00735.x
> Aim Modern biodiversity peaks in the tropics and declines poleward, a
> pattern that is potentially driven by climate. Although this latitudinal
> biodiversity gradient (LBG) also characterizes the marine invertebrate fossil
> record, distributions of ancient terrestrial faunas are poorly understood.
> This study utilizes data on the dinosaur fossil record to examine spatial
> patterns in terrestrial biodiversity throughout the Mesozoic.
> Location We compiled data on fossil occurrences across the globe.
> Methods We compiled a comprehensive dataset of Mesozoic dinosaur genera
> (738), including birds. Following the utilization of sampling standardization
> techniques to mediate for the uneven sampling of the fossil record, we
> constructed latitudinal patterns of biodiversity from this dataset.
> Results The dominant group of Mesozoic terrestrial vertebrates did not
> conform to the modern LBG. Instead, dinosaur diversity was highest at
> temperate palaeolatitudes throughout the 160 million year span of dinosaurian
> evolutionary history. Latitudinal diversity correlates strongly with the
> distribution of land area. Late Cretaceous sauropods and ornithischians
> exhibit disparate LBGs.
> Main conclusions The continuity of the palaeotemperate peak in dinosaur
> diversity indicates a diminished role for climate on the Mesozoic LBG;
> instead, dinosaur diversity may have been driven by the amount of land area
> among latitudinal belts. There is no evidence that the tropics acted as a
> cradle for dinosaur diversity. Geographical partitioning among major clades
> of herbivorous dinosaurs
> e Cretaceous may result from the advanced stages of continental fragmentation
> and/or differing responses to increasing latitudinal climatic zonation. Our
> results suggest that the modern-day LBG on land was only established 30
> million years ago, following a significant post-Eocene recalibration,
> potentially related to increased seasonality.