Something fishy...

[Tim Williams <tijawi@yahoo.com>]

No, nothing to do with what _Spinosaurus_ had for dinner; but some very 
interesting paleoichthyological papers in the latest PNAS.  (Although, for the 
first paper, not necessarily mutually exclusive).


Matt Friedman (2009).  Ecomorphological selectivity among marine teleost fishes 
during the end-Cretaceous extinction.  PNAS 106: 5218-5223. 

Abstract: 
"Despite the attention focused on mass extinction events in the fossil record, 
patterns of extinction in the dominant group of marine vertebrates - fishes - 
remain largely unexplored. Here, I demonstrate ecomorphological selectivity 
among marine teleost fishes during the end-Cretaceous extinction, based on a 
genus-level dataset that accounts for lineages predicted on the basis of 
phylogeny but not yet sampled in the fossil record. Two ecologically relevant 
anatomical features are considered: body size and jaw-closing lever ratio. 
Extinction intensity is higher for taxa with large body sizes and jaws 
consistent with speed (rather than force) transmission; resampling tests 
indicate that victims represent a nonrandom subset of taxa present in the final 
stage of the Cretaceous. Logistic regressions of the raw data reveal that this 
nonrandom distribution stems primarily from the larger body sizes of victims 
relative to survivors. Jaw mechanics are also a
 significant factor for most dataset partitions but are always less important 
than body size. When data are corrected for phylogenetic nonindependence, jaw 
mechanics show a significant correlation with extinction risk, but body size 
does not. Many modern large-bodied, predatory taxa currently suffering from 
overexploitation, such billfishes and tunas, first occur in the Paleocene, when 
they appear to have filled the functional space vacated by some extinction 
victims."

An excerpt from the Results & Discussion:

"The clearest pattern delivered by this study is the complete
extirpation of large-bodied fishes with biomechanically fast jaws
... Elevated extinction intensity among teleosts appearing to occupy higher 
trophic levels is consistent with the collapse of oceanic food webs, 
corroborating earlier hypotheses implicating diet as an important determinant 
of survivorship among fishes... The most prominent teleostean casualties of the 
end-Cretaceous extinction include the predatory pachycormids, 
pachyrhizodontids, ichthyodectiforms, enchodontids, and cimolichthyids, all of 
which are equipped with high-aspect-ratio caudal fins and fusiform bodies that 
imply fast swimming and sustained cruising."


That article is not open access, but the following article is:

http://www.pnas.org/content/106/13/5224.full.pdf+html

Alan Pradel, Max Langer, John G. Maisey, Didier Geffard-Kuriyama, Peter 
Cloetens, Philippe Janvier, and Paul Tafforeau (2009).  Skull and brain of a 
300-million-year-old chimaeroid fish revealed by synchrotron holotomography.  
PNAS 106: 5224-5228. 

Abstract:
"Living cartilaginous fishes, or chondrichthyans, include numerous elasmobranch 
(sharks and rays) species but only few chimaeroid (ratfish) species. The early 
history of chimaeroids, or holocephalans, and the modalities of their 
divergence from elasmobranchs are much debated. During Carboniferous times, 
358â300 million years (Myr) ago, they underwent a remarkable evolutionary 
radiation, with some odd and poorly understood forms, including the enig
erygians that were known until now from poorly informative flattened 
impressions. Here, we report iniopterygian skulls found preserved in 3 
dimensions in â300-Myr-old concretions from Oklahoma and Kansas. The study 
was performed by using conventional X-ray microtomography (ÎCT), as well as 
absorption-based synchrotron microtomography (SR-ÎCT) [Tafforeau P, et al. 
(2006) Applications of X-ray synchrotron microtomography for non-destructive 3D 
studies of paleontological specimens. Appl Phys A
 83:95â202] and a new holotomographic approach [Guigay P, Langer M, Boistel 
R, Cloetens P (2007) Mixed transfer function and transport of intensity 
approach for phase retrieval in the Fresnel region. Opt Lett 32:1617â1619], 
which revealed their peculiar anatomy. Iniopterygians also share unique 
characters with living chimaeroids, suggesting that the key chimaeroid skull 
features were already established 300 Myr ago. Moreover, SR-ÎCT of an 
articulated skull revealed a strikingly brain-shaped structure inside the 
endocranial cavity, which seems to be an exceptional case of soft-tissue 
mineralization of the brain, presumably as a result of microbially induced 
postmortem phosphatization. This was imaged with exceptional accuracy by using 
holotomography, which demonstrates its great potential to image preserved soft 
parts in dense fossils."