Tuesday, 21 January 2014

More SVP Snippets


 
We are often fascinated by the large, some would say outsized, eyes in animals – particularly those in extinct animals such as ichthyosaurs. But do big eyes necessarily indicate a specific adaption to perhaps avoid being hunted by larger predators or, perhaps, do they serve multiple evolutionary driven functions? Lars Schmitz, of Claremont College in California, and his colleagues and have been looking at just that.
By testing a variety of theoretical models they compared the eyes of present day squid (the animal with relatively the biggest eyes of any organism) with those of ichthyosaurs by assuming that both squid and ichthyosaurs evolved their large eyes to avoid predation. They found, however, that large eyes are extremely useful for a number of key visual adaptations in low light level conditions and    that there is insufficient evidence supporting the evolution of large eye size due to predation.
Allometric testing was also indicative that actually both squid and ichthyosaurs do not actually have outsize eyes relative to their large body size within their respective clades.  This adds further weight to the argument that large eyes evolved naturally with large body size and helped the animals perform multiple functions in a light restricted environment although nobody doubts that they would indeed be useful in spotting a possible predator.
There were a few interesting presentations concerning phylogenetics during the meeting and Robert Sansom, of the University of Manchester, and Matthew Wills, of the University of Bath, threw a bundle of tinder wood onto an already burning fire and reinforced the necessity that we must all be alert to the external forces of fossilisation and the significant impact it can have on any phylogenetic tree.
The authors suggest that inherent loss of soft tissue during the fossilisation process causes morphological error. When comparing phylogenies of extant taxa that were filtered with those that are random by removing soft tissue data they found that these taxa actually dropped down lower in their respective phylogenetic trees.
Fossil filters, therefore, can cause significant signal loss which, of course, is the raw data utilised by phylogenetics and thus moves taxa down the tree causing the character states to appear more primitive. If this is the case then the element of caution when considering “parsimonious” character states would need to be greatly intensified.
To compound the issue for those of you involved in phylogenetics, Akinobu Watanabe and Mark Norell, both of the American Museum of Natural history Museum in New York, also highlighted the impact of poor sampling when variation within species occurs. In other words, intraspecific variance within species can cause character states to appear different from what they really are due to the lack of specimens.
A secondary taxon that is part of the same species that is different since it was not sexually dimorphic or ontogentically variant is known as a polymorph. To help combat the issues of polymorphic distortion, the authors have created a new script within TNT that simulates polymorphic sampling but replaces the polymorphic scores with single character states and then cross references the simulated results with the actual data.
The results are interesting albeit somewhat discouraging as the TNT variant recovers substantial differences in the organisation of phylogenetic trees. These discrepancies confirm what many palaeontologists have known for some time and that is that there is nothing that can be described as simple phylogenetics when dealing with extinct organisms and that sampling, not only from  the “more is better” standpoint,  but particularly of multiple specimens of the same species are essential to enable a clearer understanding of phylogenetic evolution.
More ichthyosaurs now which should not be a surprise since there is an abundance of research going on into these fascinating marine reptiles. It is well known that there was a significant extinction event across the Cenomanian/Turonian boundary during the early Late Cretaceous and, despite the odd rumour of ichthyosaur elements known from the Maastrichtian, it is generally accepted that ichthyosaurs went extinct at the end of the Cenomanian.
There are many theories on the cause of this extinction which also saw the demise of the spinosaurs, pliosaurs and, at the other end of the scale, the lepidotid fishes amongst many others. Whatever sparked this event led to depleted oxygen levels in the oceans and saw a rise in both sea and atmospheric temperatures.  And yet was the extinction of the ichthyosaurs a gradual decline or as a result of a sudden catastrophic event? Valentin Fischer, of the Royal Belgian Institute of Natural Science, has been looking at just this.
Re-examining the phylogeny, taxonomy and palaeoecology of Cretaceous ichthyosaurs has revealed some interesting data. Firstly, it has always been generally accepted that ichthyosaur diversity diminished after the Jurassic but we now know that this is not the case and that ichthyosaurs were much more common and diverse in the Early Cretaceous than generally thought.
Secondly this revision has also highlighted the fact that ichthyosaur extinction lasted throughout the Cenomanian and is indicative that marine ecosystems worldwide endured significant faunal turnover that affected many groups of marine animals. This is part of an increased amount of evidence we have that ichthyosaur extinction was part of a much wider reorganisation of marine fauna at the beginning of the Late Cretaceous which, although plesiosaurs survived the extinction event,  paved the way for the mosasaurs to become the dominant marine reptiles for the last 20 million years of the Mesozoic.
Staying with marine reptiles and Neil Kelley, of Vanderbilt University, has been examining the adaptability and morphological variation of various marine forms in the Triassic period. By analysing both dietary data and morphological characteristics of extant marine taxa with those of known Triassic marine reptiles (51 species), Kelly was able to ascertain that the data recorded and observed was generally comparable.
Unsurprisingly, perhaps, marine reptiles of the Triassic pursued a variety of dietary niches although it is interesting that as they got progressively larger in the Mid-Triassic that diets moved on from being mainly piscivorous to not only becoming more carnivorous but also specialising in invertebrate prey. After the Middle Triassic the specialist forms began to disappear whilst both the fish eating forms and the larger carnivores persisted.
The fact that open ocean forms continued to flourish in the Late Triassic may be indicative that there was a shortage of food resources in shallower waters near to shore. Because larger pelagic forms were able to persist into the Late Triassic may have ultimately led to a greater taxonomic diversity at the end of the period with many of the forms that dominated the seas of the Jurassic evolving during these very important stages.
Despite the data being assembled that led to these conclusions being comparable, much of it depends on assumption and the author rightly points out that the overall lack of fossil specimens does somewhat hinder this research and that there will always have to be a degree of conjecture.


 

References

Fischer, V. 2013. The extinction of ichthyosaurs is a facet of a major Cenomanian turnover in marine ecosystems.  Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2013, pp127.
Kelley, N. 2013. Ecomorphological diversity of Triassic marine reptiles.  Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2013, pp152
Sansom, R. and Wills, M. 2013. Fossilization filters result in significant loss of phylogenetic signal and cause organisms to appear erroneously primitive.  Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2013, pp205.
Schmitz, L., Motani, R. and Wainwright, P.C. 2013. Evolutionary drivers of giant eyes in large ocean predators. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2013, pp206 – 207.
Watanabe, A. and Norell, M. 2013. Tree building from Noah’s Ark: the impact of poor sampling within species on phylogenetic reconstruction. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2013, pp235.

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