Unravelling the genealogy of dinosaurs

Up to this point, our focus has been largely, if not exclusively, tuned to exploring aspects of the anatomy, biology, and way of life of the dinosaur Iguanodon. It must be obvious that Iguanodon was just one dinosaur that fitted into far larger tableaux of life in the Mesozoic Era. One of the important tasks that falls to palaeontologists is to try to discover the genealogy, or evolutionary history, of the species that they study. To put dinosaurs as a whole into some sort of perspective, it will be necessary to outline the techniques used to do this, and our current understanding of dinosaurian evolutionary history.

One feature of the fossil record is that it offers the tantalizing possibility of tracing the genealogy of organisms not just over a few human generations (which is the ambit of modern genealogists) but over thousands, or millions, of generations, across the immensity of geological time. The primary means by which such research is carried out at present is the technique known as phylogenetic systematics. The premise of this technique is really quite simple. It accepts that organisms are subject to the general processes of Darwinian evolution. This does not require anything more profound than the assumption that organisms that are more closely related, in a genealogical sense, tend to physically resemble each other more closely than they do more distantly related creatures. To try to investigate the degree of relatedness of creatures

(in this particular case fossil creatures), palaeosystematists are most interested in identifying as wide a range of anatomical features as are preserved in the hard parts of their fossils. Unfortunately, a great deal of really important biological information has simply rotted and been lost during the process of fossilization of any skeleton, so, being pragmatic about things, we simply have to make the most of what is left. Until quite recently, the reconstruction of phylogenies had relied on hard-part anatomical features of animals alone; however, technological innovations have now made it possible to compile data, based on the biochemical and molecular structure of living organisms, that can add significant and new information to the process.

What the dinosaur systematist has to do is compile lengthy lists of anatomical characteristics, with the intention of identifying those that are phylogenetically important, or contain an evolutionary M signal. The task is intended to produce a workable hierarchy of | relationship, based on groupings of ever more closely related £ animals.

The analysis also identifies features that are unique to a particular fossil species; these are important because they establish the special characteristics that, for example, distinguish Iguanodon from all other dinosaurs. This probably sounds blindingly obvious but, in truth, fossil creatures are often based on a small number of bones or teeth. If other partial remains are discovered in rocks elsewhere from the original, but of very similar age, it can be quite a challenge to prove convincingly whether the new remains belong to, say, Iguanodon, or perhaps a new and previously undiscovered creature.

Beyond the features that identify Iguanodon as unique, there is also a need to identify anatomical features that it shares with other equally distinct, but quite closely related animals. You might say that these were the equivalent of its anatomical 'family'. The more general the characters that 'family' groups of dinosaurs share, the

The case of Baryonyx

The Early Cretaceous rocks of south-east England have been intensely investigated by fossil hunters (starting with Gideon Mantell) and geologists (notably William Smith) for well over 200 years. Iguanodon bones are very common, as are the remains of a limited range of other dinosaurs, such as Megalosaurus', Hylaeosaurus, Polacanthus, Pelorosaurus, Valdosaurus, and Hypsilophodon. Given the intensity of such work, it would be thought highly unlikely that anything new would ever be discovered. However, in 1983 the amateur collector William Walker discovered a large claw bone in a clay pit in Surrey that led to the excavation of an 8-metre-long predatory dinosaur that was entirely new to science. It was named Baryonyx walkeri in honour of its discoverer, and has pride of place on exhibition at the Natural History Museum in London.

The moral of this story is that nothing should be taken for granted; the fossil record is likely to be full of surprises.

more this allows them to be grouped into ever larger and more inclusive categories of dinosaurs that gradually piece together an overall pattern of relationships for them all.

The real question is: how is this overall pattern of relationships achieved? For a very long time, the general method that was used might be described simply as 'I know best'. It was quite literally the view of self-styled experts, who had spent much time studying a particular group of organisms and then summarized the overall patterns of similarity for their group; their methods for doing this might vary considerably, but in the end their preferred pattern of relationship was little more than just that: their preference, rather than a rigorous, scientifically debated solution. While this method worked reasonably well for restricted groups of organisms, it proved far more difficult to properly debate the validity of one interpretation compared with another because the arguments, when boiled down to their essentials, were circular, relying on one person's belief over another's.

This underlying problem was brought into sharp focus when groups of organisms were very large in number and varied in many subtle ways. Good examples are groups of insects, or some of the bewildering varieties of bony fish. If the general scientific community was happy to accept the authority of one scientist for a period of time then all was apparently fine. However, if experts could not agree, the end result was frustratingly circular debates.

M Over the past four decades, a new methodology has gradually been | adopted that has proved far more valuable scientifically. It does not £ necessarily give the correct answers, but it is at least open to scientific scrutiny and real debate. This technique is now widely known as cladistics (phylogenetic systematics). The name is treated with a fair degree of trepidation by some, but this is largely because there have been some very fierce arguments about how cladistics is done in practice and what the overall significance of the results might be in an evolutionary context. Fortunately, we do not need to consider much of this debate because the principles are actually surprisingly simple and clear-cut.

A cladogram is a branching tree diagram that links together all the species that are being investigated at the time. To create one, the researcher needs to compile a table (data matrix) containing a column listing the species under consideration and against this a compilation of the features (anatomical, biochemical, etc.) that each species exhibits. Each species is then 'scored' in relation to whether it does (1) or does not (0) possess each character, or in some instances if the decision is uncertain this can be signified as a (?). The resulting matrix of data (these can be very large) is then analysed using a number of proprietary computer programs, whose role is to assess the distribution of 1s and 0s and generate a set of statistics that determines the most parsimonious distribution of the data that are shared by the various species. The resulting cladogram forms the starting point for a considerable amount of further investigation that is aimed at determining and understanding the degree to which there are common patterns or overall similarities, and the extent to which the data might be misleading or erroneous.

The cladogram that results from this type of analysis represents no more than a working hypothesis of the relationships of the r animals that are being investigated. Each of the branches on |

the tree mark points at which it is possible to define a group J'

of species that are all connected by their sharing a number of e

characteristic features. And using this information it is possible £ to construct what is, in effect, a sort of genealogy or phylogeny °

y representing a model of the evolutionary history of the group f as a whole. For example, if the known geological times of £

occurrence of each of the species are plotted on to this pattern, | it becomes possible to indicate the overall history of the group, s and also the probable time at which various of the species may have originated. In this way, the cladogram, rather than simply representing a convenient spatial arrangement of species, begins to resemble a real genealogy. Obviously, each such phylogeny created in this way is only as good as the data available, and the data and how it is scored can change with the discovery of new, better, or more complete fossils, and also as new methods of analysis are developed or older ones are improved upon.

The aim of all this work is to help create as accurate a picture as possible of the evolutionary history of life or, in this particular case, the evolutionary history of dinosaurs.

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