1 primitive (plesiomorphies);

2 shared and derived (synapomorphies);

3 new (novelties); and

4 convergent.

Of these, synapomorphies, which are characters shared between two or more groups of organisms and derived from earlier features, are the most important for defining clades (Chapter 1). In biological evolution, a character must be genetically inheritable and not acquired during the lifetime of an animal. Characters thus relate to an organism's genotype, or how its genes were expressed (Chapter 6). In contrast, a population of mice that originally had sight but lost it, then had their tails cut off, do not constitute a clade because their blindness and tail losses are acquired traits that are not inheritable. The outward physical expression of an organism, caused by a combination of environmentally caused traits and the genotype, is called its phenotype, which can vary considerably from the potential of the genotype (Chapter 6). In other words, these mice becoming tailless and blind were traits that were acquired, not inherited.

If a group of blind mice from a population of ancestral mice evolved a genetically inheritable lack of tails, sight, or both, then these novel traits would show up in subsequent generations (descendants). The group without tails or the blind group therefore comprise a new clade, as will any successive group that shows these synapomorphies, indicating relatedness (Fig. 5.12). Consequently, cladistics is a method used to hypothesize the phylogeny (evolutionary history) of a group of organisms, which is why it is also called a phylogenetic classification (Chapter 1). Plesiomorphies (primitive features) can also help with discerning ancestry, in that descendants may have retained a trait from far back in their evolutionary

FIGURE 5.12 Hypothetical evolution of mice and how their synapomorphies (novelties) would contribute to their cladistic classification.

lineage: examples include the formation of teeth in embryonic chickens and pha-ryngeal gill slits in human embryos. A clade, because it shows the evolutionary origin for all descendants from a common ancestor, is thus monophyletic. In contrast, taxonomic groups that have multiple evolutionary origins, such as more than one clade, are polyphyletic (Chapter 3).

The result of such analyses and the consensus reached by most dinosaur paleontologists is summarized by the following clades, introduced in Chapter 1:

Chordata Tetrapoda Amniota Reptilia Diapsida

Archosauriformes Archosauria Ornithodira Dinosauria Saurischia Ornithischia

However, the mere listing of clades, even with progressive indentations given to the list, does not explain adequately the relations between them. Their interrelationships are best illustrated through a cladogram, which shows how clades branch from one another at points called nodes, where a common ancestor of all subsequent clades first developed a new synapomorphy. The influence of the Linnaean classification is retained through some taxa (i.e., Diapsida, Archosauria) that were originally based on some shared characters recognized long before the invention of cladistics. Yet another aspect of Linnaean classification that still remains is the binomial nomenclature of fossil species. As a result, the embrace of cladistics by biologists and vertebrate paleontologists has not erased colorful species names such as Triceratops horridus (Chapter 13).

Vertebrate paleontologists, who employ cladistics, attempt to be scientifically rigorous in their approach by examining evidence for the inheritability of any character observed in fossil specimens. Each cladogram is essentially a hypothesis for a phylogeny that is tested through peer review. Typically, the least complicated hypothesis (the one requiring the fewest steps for establishing the relatedness) is regarded as the most likely, and such less complicated cladograms are said to have parsimony. New evidence, such as a fossil find from the field or a museum with previously undescribed or unrecognized characters, requires re-examination of previous hypotheses about the evolutionary relationships of certain clades. Hence, use of this methodology can falsify the justification for a new clade, or argue more firmly for a previously-defined clade (Chapter 15). This situation means that cladistics, in its earliest stages, can be quite volatile as new information is added, which results in hindered communication.

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