Beginning Dinosaur Cladistics

■ Three or more sacral vertebrae.

■ Acetabulum with open medial wall.

■ Femur with ball-like head on proximal end.

■ Tibia with cnemial crest.

■ Astragalus with long ascending process that fits into the anterior part of the tibia.

■ Sigmoidally shaped third metatarsal.

Saurischians apparently are the earliest known clade of dinosaurs, and they are certainly the most abundant dinosaurs in both the body and trace fossil record of the Late Triassic (Chapter 6). The first ornithischians occur in slightly younger strata than the first saurischians, although both groups were derived from a still-unknown archosaurian ancestor that was the node from which they diverged. These two clades had other monophyletic groups branch from them that form the presently understood ancestor-descendant relationships of dinosaurs (Fig. 5.13). Familiarization with representative species from each clade (Tables 1.1 and 5.1) will help with visualizing some of the dinosaurs associated with these clades. More detailed knowledge of anatomical differences, covered in remaining chapters, is necessary to understand the scientific basis for paleontologists showing where these clades branch into more derived clades.

Of course, human factors complicate even the most scientific of classification schemes. In some cases, the best-defined clades are biased, favoring the views of the particular dinosaur paleontologists who are most active in researching them, as well as the relative abundance of skeletal material available (or not available) for character analysis in certain clades. For example, theropods are described cladisti-cally in more detail than any other dinosaur group (Chapter 9). This circumstance is at least partially attributable to the recurring fascination most dinosaur paleontologists have with theropods, but it is also related to the abundance of theropod material for study in nations where scientific methods have a long tradition (Chapter 3). Furthermore, the logistical problems associated with unrecoverable dinosaur remains add to the bias of their cladistic classification. For example, sauropods, most of which were disproportionately larger than the other dinosaurs, may not be defined as well in a clade because recovery of their extremely large body parts from remote field areas is difficult. The extended amount of time, money, and labor required for preparing those parts in a laboratory prevent proper study of their characters (Chapters 3 and 10). Yet another human factor to consider is experience. Because cladistics has been applied to dinosaurs for only about 20 years, not all dinosaur paleontologists have gained the expertise necessary for identifying all of the characters.

Marginocephalia

Marginocephalia

Main Cladogram For Dinosaurs

Chordata

FIGURE 5.13 Currently accepted cladogram for dinosaurs, beginning with Chordata and ending with the Saurischia and Ornithischia and main monophyletic groups within these clades.

Chordata

FIGURE 5.13 Currently accepted cladogram for dinosaurs, beginning with Chordata and ending with the Saurischia and Ornithischia and main monophyletic groups within these clades.

Because cladistic classifications are based on considerable amounts of descriptive data, revisions of these classifications are likely to be made with each new fossil find that either supports, refutes, or modifies an inferred node for two or more clades. Another method, independent of the fossil record, that has been used to test cla-distic classifications is molecular phylogeny, which looks at the relatedness of certain proteins or nucleic acids (which are characters, too) of extant organisms, to extrapolate evolutionary rates of some lineages (Chapter 6). In some rare instances, fossil material yields biomolecules with sufficient nucleic acids available for comparison with living counterparts. Finds of proteins (osteocalcin), amino acids related to hemoglobin, and still-elastic tissues in dinosaur bones are encouraging in this respect, although the likelihood of nucleic acid preservation in dinosaur bones is very low (Chapter 4). This constant revision of dinosaur phylogenies results from the application of scientific methods, is evidence-based, and dissuades scientists into making unsound interpretations. For these reasons, cladistics is currently the preferred method for classification and will likely continue its prominence in dinosaur studies well into the future.

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