Trilobite systematics: The last 75 years

Journal of Paleontology, Nov 2001 by Fortey, Richard

I should mention in passing phenetic methods, which have been used to analyse such families as Trinucleidae and Encrinuridae (e.g., Temple and Tripp, 1979). These treatments did not, in general, result in important new systematic insights. However, these studies were prescient in that they anticipated subsequent work on issues such as morphospace occupation of Cambrian vs. post-- Cambrian trilobites (Foote, 1991).

There are immediate and obvious conflicts between the cladistic treatment of taxa and those that derive from the stratigraphic tradition. Most importantly, the latter permits the definition of stratigraphically early members of groups united only by the possession of symplesiomorphic ("primitive") characters. These tend to comprise paraphyletic taxa as recognised on cladistic treatments, which are unacceptable. Because of the long tradition of stratigraphy-based taxonomy, some high level taxa (e.g., Redlichiina) may be paraphyletic, and include yet other paraphyletic taxa (say, genera) within them. Dismantling these cladistically may cause radical taxonomic changes and will not be easy.

SILICIFIED FAUNAS AND ONTOGENY

The discovery that silicified trilobites could be dissolved out of limestone in dilute acid added more knowledge more quickly than years of patient preparation from crack-out specimens. Whittington (1941) reported in the Journal of Paleontology splendidly preserved Middle Ordovician trilobites, originally passed to him by G. Arthur Cooper, doyen of brachiopod workers. Another pioneer, W. R. Evitt, described the preparation techniques in these pages in 1951. Whittington followed with several papers which are landmarks in the understanding of trilobite morphology. I cannot list them all, but the Harvard Bulletins (e.g., Whittington, 1956, 1959) are among the most seminal. From a stratigraphic point of view the papers on western U.S. Ordovician by Ross (1951) and Hintze (1953) are also noteworthy. These papers allowed us to see more details of ventral morphology, hypostomes, doublure and thoracic articulation than ever before. They introduced many new characters into the assessment of classification. Fine surface details-spines, organelles, apodemes and other muscle attachments-allowed a proper appreciation of the complexity of trilobite morphology. These works are just as useful now as when they were published. The tradition of publishing detailed descriptions of silicified trilobites has continued in this Journal. Particularly fine examples of this work include the ongoing accounts of faunas from the Precordillera of Argentina by Edgecombe, Chatterton and colleagues (e.g., Edgecombe et al., 1998) and those revealing a hitherto unrecognised diversity of Silurian trilobites in the Canadian Arctic (e.g., Adrain, 1994, 1998).

Silicified faunas are probably most important for what they reveal about trilobite ontogeny. As already noted, ontogenetic information had been regarded as relevant to trilobite classification since at least the nineteen twenties. However, "rock" specimens of trilobite larvae were not often well-enough preserved to see fine details. High quality silicified material was much more informative, notably by permitting us to see the underside of the protaspis for the first time, and to recognise the immature hypostome. Again, H. B. Whittington pioneered much of this important work (Whittington, 1956, 1959), which has been followed by many important monographs, especially those of Brian Chatterton (e.g., Chatterton, 1980). Careful plotting of ontogenetic trajectories allowed for the recognition of protaspides of several families for which they had not previously been identified, and for depiction of the changes which happened during the meraspis period. The discrimination of several moults within the protaspis period of development showed that in many species profound alterations occurred even at very small size. Most trilobites showed a first protaspis stage in which the protopygidium was not defined, while it appeared in later stages outlined by a transverse furrow. However, the discrimination of the former as anaprotaspis from the latter as metaprotaspides was later abandoned (see Chatterton and Speyer, 1997) because it was unlikely that they were homologous between different trilobites. A still smaller, convex phase described as a phaselus by Fortey and Morris (1978) is controversial: although now recognised from several silicified faunas it is unclear whether these small larvae belong to trilobites or to some other (?crustacean) arthropod. Most larval hypostomes are spinose, and occupy a relatively large area of the larval venter. These spines have been related to primary segmentation by Fortey and Chatterton (1988) and Speyer and Chatterton (1989). Neither Olenellina nor Agnostina had calcified protaspides, a condition which is sometimes regarded as primitive.