Ordovician shales and mudstones outcropping in the Builth-Llandrindod area of central Wales yield some of the best-preserved trilobites in the UK.[1] The sediments lack a well-developed metamorphic cleavage and tend to break along the original bedding; this is a factor in the good preservation. While investigating the validity of a graptolite biozone, Peter Sheldon collected 14,888 trilobites from seven sections in the Teretiusculus Shales.[2] The sections were divided into about 400 sampling localities, with a mean stratigraphic thickness of 23 cm. He was surprised by his frequent failure to be able to identify trilobites to the species level.
An important character used to discriminate these trilobites is the number of pygidial ribs. Rib counts were obtained for at least one side of 3,458 specimens. The existing species diagnoses for Ogygiocarella and Cnemidopyge proved unworkable, especially in two new stratigraphically-intermediate sections. Attempts to use the median glabellar ridge as a ‘presence or absence’ character in diagnosis of Cnemidopyge species failed repeatedly. Despite assertions that C. bisecta arose from C. nuda in the Gracilis Shales, specimens fitting the diagnosis of C. bisecta were found alongside ‘C. nuda’ in the Teretiusculus Shales. Many Cnemidopyge had faint or very faint ridges.
Similar problems were encountered when applying existing species diagnoses to specimens of Platycalymene, Nobiliasaphus, and the trinucleids Bergamia and Whittardolithus, and no new criteria emerged to permit specific separation. Nileids underwent sufficient change for the ends of the lineage to have been placed in different genera, Barrandia and Homalopteon. Every character difference used to discriminate these genera and their species proved, in practice, far from clear-cut.
Sheldon was able to show that every lineage underwent a net increase in the number of pygidial ribs. Significant changes, including reversals, occur at different times in different lineages. The fact that so many trilobite lineages showed the same net trend strengthened the hypothesis that selection was involved. If change in rib number was only an ecophenotypic phenomenon one would expect far closer parallelism between lineages, as is clearly the case with changes in size. The functional significance of pygidial ribs is unknown. Perhaps each rib indicates the presence of an underlying appendage carrying a well-developed ‘gill branch’. Alternatively, or additionally, extra ribs may have conferred greater strength; several lineages show thinning of already thin exoskeletons. It is possible that rib development was linked to another character which was being selected, whilst variations in rib number were adaptively neutral or even slightly disadvantageous.
The belief that many fossil species remained in morphological stasis throughout their existence prompted Eldredge and Gould [3] to invoke a pattern of punctuated equilibrium as an alternative to the pervasive paradigm of phyletic gradualism. Their proposal was based partly on observations of Middle Devonian trilobites in North America. Eldredge [4] argued that these populations, once established, were relatively stable and underwent little evolutionary change, only to be suddenly replaced by a new species without intermediates.
However, Sheldon appears to have documented parallel gradualistic change in Ordovician trilobites. He suggests that the apparent success of Linnean nomenclature (with its implications of discrete species) could easily have been misinterpreted as evidence of punctuation and stasis. Perception of many other gradualistic patterns equally may have been hindered by conventional descriptive procedures, particularly the requirement to apply binomial taxonomy to fossils and the practice of lumping together specimens collected from different horizons in order to amass enough material for full ‘species’ descriptions. In the case of Sheldon’s trilobites, to unite all the components of each lineage into single species would submerge the observed trends under one taxon. But to subdivide a lineage into two or more arbitrary species or subspecies would give a false impression of punctuation and stasis.
Sheldon does not necessarily reject punctuated equilibrium. Rather, he suggests that studies of the fossil record reveal a wide spectrum of microevolutionary patterns.[5] Punctuated equilibrium and phyletic gradualism should, he argues, be viewed as just two theoretical versions of many possible evolutionary patterns and the temptation to force poorly documented cases to fit one or other of these models must be resisted. Often there is simply too little data to assess patterns of change. We are not yet in a position to assess accurately the relative frequency of particular patterns and the domain of their expected settings. Individual taxa may have exhibited different patterns at different times, and different morphological characters in the same species may evolve at different rates.
Peter Sheldon’s rigorous and systematic collecting confirms that the fossils occur in genuine successions.[6] His high resolution collecting of these Ordovician trilobites revealed gradual change in characters that had been used to diagnose species. Consequently, there was an inability to successfully identify fossils to the species level. This pattern of morphological change can be convincingly interpreted as the result of evolutionary change – at the species or genus level. This is consistent with the hypothesis that the biblical kind is usually broader than these modern taxonomic categories. We can interpret this evolutionary change as diversification in post-flood lineages of trilobites.
The shale sequences are thick and monotonous and in the conventional scheme this is interpreted as stability of environments over long periods of time. However, the frequency of ‘fluctuating assymetry’ in pygidial rib counts in Cnemidopyge is difficult to reconcile with this view. In this species, the right and left sides of the pygidium often have different numbers of ribs. This phenomenon is an abnormality of development and is associated today with environmental stress in hostile environments. Perhaps, contrary to the conventional view, these trilobites were in an environment of rapidly accumulating sediments? Intuitively, the conventional time scale of 3 Ma seems excessively long for the documented changes in pygidial rib counts.
Other field evidences suggest rapidity, especially in the build-up of the volcanic sequences. The succession exposed in Llanelwedd quarry is interpreted as the build up and subsidence of a volcanic island. The basalt lavas, though undoubtedly erupted underwater, are sheet flows and only rarely pillowed. This indicates rapid extrusion. The flows are separated by shale bands that are thin (only a few centimetres) and unfossiliferous. This places a constraint on the time that may be inserted between each lava flow. Interbedded with the basalts is a 10-metre-thick pyroclastic breccia that probably formed from a single violent event.
1 The Builth-Llandrindod area is dominated by an Ordovician inlier composed of sedimentary and igneous rocks spanning the Didymograptus bifidus Biozone to the Nemagraptus gracilis Biozone. By conventional dating, the sequence represents about 10-15 Ma. The stratigraphy of the inlier is as follows. The Didymograptus bifidus Beds are a 250 m sequence of graptolite-bearing shales. Above these beds is a sequence of volcanic rocks, with intercalated shales yielding Didymograptus murchisoni and other fossils. The overlying Glyptograptus teretiusculus Beds are estimated as up to 600 m thick and composed of shales with occasional thin tuff or limestone bands. In addition to the zone fossil and other graptolites, trilobites are common and include Ogygiocarella, Cnemidopyge, and Whittardolithus. The sediments grade up into the overlying Nemagraptus gracilis Beds, again dominantly dark grey shales and mudstones. Trinucleus, Ogygiocarella, and Platycalymene are the most common trilobites in a rich assemblage dominated by trilobites, graptolites and inarticulate brachiopods. (return to text)
2 Sheldon, P.R. Parallel gradualistic evolution of Ordovician trilobites. Nature 1987;330:561-563. (return to text)
3 Eldredge, N. and Gould, S.J. Punctuated equilibria: an alternative to phyletic gradualism, pp.82-115 in: Schopf, T.J.M., editor. Models in Paleobiology. San Francisco: Freeman, Cooper & Co; 1972. (return to text)
4 Eldredge, N. Systematics and evolution of Phacops rana (Green 1832), and Phacops iowensis (Delo 1935) (Trilobita) from the Middle Devonian of North America. Bulletin of the American Museum of Natural History 1972;147:49-113. (return to text)
5 Sheldon, P.R. Microevolution and the fossil record, pp.106-110 in: Briggs, D.E.G. and Crowther, P.R., editors. Palaeobiology: A Synthesis. Oxford: Blackwell Scientific Publications; 1990. (return to text)
6 An interesting by-product of his collecting was incidental confirmation of a graptolite zone constructed by earlier workers. (return to text)
Document date: April 2004
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