Fortey (1974) discussed the phylogenetic implication of early meraspid cranidia within Olenidae. According to Westergård (1922) and Henningsmoen (1957), the genus Protopeltura lasted into the Peltura minor Zone. Here, it lost its genal spines and developed into Peltura, which ranges into the Acerocare Zone. Protopeltura is not found however in the Leptoplastus Zone.
Summary of ontogenetic trends in Protopeltura aciculata
Glabellar furrows reduce from 3 to 1, and S1 becomes confined to the lateral regions only, S2 and S3 become faintly impressed elongated pits. The occipital ring is quite convex/inflated and triangular in the earlier stages, but becomes less so, and strap-shaped subsequently. The occipital spine, where present, becomes longer and slender. The coarse granulation on the fixigenae in early to mid-meraspides becomes finer until it disappears completely in the holaspides. The pygidium decreases in size relatively and becomes triangular in outline, and the spines likewise become shorter and broader at the base until they disappear. The relative dimensions of the cranidium change during growth, more so in the late meraspid to holaspid stages.
In many groups of trilobites, morphology and size are closely coupled during ontogeny, and the forms and sizes of successive developmental stages are tightly constrained. Thus, it may be possible to recognise successive instars by measuring the dimensions (usually length and width) of individuals within a population and plotting these on a bivariate scattergram. Often, these graphs show distinct instar groupings or clusters, within which individuals are very similar in size and shape. Normally, it is the earlier growth stages that plot out most clearly in such clusters, and natural variation in the size of later stages obscures the distinction between them. Such patterns emerged clearly in an early work by Hunt (1967) on the agnostoid Trinodus elspethi (Raymond, 1924), in eodiscoids (Jell 1975, Zhang 1989, Zhang and Clarkson 1993; Cederstöm et al. 2009) and in many silicified trilobites, such as Scutellum (Chatterton 1971) and Dimeropyge (Chatterton and Speyer 1997), to name only a few. Such clustering of successive instar groupings on ontogenetic development seems to be common in trilobites generally, especially those of the Ordovician and later.
Within the Family Olenidae, certain genera and species follow the same pattern. Thus, clear instar groupings are apparent in Ctenopyge ceciliae (Clarkson and Ahlberg, 2002) and C. (Eoctenopyge) angustaWestergård, 1922 (Clarkson et al. 2003), and there is some tendency to grouping also in C. (Ctenopyge) gracilisHenningsmoen, 1957 (Clarkson et al. 2004). It is less distinct in P. scarabaeoides westergaardiHenningsmoen, 1957 (Bird and Clarkson 2003). Somewhat broad instar groupings are evident in J. keideliKobayashi, 1936 (Tortello and Clarkson 2003), but both in P. spinulosa (Wahlenberg, 1818) from Sweden and in Parabolina frequens argentina (Kayser, 1876) from Argentina, ontogenetic variation is considerable, some character states being much more advanced than others so that it is not possible to assign disarticulated sclerites to their correct meraspid degree. Variation among juveniles in P. frequens argentina was described as ‘formidable’ (Tortello and Clarkson 2008). And the same is very much true of P. aciculata and possibly even more so.
Perhaps the most arresting case of extreme variability in trilobites is that of the Furongian Dikelocephalus minnesotensisOwen, 1852 from the upper Mississippi valley, north central USA. This large and quite widespread asaphide can reach 40 cm in length, and so strong is the variability among adults that Ulrich and Resser (1930) erected no less than 25 species, which Hughes (1993, 1994) reduced to a single one. Here, there is no consistent pattern of clustering, but a continuous morphological variation between specimens both from the same bed and from different localities. There is no relationship between the variation in the trilobites and the enclosing lithologies. Only holaspides were available for study, because earlier stages were not preserved.
Hughes (1994, p. 58) discussed possible controls on variation in Dikelocephalus. He distinguished growth-related variation as one factor, in other words size/volume developmental constraints as the trilobite grew. A second factor was population-related variation, posing the question as to whether developmental plasticity in this case is a result of the palaeoenvironmental setting. But as there is no evidence of an unusually high environmental stress level in the northern Mississippi valley area during late Cambrian times, this does not seem very likely. From the genetic point of view, it seems that Dikelocephalus had a very flexible, in other words a poorly canalised, genotype – the alternative possibility of a series of genetically canalised polymorphs is not borne out by the evidence. A further factor is that in any case, Cambrian trilobites are generally much less developmentally constrained than those of the Ordovician. Olenellid genera, for instance, as has long been known, tend to intergrade, and many other Cambrian trilobites show a relatively high degree of morphological plasticity – this has been called the ptychopariid problem – where to draw the boundaries between species.
All these issues are important for interpreting the unconstrained variation in P. aciculata. There is, however, another possibility which we tentatively propose; that is, the high developmental plasticity that we see is actually a survival strategy for the species. The Alum Shales sea floor, by contrast with that in which Dikelocepahus lived, was undoubtedly a stressed environment for its inhabitants, being dysoxic and probably anoxic just below the surface. It may well have been toxic from time to time. If some potentially lethal event took place, a local spread of anoxic or poisoned water for example, then populations in which the individuals were all of very similar kind would be more likely to be wiped out completely than if they were highly variable. Some morphs in a more developmentally plastic population might survive by pure chance, and thus, the species would continue. There is a possible parallel here, albeit a distant one, with both the land snail Cepea nemoralisLinnaeus, 1758 and the Silurian mollusc Pterotheca (Clarkson et al. 1995). Cepea exhibits striking genetic polymorphism in its colour banding; it is actively preyed upon by thrushes which are highly selective about which morph they look for. They develop a ‘search image’ disregarding those morphs that fall outside this range, until the preferred morph become too scarce. Then, they have to begin to prey on another morph. In the long term, the balance between morphs remains stable, because a sequence of morphs is preyed on in turn, and the population of a previously sought morph will build up again when it is no longer the prime target. It was argued that the marine bellerophontiform Pterotheca was highly sought after by cephalopods (Clarkson et al. 1995), and marked polymorphic asymmetry confused their ‘search image’ in a very similar way. The difference between this kind of polymorphism and that of Protopeltura is that the stresses on the molluscs, with their voluminous flesh, are biological while those on the trilobites are more likely to be physicochemical.
If morphological plasticity, at all stages in development of Protopleltura, is indeed a response to stressed sea floor conditions, then why should the development of C. ceciliae, C. (Ctenopyge) gracilis and J. keideli be more ‘normal’, in other words why can discrete instar groupings be recognisable? Ctenopyge ceciliae is considered, on various grounds, to have been planktonic (Clarkson and Ahlberg 2002, Schoenemann et al. 2010). Jujuyaspis keideli was most likely pelagic, at least during the early meraspid stages; the later stages were probably nekto-benthic (Tortello and Clarkson 2003) as may have been C. (Ctenopyge) gracilis. Might it have been that the upper waters of the sea represented a less hazardous environment than the floor of the Alum Shale Sea, and thereby conduced to developmental ‘normality’? Such a concept is at least worthy of consideration.