The family Raphiophoridae were a diverse clade of predominantly Ordovician trilobites, which were common in outer shelf palaeoenvironments worldwide. Raphiophorids are small, blind trilobites, and all of them have slender genal spines that are usually much longer than the rest of the body, excluding any frontal spine. They are considered to have had benthic or nektobenthic habits, which are matched by their inferred weak thoracic musculature in relation to their peripherally extended exoskeletal structures. Within the family there is a variety of median glabellar structures, mostly variations on spines (), although the existence of several genera (Mendolaspis
), in which the median spine is reduced or absent, demonstrates that it was not a sine qua non
for successful life as a raphiophorid.
- Anterior spine in Ampyx is long and slender, tapering slowly, with a round cross-section. The silicified species from the Middle Ordovician of North America, including Ampyx virginiensis (a,b), have distally upturned spines, but Ampyx spongiosus (c) has a spine that is declined gently distally. Here, we figure an undescribed species of Ampyx in which the spine rises vertically from the mid-part of the glabella and curves backwards distally (d).
- Anterior spine in Lonchodomas has a prismatic cross-section, and the glabella often protrudes towards it, making a lance-like structure.
- Anterior spine of Bulbaspis (Chugaeva 1958), from the Ordovician of Kazakhstan and China is also directed upwards, but distally it is inflated into a balloon-like structure (e).
- Anterior spine of Ampyxoides is short and straight.
Figure 1 Diversity of anterior structures in the Raphiophoridae compared with similar structures in the Coleoptera. (a) Reconstruction of A. virginiensis, dorsal view, after Whittington & Campbell (1967). Scale bar represents 5mm. Note that the (more ...)
As to function, several possibilities might be suggested for use of the anterior spine, which can be largely discounted by a consideration of the functional morphology of the animal as a whole, or by comparison with related species. The production of such a striking structure is likely to have entailed a considerable metabolic cost, and it surely played an important role in the life of the trilobite.
(a) Hydrodynamic streamlining
Glabellar extensions that reduce surface turbulence are known from a number of actively swimming trilobites (Fortey 1985
), but in these cases, the rest of the exoskeletal morphology is completely different. Symphysops
, for example, has very large eyes, a compact thorax and pygidium with evidence of strong musculature—all features associated with pelagic habits. Raphiophorids are, by contrast, blind, flattened and weakly muscled, and their occurrence seems to be closely related to substrate type. Furthermore, although some of the structures may seem hydrodynamically efficient, it is evident that the balloon structures of Bulbaspis
, or the vertical spines of some Ampyx
species, are anything but. This does not rule out other hydrodynamic effects of spines, such as increasing drag to allow greater velocity gradients during filter feeding, but these effects are most pronounced in small planktonic animals (less than 0.5
mm in length; Emlet & Strathman 1985
) and would be unlikely to be important in these trilobites, which were usually between 10 and 30
mm in length.
It is possible the anterior spines of Ampyx
, in conjunction with the genal spines, may have served to present a comparatively large, spiny target for any potential predator, in a similar fashion to the inducible spines of modern Daphnia
that limit the ability of gape-limited predators (Dodson 1989
). However, such adaptations in modern animals are mostly found on free-swimming animals, and anti-predator defences on benthic arthropods, such as odonate nymphs (Arnqvist & Johansson 1998
) and also many trilobites (Fortey & Owens 1999
), often consist of more robust spines carried on the back. The diversity of the spines of the Raphiophoridae also argues against a defensive role: it is not easy to imagine a defensive role for the full variety of spines found in the family. Thus, in the case of raphiophorids it does not seem likely that the prime function of the spines was for protection, although it is not possible to rule it out completely. The fact that there are several ‘spineless’ genera in the same family indicates that such putative protection was not invariably required.
(c) Sensory apparatus
The known examples of raphiophorid frontal spines show no evidence of distal perforation or other openings for subcuticular organs, or sensory cells. Small spines on the dorsal cephalic surface of many trilobites frequently do have such terminal perforations, which are often interpreted as marking the site of sensory setae. As far as one can tell, the raphiophorid spine is an extension of normal cuticle, and carries no such structures. This does not rule out any use of the spine as a sensory apparatus, of course, and even without sensory setae on the spine, sensory information could be transmitted to receptors at the base of the spine, but it does make a primary sensory function unlikely.
It has been suggested (G. Kloc, personal communication) that strongly spinose trilobites such as odontopleurids may have encouraged dorsal epibionts that served as camouflage. There is no evidence of attachment scars for epibionts on any raphiophorid known to us.
These functional explanations are all unsatisfactory. The remaining explanation is that the anterior structures are secondarily sexual in character.