Cornulites, Schlotheim, 1820

Herringshaw, Liam G., Thomas, Alan T. & Smith, M. Paul, 2007, Systematics, shell structure and affinities of the Palaeozoic Problematicum Cornulites, Zoological Journal of the Linnean Society 150 (4), pp. 681-699 : 691-695

COMPARISON OF CORNULITES WITH OTHER

PROBLEMATICA

The likely position of Cornulites within the Metazoa is discussed below, but there are other problematical taxa, notably Tentaculites Schlotheim, 1820 , and Cloudina Germs, 1972 , with which it shares distinct similarities. Schlotheim (1820) erected Tentaculites to include a different group of annulated, conical, calcareous shells from the Palaeozoic and described two species, T. scalaris and T. annulatus . As with Cornulites serpularius , the specimens figured by Schlotheim (1820) cannot be traced ( Larsson, 1979b), but based on the original illustrations the two genera are morphologically similar. Like Cornulites , Tentaculites is radially symmetrical about its long axis, but the shell is more slender, with a narrower aperture, rarely shows any sinuosity, and normally lacks obvious longitudinal ornamentation. However, having suggested that Cornulites might be most closely related to annelids, Schlotheim (1820) interpreted Tentaculites as a crinoid appendage. Subsequent assessments of the biological affinities of Tentaculites have been varied (for a summary see Larsson, 1979b): whilst some authors (e.g. Nicholson, 1872a, b, 1873; Fisher, 1962) have followed Schlotheim in regarding Tentaculites and Cornulites as belonging to separate phyla, others (e.g. Murchison, 1859; Vine, 1882; Bouček, 1964; Dzik, 1991, 1993) have argued that the two genera are closely related. Indeed, Bouček (1964) erected the Order Cornulitida as a constituent group of his Class Tentaculita, a classification followed by Vinn (2005).

As with cornulitids, the earliest growth stages of the tentaculitid shell are very rarely preserved ( Larsson, 1979b), but the shell structures of some tentaculitids show a number of similarities to Cornulites , particularly towards the shell apex. Most prominent are the transverse shell layers that, in many tentaculitids, divide the apical region into distinct chambers, or camerae (see Larsson, 1979b: fig. 12). These are similar in arrangement to cornulitid tabulae and camerae, although the transverse layers do not continue up the interior shell surface to form lamellae. Instead, they taper distally, and the shell wall is formed of separate lamellae, unconnected to the transverse layers, producing a bipartite division of primary (outer) and secondary (inner) layers (Bouček, 1964; Larsson, 1979b). Larsson (1979b: 27) noted also that tentaculitid lamellae were ‘not persistent along or around the conch’, making them unlike the continuous lamellae of cornulitids. However, he did show that the lamellar part of the conch contained pseudopuncta orientated perpendicular to the surface of the wall. The phylogenetic position of tentaculitids is unresolved but, based on similarities in shell microstructure, Towe (1978) raised the possibility that they were most closely related either to the brachiopods or their sister group, the phoronids. Larsson (1979b: 59) suggested that tentaculitids were perhaps more closely related to phoronids, but noted that the lophophorate feeding system of phoronids was incompatible with the planktonic mode of life proposed for some tentaculitids. Vinn (2005) and Vinn & Mutvei (2005) have interpreted cornulitids and tentaculitids as closely related groups of probable lophophorates but, as discussed below, comparable morphologies and shell structures are found also in cnidarians. Further work is required to resolve both the affinities of Tentaculites and its systematic relationship to Cornulites .

Cloudina is a genus of tubular, calcareous fossils found in rocks of Ediacaran age. The first detailed study of the shell structure of Cloudina was that of

Grant (1990), who described a cone-in-cone structure of stacked tubes, a layer of calcium carbonate having been deposited over the entire surface at each growth stage, and the presence of vacuities between the layers both apically and aperturally (see Grant, 1990: figs 5,

7, 9). He described also that each new tubular cone of calcium carbonate was deposited eccentrically within sections through shell wall, showing lamellar structure

(lam) separated by occasional cellulae (cel), aperture towards top in both images. A, shell interior to right, ×20.

B, shell interior to left, ×23.

morphology, based on shell structures seen in C. cellulosus

(approximately × 5). Transverse annulations (ann) are visible externally; cutaway sections show major aspects of internal morphology. Narrow apical end of shell possesses tabulae (tab) internally (upper surface of most apertural tabula shaded black; more apical tabulae shown only in section). Shell wall in apical region composed of internally straight, but externally undulating, lamellae (lam).

Towards aperture, convex-inwards cellulae (cel) become abundant outside inner lamellar layer; in vertical section,

cellulae are convex-upwards and bounded by well-

separated undulating lamellae. In this orientation, the undulating lamellae form a shell wall of outwardly and downwardly dipping zones. Towards aperture, cellulae are arranged in crescentic transverse arcs, indicating an eccentricity to the cone-in-cone growth pattern. In upper cutaway, pale grey shading represents inner surface of outermost shell layer; medium grey indicates inner surface of whole skeleton with faint transverse annulations visible

(broken lines). Apertural region has prominent apertural groove (ag).

earlier layers, and stated that he was unaware ‘of any Cambrian or younger fossils that share features of the Cloudina shell structure’ ( Grant, 1990: 286). However, although simpler and lacking such features as pseudopuncta and cellulae, the overall pattern of shell growth and structure in Cloudina is very similar to that of Cornulites . In particular, the placement of each layer eccentrically within earlier layers gives Cloudina a transverse section view similar to that of Cornulites cellulosus (compare Fig. 5F with Grant, 1990: fig. 5E). It is possible therefore that the more complex skeleton of Cornulites developed from that of a Cloudina -like ancestor.

Figures

Figure 5. Shell structures in Cornulites cellulosus sp. nov. A–C, BU 4371, pseudopuncta. A, transverse view, ×45, B, longitudinal view, ×45. C, ESEM image of transverse section (scale bar = 20 µm), shell interior towards top in all images. D, BU 4371, ESEM image of transverse section across mid-region shell wall, showing bipartite structure within lamellae, scale bar = 10 µm. E, BMNH A455, transverse section across apertural region showing concentric lamellae and apertural groove filled with cellulae, ×4. F, G, BMNH A459. F, transverse section close to aperture, ×3.25. G, close-up of concentric lamellae, separated by zones of cellulae, ×4.