Ice-cream Cones of the Early Palaeozoic

It's time for something I haven't done in a very long time... (credit to Niel from Microecos):

I briefly described tentaculitoids on this site way back in September 2007. These narrowly conical shells of uncertain affinities were prominent members of the marine fauna during the Silurian and the Devonian, only to then disappear without a trace. No direct evidence is available for the soft-body appearance of the animals that produced them nor are we overly certain on their lifestyle. But at least one of the major subgroups of the tentaculitoids, the Dacryoconarida, are held to be of palaeontological significance due to their ubiquity and cosmopolitan distribution at the species level making them of use in biostratigraphy.

Reconstruction of Nowakia elegans, from Berkyová et al. (2007).

Dacryoconarids have generally been presumed to be planktonic in some way, owing to the aforementioned tendency of individual species to be found more or less worldwide, together with their small size (generally about the centimetre range). Dacryoconarids are distinguished from other tentaculitoids by the apical portion of their shell ending in a small globular bulb, presumed to represent the embryonic or larval shell of the original animal (Farsan 2005). A more or less distinct constriction or 'neck' separates this embryonic bulb from the remainder of the shell. In those forms with more heavily ornamented shells such as the genus Nowakia, a distinct juvenile section of the shell is visible immediately following the embryonic bulb in which the adult ornament is absent or weakly developed; said adult ornament, when it appears, takes the form of rounded transverse ridges and troughs, often associated with longitudinal and/or transverse striae. In other forms, such as the genus Styliolina, the outside of the shell is flat and ridgeless, with at most the only ornamentation present being striae. The inside of the shell may be rippled to follow the exterior ornamentation or it may be perfectly smooth (Fisher 1962).

Dacryoconarids are first recorded from the Late Ordovician but they remained at relatively low diversity until the Devonian which saw a notable radiation (Wittmer & Miller 2011). Nevertheless, they declined rapidly towards the end of the Devonian. It has been suggested that their extinction by the end of that period may be related to the appearance of more actively swimming predatory fish before which the tentaculitoids may have been relatively defenceless. Other early Palaeozoic planktic groups such as the graptoloids experienced a similar collapse at about this time, though the disappearance of the dacryoconarids may have lagged behind that of the graptoloids.

Styliolina clavulus, from Fisher (1962).

Over the years, a wide range of suggestions have been made about the affinities of the tentaculitoids, ranging from jellyfish to annelids. Perhaps the most persistent association has been made with molluscs but there really is little to support such a premise than the possession of a calcareous shell, a feature that is hardly unique to molluscs even among living animals. The structure of the tentaculitoid shell is most similar to that of some brachiopods (Fisher 1962) and some sort of brachiozoan affinity is perhaps the currently most favoured concept. As noted above, we know nothing about the tentaculitoid anatomy other than what we can infer from the nature of the shells themselves. In some larger tentaculitoids (though not among the dacryoconarids so far as we know) the apical parts of the shell may become walled off by solid septa so the living animal presumably didn't occupy the entire shell. Fisher (1962) described the tentaculitoids as "presumably tentacle-bearing" but I have no idea on what basis he made that statement (as I've noted before, the name 'tentaculitoid' itself comes not from a belief that they possess tentacles but from the mistaken interpretation of the first specimens named as being themselves the tentacles of larger animal). Tentacles would be a not unreasonable method of capturing the smaller micro-plankton on which the dacryoconarids presumably fed but it is not impossible that some other structure served this purpose.

REFERENCES

Farsan, N. M. 2005. Description of the early ontogenetic part of the tentaculitids, with implications for classification. Lethaia 38: 255–270.

Fisher, D. W. 1962. Small conoidal shells of uncertain affinities. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt W. Miscellanea: Conodonts, Conoidal Shells of Uncertain Affinities, Worms, Trace Fossils and Problematica pp. W98–W143. Geological Society of America, and University of Kansas Press.

Wittmer, J. M., & A. I. Miller. 2011. Dissecting the global diversity trajectory of an enigmatic group: the paleogeographic history of tentaculitoids. Palaeogeography, Palaeoclimatology, Palaeoecology 312: 54–65.