The Crossostomatinae of the Mesozoic

The hard shell of many molluscs has left them with an excellent fossil record, one with few rivals among other groups of organisms. As a result, we are aware of a great many molluscan lineages that have inhabited this planet in the past, only to fade away long before the present day. One such group is the gastropod subfamily Crossostomatinae.

Crossostoma specimen, from Szabó et al. (1993).

The Crossostomatinae were Mesozoic representatives of the vetigastropods, one of the major subdivisions of gastropods corresponding to what used to be referred to as the archaeogastropods. Vetigastropods are primarily marine (off the top of my head, I can't think of any that are found in freshwater or terrestrial habitats, though I'm happy to be corrected) and crossostomatines were no exception. The classification of vetigastropods has tended to be rather unsettled but crossostomatines were definitely part of the lineage that includes the modern top shells (Trochidae) and cat's-eyes (Turbinidae), recognised as the superfamily Trochoidea in the recent synoptic classification of Bouchet et al. (2017). Within this lineage, the crossostomatines belong to the group of families possessing a calcareous operculum (sometimes treated as a separate superfamily Turbinoidea, but the significance of the calcareous vs horny operculum division in the trochoids seems to be the subject of debate). In recent treatments, the Crossostomatinae have been included within the family Colloniidae, characterised by the lack of a nacreous layer on the inside of the shell (Monari et al. 1996).

In general, crossostomatines were small shells with a smooth outer surface and broadly rounded whorls. They varied in shape from forms resembling modern cat's-eyes to lower-coiling, almost planispiral forms. A notable feature of the group is a tendency for the top of the aperture to be filled by a callus so the aperture appears almost perfectly circular. Other modifications of the mature shell opening are also common: Crossostoma, for instance, has the outer lip strongly thickened (Knight et al. 1960) whereas the final whorl of Adeorbisina turns away slightly from the regular coiling axis so that in top-down view the shell appears to bulge outwards before the terminus (Szabó et al. 1993).

Though they persisted through most of the Mesozoic, the number of known crossostomatine genera does not appear to be large. They seem to be associated with hard-ground deposits (Conti & Szabó 1987) so it is possible the group was more diverse in high-energy environments (organisms living in such environments, for instance along rocky shores, tend not to get preserved in the fossil record because their remains are broken up by wave action). It is possible that their lineage did not truly go extinct in the Mesozoic: Szabó et al. (1993) allude to the possibility of crossostomatines being ancestral to the subfamily Colloniinae, members of which may have survived to the Pliocene. Nevertheless, the Colloniidae as a whole did not survive to the present day, and it seems the line of the crossostomatines may have entirely passed from this Earth.

REFERENCES

Bouchet, P., J.-P. Rocroi, B. Hausdorf, A. Kaim, Y. Kano, A. Nützel, P. Parkhaev, M. Schrödl & E. E. Strong. 2017. Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia 61 (1–2): 1–526.

Conti, M. A., & J. Szabó. 1987. Comparison of Bajocian gastropod faunas from the Bakony Mts. (Hungary) and Umbria (Italy). Annales Historico-Naturales Musei Nationalis Hungarici 79: 43–59.

Knight, J. B., L. R. Cox, A. M. Keen, R. L. Batten, E. L. Yochelson & R. Robertson. 1960. Gastropoda: systematic descriptions. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt I. Mollusca 1: Mollusca—General Features, Scaphopoda, Amphineura, Monoplacophora, Gastropoda—General Features, Archaeogastropoda and some (mainly Paleozoic) Caenogastropoda and Opisthobranchia pp. I169–I331. Geological Society of America, and University of Kansas Press.

Monari, S., M. A. Conti & J. Szabó. 1996. Evolutionary systematics of Jurassic Trochoidea: the family Colloniidae and the subfamily Proconulidae. In: Taylor, J. D. (ed.) Origin and Evolutionary Radiation of the Mollusca pp. 199–204. Oxford University Press: Oxford.

Szabó, J., M. A. Conti & S. Monari. 1993. Jurassic gastropods from Sicily; new data to the classification of Ataphridae (Trochoidea). Scripta Geologica, Special Issue 2: 407–416.

The Osmiin Mason Bees

As I'm sure I must have had cause to say before, the world of solitary bees is a spectacularly diverse. Literally tens of thousands of species have been described to date, and no doubt many more remain. The classification of bees was reviewed by in great detail by Charles Michener (2007) in his monumental Bees of the World, and it was there that I turned to learn about the subject of today's post, the osmiins.

Female Osmia ferruginea, copyright Gideon Pisanty.

The Osmiini are currently recognised as a tribe of the Megachilidae, one of the two families of long-tongued bees (the other is the Apidae, including, among others, the majority of social bees). Megachilids are most easily characterised by the position of the scopa, a dense array of hairs used by bees for carrying pollen. In most bees possessing a scopa (it tends to be reduced or lost in kleptoparasitic forms), it is located on the hind legs but in megachilids it covers the underside of the metasoma. Osmiins are distinguished from other megachilids by the combination of a well developed sting, elongate stigma on the fore wing, arolia between the claws, and the lack of a pygidial plate. They are often smaller bees, less than a centimetre in length, though the largest osmiins grow close to two centimetres. Some osmiins are also more or less metallic in coloration, an unusual condition for megachilids. No feature has been identified that is unique to osmiins as a whole and their monophyly relative to other megachilid tribes (particularly the Megachilini) has long been called into question. A number of authors have recognised a division of living osmiins between two subtribes, the Osmiina and Heriadina. Osmiina have generally been distinguished from Heriadina by features such as a smaller stigma in the fore wing, a mesopleuron (a plate forming much of the side of the mesosoma) that is shorter ventrally than dorsally, and a propodeum that generally slopes downward from the base (rather than being initially flat). Again, however, the validity of this division has been questioned as no one feature uniformly distinguishes the two groups. A phylogenetic analysis of the Megachilidae by Gonzalez et al. (2012) did not support monophyly for Osmiini or either of its subtribes, but a proper revision of the group's higher classification remains to be done.

Female Hoplitis parana, copyright Gideon Pisanty.

Like other solitary bees, osmiins nest in cavities (a handful are kleptoparasites that do not construct their own nests). They often do not construct these cavities themselves but occupy pre-existing ones such as abandoned beetle burrows and hollows in wood, or crevices between rocks. Some species of Osmia have a predilection for nesting in empty snail shells. Cells are most commonly demarcated in the nest by walls constructed of chewed leaves, often held together with a sticky substance such as mud, resin or (more rarely) nectar. In some cases, the amount of leaf material used is reduced or abandoned, so the cell walls are made entirely of mud or resin. In some European species of Hoplitis, the cells are lined with petals; the species H. papaveris, for instance, lines its cells with bright red poppy petals. Osmia brevicornis, a species found in southern Europe and central Asia, is unusual in that its nest is not divided into cells. Instead, the nest cavity (an abandoned beetle burrow) is uniformly packed with pollen, with eggs being progressively inserted into the pollen mass as it is laid down. The larvae feed on the pollen around them after they hatch, and cocoons end up randomly scattered through the remains of the mass as they mature.

REFERENCES

Gonzalez, V. H., T. Griswold, C. J. Praz & B. N. Danforth. 2012. Phylogeny of the bee family Megachilidae (Hymenoptera: Apoidea) based on adult morphology. Systematic Entomology 37: 261–286.

Michener, C. D. 2007. The Bees of the World 2^nd ed. John Hopkins University Press: Baltimore.