Fossil Record

Despite having a calcified exoskeleton, hermit crabs are not particularly well represented in the fossil record. However, some epibionts only settle on shells occupied by hermit crabs and so the presence of certain epibionts on fossilised gastropod shells (which are more common in the fossil record) can be taken to infer the presence of hermit crabs (Walker 1988).

There have been two significant discoveries very recently which have empowered understanding of paguroid evolution. Fraaije et al. (2012) have described what is currently the earliest known hermit crab of the Diogenid family, of which Dardanus is a genus. The new species, Eopaguropsis nidiaquilae, was recorded from Poland and has been dated to the late Jurassic (Fraaije et al. 2012). The specimen consisted of a near complete carapace and was identified as a diogenid chiefly because the characteristic deep cervical groove of the shield (Fraaije et al. 2012).

Hermit crabs are thought to have first arisen in the Jurassic (Fraaije 2003). However, this has not been well supported (Fraaije 2003), chiefly because of a lack of in situ fossil hermit crabs from this period, that is, hermit crabs fossilised within their host shell. A significant discovery from the Netherlands in 2003 has however changed the approach of palaeontologists studying hermit crab evolution. Previously the earliest in situ hermit crab was dated to the mid-Cretaceous. This fossilised hermit crab was found associated with a gastropod mollusc in Germany. But the Netherlands discovery was a perfectly intact hermit crab within an ammonite shell, an extinct lineage of cephalopods very common in fossil deposits of the period.

This finding may explain the absence of in situ hermit crabs from the Jurassic, or more precisely, the absence of hermit crabs associated with gastropods. The discovery has been suggested to provide evidence of a shift of hermit crab host use, from ammonites to gastropods, in the Cretaceous (Fraaije 2003). Such a shift coincides with a marked increase in diversification of Gastropods during the Cretaceous. The increased diversity and abundance, increased shell strength and ornamentation of gastropods, and the subsequent extinction of the ammonites at the Cretaceous-Tertiary boundary, all support this ecological transition (Fraaije 2003). Gastropod shells also perform better than those of ammonites, the differences in shape conferring less resistance when dragged across the substrate (Fraaije 2003).

The hermit crab discovered within the fossilised ammonite shell had a different cheliped morphology to that of modern hermit crabs and it has subsequently been hypothesised that the ecological host transition discussed above has resulted in evolutionary adaptation of the cheliped to better fit the aperture of a gastropod shell for protection (Fraaije 2003).

The appearance of hermit crabs in the Jurassic and their subsequent success has had substantial ecological impacts for other faunal groups. Most importantly, hermit crabs greatly extend what can be considered the ‘ecological life’ of a single shell. For many years after a gastropod is deceased its shell continues to act as a defended, moving substrate, carried about the environment by the hermit crab (Pretterebner et al. 2012). The diversity of symbionts associated with hermit occupied shells, some obligatory, is far from trivial. (see symbionts). Further the shell continues to hold a large food item, the hermit crab, for potential predators. This increase in shelled prey, in addition to the Cretaceous diversification of gastropods, has been suggested to have fueled the evolution of shell-specialist predators. Similarly the recording of the aforementioned ammonite dwelling hermit crab has been suggested to explain the few discoveries of fossilised ammonite shells dated to after their accepted demise in the Cretaceous-Tertiary extinction. That is, hermit crabs extended the ‘life’ of these ammonite shells, carrying them through the extinction to be deposited early in the Tertiary period, a substantial feat. This is an example of the significance of hermit crabs to palaeontology, influencing when, where and which gastropod shells are likely to be deposited (Walker 1988).