Pagurus hazenorum, Wallaard & Fraaije & Van Bakel & Nance & Lindholm & Jagt, 2023

Wallaard, Jonathan J. W., Fraaije, René H. B., Van Bakel, Barry W. M., Nance, John R., Lindholm, Adam & Jagt, John W. M., 2023, New hermit crab species (Anomura, Paguroidea) from the upper Miocene St Marys Formation of Maryland (USA), preserved in their host shells, Zootaxa 5227 (3), pp. 389-397 : 391-392

Pagurus hazenorum n. sp.

( Fig. 1 )

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Diagnosis. Major carpus (right) covered with small, randomly scattered tubercles, increasing in size toward outer margin, there becoming setose. On minor carpus (left), tubercles arranged in rows and slightly increasing in size toward distal end. Both claws densely covered with large granules, decreasing in size toward outer margin. Outer margins of propodus and dactylus covered with blunt teeth, smaller on dactylus. Outer lateral surface of dactylus with longitudinal, medially elevated surface. Walking legs covered with row of teeth, decreasing in size toward distal end. Dactylus of walking leg only covered by blunt teeth and surface with some longitudinal grooves.

Type material. The holotype, and sole specimen known to date, is CMM-I-4785. It is preserved inside its gastropod host shell ( Buccinofusus parilis Conrad, 1832 ), which is severely damaged and allows the hermit crab inside to be observed in more detail. The right and left chelipeds are preserved within the gastropod aperture. A relatively complete walking leg, consisting of carpus, propodus and dactylus, is seen posterior of that aperture. A second walking leg, of which only the carpus is preserved, is found anterior of the aperture, while several fragments of other legs are present just behind the chelipeds. The carapace should have been situated here, but this part of the individual has suffered considerable damage and there is no trace of a carapace.

Etymology. In honor of Dr Robert M. Hazen, senior staff scientist at the Carnegie Institution for Science, and his wife, Margaret Hazen, writer and historian.

Locality and stratigraphy. Driftwood Beach, Calvert County, Maryland, from the upper Miocene (Tortonian) Little Cove Point Member of the St. Marys Formation in a silty lens within Bed E ( Ward & Andrews 2008). Kidwell et al. (2015) identified this level as belonging to “SM-C,” assigned to Shattuck zones 22–23 and part of dinocyst zone 8.

Description. Chelipeds stout, broad, with propodus of major (right) claw measuring 21 mm by 14 mm; that of minor (left) claw measuring 16 mm by 12 mm. Entire surface of carpus of major claw covered with small, randomly scattered tubercles, increasing in size and setation toward outer margin. Carpus of minor claw with tubercles arranged in rows and slightly increasing in size toward distal end. Both left and right propodi densely covered with large granules, decreasing in size toward outer margin. Outer margin of propodus arcuate; that of dactylus almost straight. Outer margin of both propodus and dactylus covered with blunt teeth, smaller on latter. Outer lateral surface of dactylus covered with longitudinal, medially elevated ridge.

Walking legs covered with row of teeth, decreasing in size toward distal end. Dactylus of walking leg with blunt teeth; surface with some longitudinal grooves.

Remarks. The new species compares fairly well with the extant Pagurus impressus (Benedict, 1892) , from the west coast of Florida ( Provenzano 1959), as well as with Diacanthurus rubricatus Henderson, 1888 , from the coast of New Zealand and P. bernhardus ( Linnaeus, 1758) from the eastern North Atlantic.

Discussion. The assignment of the Maryland material to the genus Pagurus is based on several morphological features which P. hazenorum n. sp. has in common with extant congeners, as described below.

Although P. hazenorum n. sp. is preserved in situ within its gastropod shell, the carapace appears to be missing, most likely as a result of the damage to the host shell. Overall, the shape of the dactylus is more elongated and bears a closer cover of granules in P. impressus , whereas that of P. hazenorum n. sp. is stout, with blunt teeth along the outer edge and a cover of large granules. Chelipeds of the present-day D. rubricatus bear small spines, in equal density as in P. hazenorum n. sp. Pagurus bernhardus is comparable as well; this has a coarse ornament which, however, is less dense than that of P. hazenorum n. sp. (see Hyžný & Dulai 2021: fig. 35.8).

Our comparison of P. hazenorum n. sp. with both extant and extinct species has yielded numerous forms with closely comparable anatomical features. Molecular and genetic research carried out recently on extant representatives of the genus Pagurus has shown that this is in fact a wastebasket taxon, comprising forms with closely comparable morphologies, but widely divergent genetic structures (e.g., Olguin & Mantelatto 2013; Sultana et al. 2018). Naturally, genetic research cannot be carried out on extinct forms, which makes any workable subdivisions of the genus Pagurus even more difficult. This morphological similarity amongst genetically diverse species is most likely a reflection of functional morphology. Particularly in paguroids, the shell has a marked impact on cheliped shape, and most hermit crabs inhabit comparable mollusks, which explains the closely comparable morphology of the chelipeds.

In the fossil record, isolated paguroid chelipeds (mostly propodi) are quite common, whereas carapaces are extremely rare. In view of this, it is highly unlikely that the difficulties surrounding the ‘lump’ taxon Pagurus can be resolved on the basis of extinct forms.

Figures

FIGURE 1. Pagurus hazenorum n. sp., holotype, CMM-I-4785, from the upper Miocene St. Marys Formation in the cliff south of Little Cove Point, Calvert County, Maryland, USA, preserved in its host shell, and in dorsal (A), ventral (B), and dorsoventral (C) aspects.