Haplorygma, Balinski & Sun, 2010

Baliński, Andrzej & Yuanlin, Sun, 2010, Tubular shell infestations in some Mississippian spirilophous brachiopods, Acta Palaeontologica Polonica 55 (4), pp. 689-694 : 693

Ichnogenus Haplorygma nov.

Ichnospecies type: Haplorygma dorsalis ichnosp. nov., by monotypy.

Etymology: Greek haplóos, simple, onefold, single; orygma, tunnel; from the simple tubular structure of the trace fossil.

Diagnosis.—Simple, posteromedianly inclined with distal part upturned ventrally, microscopic tubular outgrowth of the inner surface of the secondary organocalcitic shell layer of the dorsal valve of spiriferide and spiriferinide brachiopods.

Remarks.—The name of the trace fossil here described is treated in the meaning of Articles 10.3 and 42.2. of the International Code of Zoological Nomenclature (1999; see also Dzik 2005) and thus refers to the animals that are responsible for formation of the named structure. Because homeomorphy of the Cambrian Eodiorygma and Carboniferous Haplorygma is very probable (in contrary to their monophyly), we prefer to distinguish these trace fossils as separate genera.

This new genus of the trace fossils attains about 20 times smaller size of the tube than that in Diorygma Biernat, 1961 found inside shells of Desquamatia subzonata Biernat, 1964 . What is more, the tube of Haplorygma is simple, inclined posteromedianly and sub−ventrally, and has only one distal aperture whereas that in Diorygma encloses two long U−turned contiguous tunnels which open distally by two round or slightly elliptical apertures and is anterodorsally inclined. The tube of the latter occurs exclusively in ventral valves of the atrypide host while the former was found in dorsal valve of Tylothyris laminosa and Spiriferellina cf. insculpta .

Haplorygma ichnogen. nov. reveals similar preference for dorsal valves of spiriferide (and spiriferinide) hosts as Burrinjuckia Chatterton, 1975 described from the Early Devonian of southern New South Wales ( Chatterton 1975). The tube of the latter, however, is several times larger, shows somewhat irregular morphology, and is always located in the anteromedian part of the valve projecting sub−ventrally or anteroventrally whereas Haplorygma was found on the side of the valve and is clearly posteromedianly and sub−ventrally inclined. The polychaete tubes (not formally named) inside shells of the Cretaceous Peregrinella multicarinata Lamarck, 1819 from southeastern Crimea ( Kiel 2008) are also largesized attaining up to 75 mm in length.

In terms of size range and general appearance of the tubular outgrowth, the new genus is very similar to Eodiorygma Bassett, Popov, and Holmer, 2004 described from the late Early Cambrian of Kazakhstan ( Bassett et al. 2004). Both structures do not exceed 1 mm in length. The main and obvious difference is that the structure of Eodiorygma occurs in a phosphatic−shelled brachiopod while Haplorygma is built in calcitic−shelled hosts. They differ also in the location and inclination of the tube within the dorsal valve: the Chinese form is positioned closer to the median sector of the valve whereas Eodiorygma seems more laterally located and has lateral inclination.

Stratigraphic and geographic range.—As for the type species.