Hyolithellida

Life reconstruction of Hyolithellus micans, from Fisher (1962).

Belongs within: Hyolitha.

The Hyolithellida are a group of small, tubular fossils known from the Cambrian and Ordovician periods. Their affinities are uncertain; though often classified with the hyoliths (and here placed with them for convenience), their shells are phosphatic rather than calcitic, making such a relationship unlikely. Hyolithellidans may be divided between the Hyolithellidae, whose tubes have a circular cross-section and were sealed with an operculum bearing paired muscle scars, and the Torellellidae with elliptical or biconvex tubes for which an operculum is unknown (Fisher 1962).

Characters (from Fisher 1962, as Hyolithelminthes): Small (length usually 5–15 mm), almost cylindrical, brownish-black narrow, conical tubes (growth angle in adult region one to four degrees); curved and irregular near closed apex, straightening toward aperture. Cross section circular to elliptical. Exterior surface sometimes covered by small transverse ridgelets and striae; shell laminated, usually thick, with smooth interior surface. Body cavity elongate, devoid of septa or any other structure. Circular to subcircular operculum composed of many laminae, with eccentrically situated apex surrounded by closely spaced concentric growth lines; adult opercula thicker than translucent juvenile ones; underside of operculum with paired muscle scars originating form point opposite apex, showing bilateral symmetry. Tube and operculum composed of calcium orthophosphate.

<==Hyolithellida [Hyolithelminthes]
    |--Hyolithellidae F62
    |    |--Barella Hedström 1930 F62
    |    |--Hyolithellus Billings 1871 F62 [incl. Discinella Hall 1872 F62, H75]
    |    |    `--*H. micans (Billings 1871) [=Hyolithes micans] F62
    |    `--Mobergella Hedström 1923 H75
    |         `--*M. holsti (Moberg 1892) [=Discinella holsti] H75
    `--Torellellidae F62
         |--Torellella Holm 1893 F62
         |    `--*T. laevigata (Linnarsson 1871) [=Hyolithes laevigatus] F62
         |--Pseudorthotheca Cobbold 1935 F62
         |    `--*P. acuticincta Cobbold 1935 F62
         `--Rushtonia Cobbold & Pocock 1934 F62
              `--*R. lata Cobbold & Pocock 1934 F62

*Type species of generic name indicated

REFERENCES

[F62] 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.

[H75] Häntzschel, W. 1975. Treatise on Invertebrate Paleontology pt W. Miscellanea Suppl. 1. Trace Fossils and Problematica 2^nd ed. The Geological Society of America: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).