Biology of the Hyolitha

Abstract

Hyoliths are Paleozoic fossils that have a calcareous exoskeleton consisting of an elongate, usually bilaterally symmetrical cone, a close fitting operculum, and a pair of curved appendages. Their skeletal ultrastructure resembles the crossed‐lamellar shell layers of some molluscs. Several specimens from the Ordovician of France and the Cambrian of Antarctica have parts of the gut preserved by infilling matrix, showing that both mouth ad anus were located near the cone aperture. Muscle scars in other hyolith shells indicate that the animal had a series of dorsoventral and longitudinal, or longitudinal and circular muscles, which operated through a hydrostatic skeleton to protract and retract the head, to open and close the operculum, and to move the appendages. Although the shell form and skeletal ultra‐structure of hyoliths are of a molluscan type, the muscle insertions suggest that the hyolith cone is not homologous with the dorsal exoskeleton of primitive molluscs. Hyoliths probably constitute a small extinct branch of phylum size, related to the Mollusca and the Sipunculoidea. All three groups may have had common ancestors in the late Precambrian.

An orthothecid hyolith with a digestive tract from the early Cambrian of Bornholm, Denmark

“…The importance of such a specimen in understanding hyolith palaeobiology and palaeoecology cannot be overstated, given that few other such specimens have been found. Its significance may be judged in part from the general reconstruction of hyolith soft part morphology given by Runnegar et al (1975), who conferred separate phylum status on the hyoliths because of presumed differences between them and the Mollusca, where hyoliths had been traditionally placed (Marek & Yochelson 1964, 1976.…”

“…The discussion of dorsal/ventral position in hyoliths is closely related to the mode of life and in several hypotheses, among others discussed by Yochelson (1961), Fisher (1962), Marek (1963), Runnegar et al (1975), Marek & Yochelson (1976), Sysoev (1973Sysoev ( , 1976Sysoev ( , 1984, Dzik (1980), Kruse (1997), and Marek et al (1997). The latter authors regarded the inferred dorsal/ventral position and orientation of the gut in the single Australian specimen described by Kruse (1997), where the flat side was interpreted as dorsal, as a 'preservational oddity' .…”

Chronology of early Cambrian biomineralization

“…These fossils are usually classified either within the Phylum Mollusca or Annelida, or in a separate phylumlevel group (e.g. Runnegar et al 1975;Runnegar, 1980;Kouchinsky, 2000).…”

Chronostratigraphic and palaeogeographic significance of an early Cambrian microfauna from the Heraultia Limestone, northern Montagne Noire, France

“…The assignment of the class Hyolitha to the phylum Mollusca is still debated. They are either considered molluscs by Sysoev (1960), Marek (1963), Marek and Yochelson (1964), Missarzhevskiy in Rozanov et al (1969), Marek and Yochelson (1976), Dzik (1978), Malinky (1987), and Kruse (1997) or a distinct phylum by Runnegar et al (1975), Valkov et al (1983), and Pojeta (1987). Therefore no phylum assignment is suggested herein.…”