The Morphology of Hyolithids and Its Functional Implications

Abstract

The exceptionally preserved hyolithids Gompholites striatulus, Maxilites robustus, Maxilites snajdri and Maxilites sp. are described with particular emphasis on helen and muscle scar morphology. These two aspects of hyolithid morphology have remained controversial. In life position, each helen curved ventrally. When the operculum closed the aperture of the conch, each helen was locked at the commissure slit with its dorsal edge tilted forward. Inside the conch, it was held in the dorsal apertural plane and clear of the inner surface of the operculum. Previously unidentified muscle scars are described from both the operculum and the conch. Dorsal scars on the conch aperture held muscles directed to the operculum. Comparative study of the muscle insertion pattern indicates that hyolithids did not have serially arranged muscles and that all hyolithids may have had a common skeletomuscular system. The arrangement of the muscle scars with respect to the helens suggests that the latter were capable of relatively complex movements and could have been used to propel the organism over the substrate. The general morphology and orientation of the helens suggests that in addition they functioned to stabilize the organism on the sea-floor.Key words: hyolithids, Gompholites, Maxilites, muscle scars, helens, functional morphology.Hyoliths are a group of extinct Palaeozoic bilaterians with a calcareous exoskeleton. Their taxonomic level is contentious, being variously regarded as an order within the class Monoplacophora (Dzik 1980), a class of mollusc (Marek 1963(Marek , 1967 Marek and Yochelson 1976) or a phylum (Runnegar et al. 1975; Runnegar 1980; Pojeta 1987). The group Hyolitha is divided into two morphologically distinct subgroups, Hyolithida and Orthothecida. The Orthothecida is a heterogeneous and relatively poorly understood group including forms with a conical conch of varied cross-section and a retractable operculum. The Hyolithida is species-rich, but relatively conservative morphologically. The skeleton of hyolithids consists of four elements (Text- fig. 1): a conical shell with a protruding ventral shelf (ligula), an external operculum and a pair of curved spines named helens. The different skeletal elements were not articulated by hinges, but must have been articulated exclusively by soft tissues. The anatomical relationship between conch and operculum is relatively straightforward but the articulation and precise orientation of the helens are unresolved. Furthermore, their detailed morphology and structure are also largely unknown.The relatively frequent preservation of muscle scars provides an important source of information on the organisation and functional morphology of the skeletomuscular system. Some of the specimens discussed herein have been described previously. They are among the best preserved hyolithids known and in re-describing them we focus on new observations as well as the relevance of features known previously. HelensHelens are relatively long, narrow, curved and tapering skeletal e...

Orthrozanclus elongata n. sp. and the significance of sclerite-covered taxa for early trochozoan evolution

“…Looking more widely, the paired muscle scars and shelly internal projections evident in Morph A valves of Oikozetetes 51,52 , some of the best documented halkieriid shells, have possible parallels in the equivalent paired muscle scars and internal processes present in the mitral sclerite of the tommotiids Micrina 44 and Dailyatia 35 and the operculum of hyolithids 53 (potential relatives of tommotiids 54 ).…”

The Buen Formation (Cambrian Series 2) biota of North Greenland

“…In its simplicity, the outer surface of this operculum is reminiscent of Maxillites Marek, from the middle Cambrian of the Czech Republic, but there is no indication of the complex structure of the inner surface described by Marek () and Martí Mus & Bergström (). Although flattened and broken, it is slightly wider than tall, with the widest point at just below mid‐height; the initial growth stage and summit is at just above two‐thirds height.…”

“…In Maxillites from the Czech Republic, the summit is located lower on the shield with the cardinal surface up to half of the height (Marek ; Martí Mus & Bergström ). An operculum of Hyolithus oelandicus Holm, from the middle Cambrian of Borgholm, Sweden, referred to Maxillites by Marek (), was only tentatively placed within that genus by Malinky & Berg‐Madsen () on account of the lack of information concerning its interior surface.…”

The problematic fossilTriplicatellafrom the Early Cambrian of Greenland, Canada, and Siberia

“…stationary at the aperture of the conch, only opening at the ventral margin (Marek 1967;Marti-Mus and Bergström 2001), the opercula of orthothecids were small enough to fit inside the conch and apparently were capable of vertical movement within it (Marek 1966;Marek and Yochelson 1976). While hyolithids apparently relied on muscles to move the operculum and conch in relation to each other (Marti-Mus and Bergström 2001), orthothecids may have relied on hydrostatic mechanisms to manipulate their skeletal parts.…”

“…The opercula of hyolithids also have two sets of internal processes; a set of teeth-like structures (cardinal processes) situated close to the dorsal margin and a set of ridge-like structures (clavicles) running from the cardinal processes towards the ventral side. Diverse muscle scar patterns are present in certain hyolithid opercula, and Marek (1963Marek ( , 1967 and Marti-Mus and Bergström (2001) suggested that some at least (including scars associated with the cardinal processes) were involved in controlling the movement of the operculum with reference to the conch. Other scars were interpreted by Marti-Mus and Bergström (2001) as structures involved in controlling the position and movement of the helens.…”