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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...

Trilobites from the upper unit of the Lower Cambrian Pusa Formation (south-central Spain) substantially antedate other Iberian trilobites, according to widely published correlation schemes, and arguably would be among the earliest trilobites globally. These trilobites, previously only briefly mentioned in texts, are here described and illustrated, and their biochronological context examined. The Pusa Formation trilobites are treated in open nomenclature but with suggested affinity to the genusAbadiella.They are associated with small shelly fossils, includingPelagiellasp., chancellorid spicules andCupithecasp., and unidentfied archaeocyathans. Trace fossils from the upper unit of the Pusa Formation, down-section of the trilobites, includeDactyloiditesisp. andRusophycusisp., the latter representing the lowest occurrence of this ichnogenus in the region. This biostratigraphical context demonstrates that the Pusa Formation trilobites are substantially younger than had traditionally been thought. In terms of Iberian regional stages they are Ovetian, not Corduban as previously thought. As a consequence of the data presented here, the definitions of Iberian Lower Cambrian regional stages are discussed and a substantially revised correlation between key Lower Cambrian strata of the Central Iberian and Ossa Morena zones is proposed.

Hyolithids are a group of Paleozoic lophotrochozoans with a four-pieced skeleton consisting of a conch, an operculum, and a pair of lateral ‘spines' named helens. Both the conch and operculum are relatively well known and, to a certain extent, have modern analogues in other lophotrochozoan groups. The helens, on the other hand, are less well known and do not have clear modern analogues. This has hindered the knowledge of the complete morphology of the hyolithid skeleton, as well as other aspects of hyolithid biology, such as the organization of soft parts, and their ability to move. The material studied herein, consisting of disarticulated skeletal elements from the Silurian of Gotland, Sweden, illustrates a complete developmental sequence of a hyolithid species and includes the first complete, three-dimensionally preserved helens. Our material confirms that helens were massive skeletal elements, whose growth started proximally with the deposition of a central, coherent lamella. Further shell accretion took place around this lamella, but followed a particular accretion pattern probably constrained by the presence of marginal muscle attachment sites on the proximal-most portion of the helens. These muscle attachment sites were ideally located to allow a wide range of movements for the helens, suggesting that hyolithids may have been relatively mobile organisms.

The area of western Montes de Toledo to Guadalupe boasts a thick succession of mainly siliciclastic sediment spanning terminal Ediacaran to lower Cambrian strata as indicated by a relatively sparse but diverse palaeontological record. A terminal Ediacaran age is based on the occurrence of Cloudina in platform carbonates of the lower part of the Ibor group and in correlative levels of olistostromes at the base of the Río Huso group. Higher in the Río Huso group are found trace fossils which indicate a Cambrian age, notably Treptichnus bifurcus, which overlaps the local stratigraphic range of macroscopic carbonaceous disc-shaped fossils identified as Beltanelliformis. Strata underlying the Río Huso group contain treptichnids. The fossil record of the terminal Ediacaran-Lower Cambrian of this area is in part comparable to Cloudina-bearing sediments from other regions, in particular Namibia, where treptichnid trace fossils also overlap the range of Cloudina. The possibility of a wider biostratigraphic significance of this should be further tested, including its relation to the base of the Cambrian.