The Hyoliths are an enigmatic group of shelled invertebrates known from
the Earliest Cambrian until the End Permian Extinction. They had conical
shells with opercula (lids), and sometimes a pair of curved horns
called 'helens'. The exact nature of Hyoliths was for a long time
considered a mystery, with most palaeontologists considering them to be
either a form of Mollusc or an extinct phylum of animals of unknown
affinities. However, recent studies of Hyoliths with preserved soft
tissues and shell microstrucure has led to the conclusion that they were
lophophorate animals closely related to Brachiopods.
In a paper published in the Journal of Paleontology on 2 April 2020, John Peel and Sebastian Willman of the Department of Earth Sciences at Uppsala University and Steven Hageman of the Department of Geological and Environmental Sciences at Appalachian State University, describe a series of Hyloliths from the Early Cambrian Murray Shale of Chilhowee Mountain, Tennessee.
Outcrops of Laurentian Early Cambrian (Cambrian Series 2; roughly 521 to 509 million years ago) strata extend from Alabama and Tennessee to North-East Greenland along the Eastern Seaboard of North America. Traditionally, their age has been determined by occurrences of trilobites indicative of the Dyeran Stage of North American usage (Cambrian Stage 4; roughly 514 to 509 millio years ago), with widespread records of Olenellus and related Trilobites in the literature. By contrast, extensive faunas of older Cambrian Trilobite faunas indicative of the Montezuman Stage (Cambrian Stage 3; roughly 521 to 514 million years ago) are well known from the opposite side of Laurentia, in the western United States. However, fossiliferous Avalonian successions in Rhode Island and Massachusetts, New Brunswick, Nova Scotia, and eastern Newfoundland comprising the Terreneuvian (Cambrian Series 1; roughly 541 to 521 million years ago) and Cambrian Series 2 to Lower Ordovician, are juxtaposed against the Laurentian of the Eastern Seaboard of present-day North America.
Localities and stratigraphy. (1) Peary Land region of North Greenland showing outcrops of the Buen Formation (black) and localities with Montezuman Stage fossil assemblages; (2) Eastern Seaboard of North America with Greenland displaced southward to its approximate position in the Cambrian. NEG indicates Dyeran occurrences in North-East Greenland. (M), (NB + CBI), and (N) locate Avalonian successions in Massachusetts, New Brunswick and Cape Breton Island, and eastern Newfoundland, respectively; (3) early Cambrian stratigraphy in southern Peary Land showing derivation of fossil assemblages with the Buen Formation; (4) early Cambrian stratigraphy at Chilhowee Mountain, Tennessee, indicating location of fossiliferous samples (F) in the Murray Shale (Montezuman Stage) and the established Dyeran Stage faunas of the Shady Dolomite in Virginia. Peel et al. (2020).
The recent description of the Nevadioid Trilobite Buenellus from the upper Murray Shale of Chilhowee Mountain, eastern Tennessee, is significant in providing evidence of the Montezuman Stage in the Laurentian terrane of the eastern United States. Buenellus is otherwise known only from its type locality in the Sirius Passet Lagerstätte of Peary Land, North Greenland.
The Sirius Passet Lagerstätte is the oldest known fossil assemblage within a succession of Montezuman–early Dyeran (Cambrian Stages 3–4) age assigned to the Buen Formation. The Lagerstätte is known only from a single locality, but the siliciclastic sediments of the Buen Formation are otherwise widely distributed in eastern North Greenland. The Buen Formation lies within the transarctic Innuitian Orogen, facing the Arctic Ocean. By contrast, the Murray Shale at Chilhowee Mountain, and other Laurentian outcrops along the Eastern Seaboard between Alabama and North-East Greenland accumulated along the shore of the former Iapetus Ocean. In Cambrian times, however, Laurentia occupied a tropical position, and this Iapetan margin faced to the south.
About 35 Hyolithid specimens preserved in pale buff weathering shale were examined from locality CM3 on Chilhowee Mountain where they occur together with Buenellus chilhoweensis in the upper Murray Shale. The specimens are crushed, but not completely flattened, and some are preserved as external and internal molds. Conchs dominate but most are broken. Opercula occur as isolated fossils, and in rare partially articulated associations with conchs Rare broken fragments of the paired appendages (helens) have been observed as isolated fossils.
The specimens recovered were sorted into four species, one of which was tentatively placed in the genus Burithes, the other three being of uncertain affinities and are instead numbered. None of the material is considered well enough preserved to name a species from.
The species placed in the genus Burithes has a conch with incremental angle 20°–25° and ligula about one-sixth of total length, with a slight longitudinal curvature such that the ventral surface may have been shallowly convex. The width of the ligula is about half its length, but the length increases proportionately with growth Shallow lateral sinuses for the likely passage of helens lie on the dorsal side of the angular transition from the convex dorsal surface to the almost flat ventral surface. The dorsal surface is seemingly uniformly shallowly convex, but the degree of inflation uncertain due to crushing. Ornamentation on the ventral surface consists of fine comarginal growth lines with occasional growth halts that may appear periodic. Ornamentation on the dorsal surface is poorly known, seemingly almost orthocline. Operculum and helens not certainly known, but associated isolated opercula are wider than long, supporting the interpretation that the dorsal surface of the conch was not strongly inflated.
Burithes? sp. Hyolith conchs from the Murray Shale, Chilhowee Mountain, Tennessee. (1) PMU 35718, ventral surface; (2) PMU 35719, ventral surface; (3) PMU 35720, obliquely crushed ventral surface; (4) PMU 35721, central surface, above, with external mold of dorsal surface, below; (6) PMU 35722, crushed, with dorsal surface overlying internal surface of ventral surface with ligula; (7) PMU 35723, ventral surface with characteristic fractures; (8) PMU 35724, ventral surface. Peel et al. (2020).
All specimens are compressed, although a degree of separation between the dorsal and ventral surfaces may be maintained. Crushing has often produced Y–shaped cracks that extend from the ligual margin down the median line as a raised, irregular ridge or angulation to near the apex. The median fracture is commonly expressed as a ridge on the ventral surface, but this is a preservational artifact. However, a broad, rounded ridge in some specimens likely represents compaction around a solid object within the conch interior, possibly an early mineralized burrow or sediment-infilled section of gut.
In terms of its overall shape, the Murray Shale conchs are similar to Burithes erum from the Tommotian of the Anabar Massif of Siberia. Nevadotheca whitei from the Pioche Shale (Cambrian Series 2) of Nevada, the type species of Nevadotheca, differs in having a high, inflated dorsum and narrowly rounded lateral margins. This is also the case in Nevadotheca boerglumensis and Kalaallitia myliuserichseni, described from the early Olenellus Biozone (Dyeran, Cambrian Stage 4) of the Buen Formation of southern Peary Land, North Greenland, but Kalaallitia is distinguished by its fine longitudinal lirae and longer ligula. Opercula referred to these Peary Land species are proportionately longer than Murray Shale specimens, suggesting that the dorsal surface of their conchs was more strongly inflated than in Burithes? sp.
Some researchers have noted some similarity between specimens from the Dyeran of North-East Greenland and Burithes, but the acute dorsum of their material suggested assignment to Grantitheca. In contrast to material from the Murray Shale and Buen Formation, hyoliths from North-East Greenland are preserved mainly as internal molds in limestone or as phosphatic residues from limestones.
The first numbered species 'Hyolithid sp. 1' is known from the illustrated specimens and two additional fragments, is characterized by two or three prominent transverse folds or corrugations on the adapertural part of the shallowly convex ventral surface of the conch and ligula. The incremental angle is about 30°, and the length of the ligula is slightly more than half its width. Lateral sinuses are present at the transition from the shallowly convex ventral surface to the dorsal surface, but the degree of inflation of the latter is not known. Ornamentation consists of fine comarginal growth lines, although these are more strongly developed at the preserved aperture.
Hyolithid sp. 1, PMU 35725: (5) external and (9) internal molds showing prominent corrugation. Peel et al. (2020).
Corrugation of the latest growth stage of the ventral surface and ligula is common in hyolithids, but not with the high degree of emphasis seen in the Murray Shale specimen. Examples have been illustrated in specimens from Siberia referred to Trapezovitus sinscus and Burithes cuneatus, in Nitoricornus wushiensis from the lower Cambrian of Xinjiang, China, and in specimens from New Brunswick.
The second unnamed species, 'Hyolithid sp. 2' is described from three overlapping incomplete specimens in ventral aspect, which Peel et al. suggest may form part of a gut fill or coprolite. One of the specimens preserves the operculum in place. The incremental angle is about 20°, and the ligula is short. The ventral surface is shallowly convex, but the lateral edges are rounded and delimited on their axial edge by a shallow longitudinal furrow. The dorsal surface is not known, but lateral sinuses seem to be well developed The impression of the dorsal exterior of the operculum shows comarginal growth lines and a suggestion of a radial furrow. While similar in shape to other opercula from the Murray Shale, it is too poorly preserved for closer comparison.
Hyolithid sp. 2; (11), (12) PMU 35727 with three fragmentary specimens in ventral aspect, probably within a gut fill, coprolite, or burrow; arrow in (12) locates external mold of operculum shown in detail in (11); (13) PMU 35728, external mold of ventral surface. Scale bars 2 mm.
The third unnamed species, 'Hyolithid sp. 3', is described from a single poorly preserved specimen which has an incremental angle of 15° and appears to have an oxygonal aperture. Adaperturally, shallowly convex comarginal growth lines on the ventral surface are cord-like and laterally discontinuous, resembling the pattern seen in Nitoricornus danianum and ?Crestjahitus danianus from New Brunswick, Canada, whereas the dorsal surface is ornamented with barely discernible fine growth lines.
Hyolithid sp. 3, PMU 35726, dorsal view. Peel et al. (2020).
Three poorly preserved specimens of partially articulated Hyolithid skeletons occur, but several isolated opercula are known. The length of the best-preserved specimen is about four-fifths of its width. Its conical shield is hemispherical in plan view, and the summit lies at about one-quarter of the distance from the adapical margin to the adapertural margin. The folds separating the conical and cardinal surfaces delimit an angle of about 130° in plan view In lateral perspective, the cardinal shield rises high above the summit of the operculum, with the inclination of its adapical margin suggesting that the conch had an amblyogonal margin. The cardinal area is ornamented with radial ridges transverse to the comarginal growth lines present over the entire conch. The latter are most conspicuous on the conical shield where radial ornamentation consists of fine lines.
Hyolithid opercula from the Murray
Shale, Chilhowee Mountain, Tennessee. (1)–(3) PMU 35729: (1) oblique
apertural, (2) dorso-lateral, and (3) dorsal views. (4) PMU 37730
external mold with arrow indicating lateral sinus for passage of helen;
(5)–(7) PMU 35731: (5) dorsal, (6) dorso-lateral, and (7) lateral views.
(1)–(3) Scale bars 1 mm; (4)–(7) scale bars 2 mm. Peel et al. (2020).
A second specimen, preserved as an external mold, has a well-defined convex adapical border with prominent coarse ridges between the summit and the adapical margin. A broad shallow sinus in the margin marks the exit point for the helen. A third specimen has a more elliptical shape than the other two specimens and develops a series of short comarginal rugae located medially on the conical shield.
The cardinal surface is proportionately longer in opercula of Nevadotheca boerglumensis and Kalaallitia myliuserichseni from the early Olenellus Biozone at Brillesø, southern Peary Land, and the folds separating the conical and cardinal surfaces delimit an angle of about 90°, much smaller than in the specimens from the Murray Shale. In this respect, the opercula from the Murray Shale more closely resemble the opercula associated with articulated specimens from the Sirius Passet Lagerstätte, but these are too poorly preserved for close comparison. A similar wide angle is seen in Nitoricornus danianum illustrated from New Brunswick.
Articulated Hyolithids from the Sirius Passet Lagerstätte, Buen Formation, Peary Land, North Greenland. (1) MGUH 29260; (2), (4) MGUH 29691: (2) ventral surface with operculum displaced and inverted to show conical surface; (4) enlarged view. (3) MGUH 29258, ventral surface. (1)–(3) Scale bars 2 mm; (4) scale bar 1 mm. Peel et al. (2020)
The Murray Shale Hyolithids are the oldest Hyoliths known from present–day eastern Laurentia but not from eastern North America. Hyoliths considered to be of Terreneuvian and younger age have been reported from Avalonian terranes, although some researchers have referred some of this material to the Montezuman, citing the occurrence of Aimitus and Notabilitus.
Previous work has described Hyolithids and Orthothecids from the Sirius Passet Lagertstätte occurring together with Buenellus higginsi. Orthothecids have not been recognized from the Murray Shale. Although several Hyolithid specimens from the Sirius Passet Lagerstätte preserve the operculum and paired helens together with the conch, their poor state of preservation precludes more precise identification. However, opercula from the Murray Shale are similar in shape to a Sirius Passet operculum and unlike those occurring in the Dyeran Buen Assemblage 2 from southern Peary Land. Variation in the incremental angle of the conch suggests that several taxa may be present in the Sirius Passet articulated material. Some Sirius Passet specimens have an angular dorsum not seen in Murray Shale material and thereby resemble Grantitheca. The articulated specimens are associated with longitudinally ribbed conchs assigned to Trapezovitus and an Orthothecid, but specimens similar to these have not been observed from the Murray Shale.
The Bradoriid Arthropod Indota, represented by Indota tennesseensis in the Murray Shale, has not been described from the Sirius Passet Lagerstätte. Poorly preserved specimens from the upper Buen Formation (Dyeran Stage) have been tentatively referred to the genus, but the assignment has been questioned. Isoxys chilhoweanus from the Murray Shale is similar to Isoxys volucris, which is the most abundant fossil in the Sirius Passet Lagerstätte.
Two crushed hyolithids from Buen Assemblage 1 at Brillesø, southern Peary Land have been illustrated in association with the Nevadioid Trilibite Limniphacos perspicullum, of presumed Montezuman age. The rugose growth ornamentation of one of these is reminiscent of Hyolithid sp. 1 from the Murray Shale but is much less strongly expressed. Strata within the Buen Formation of probable Montezuman age occur in Hans Egede Land, eastern Peary Land; they are not well known, but unidentifiable hyolith fragments are associated with poorly preserved Trilobites.
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