Hyolitha, Marek, 1963

Malinky, John M. & Skovsted, Christian B., 2004, Hyoliths and small shelly fossils from the Lower Cambrian of North-East Greenland, Acta Palaeontologica Polonica 49 (4), pp. 551-578 : 562-564

publication ID	https://doi.org/ 10.5281/zenodo.13522197
persistent identifier	https://treatment.plazi.org/id/03F8878E-FF99-FF90-FF94-FE1BFB97F91C
treatment provided by	Felipe
scientific name	Hyolitha
status

Treatment

Class Hyolitha incertae sedis

Operculum A

Fig. 6C–E View Fig .

Material.—MGUH 27103–27105 and 67 additional specimens from GGU samples 314807, 314835, 314904, 314906, 314908, 314910, and 314933.

Description.—Operculum of either sub−circular ( Fig. 6D View Fig ) to rounded trapezoidal outline ( Fig. 6E View Fig ) with the dorsal edge approximately three times the width of the ventral edge; ventral edge is slightly elevated above dorsal part, but otherwise there is no indication of clear dorso−ventral differentiation; faint rounded summit is offset toward ventral edge but exact location uncertain. Exterior has concentric ribs that are best developed along ventral edge but yet remain visible around entire surface. Interior with prominent extended cardinal processes with rounded summit and steeply dipping sides; processes are slightly higher than clavicles and diverge at an angle of approximately 50°; extending in ventral direction from cardinal processes are flattened, blade−like clavicles with steeply dipping sides although the sides facing the middle of the operculum dip more gently than those on the opposite side. The interior appears to be smooth, lacking any indication of muscle scars or ornament.

Remarks.—This taxon is represented by opercula with similar clavicles on the interior but with a slightly differing outline. These opercula are unusual in that they possess certain traits associated with the Hyolithida but lack others thought to be critical in defining and recognizing that group. The structures that resemble clavicles and cardinal processes are elements that partly define the Hyolithida , but the lack of any dorso−ventral differentiation and the rooflets and furrows that are associated with the boundaries between these sides are lacking. This trait is a common feature among the Orthothecida Marek, 1966 . Hyolithes communis Billings, 1872 from the Lower Cambrian of New York has a tubular conch, but its operculum, which otherwise resembles the Greenland taxon in some aspects, can be subdivided into dorsal and ventral portions, and it has demonstrable rooflets and furrows, even though they are faint and not well−developed. The conch of Operculum A is unknown. Given that only the operculum is available, these individuals are best left in open nomenclature, although retained provisionally under Hyolithida .

Stratigraphic range and distribution.—Lower Cambrian, North−East Greenland.

Operculum B

Fig. 7 View Fig .

Material.—MGUH 27106–27107 and 16 additional specimens from GGU samples 314809, 314836, 314901, 314906, 314908, and 314933.

Description.—Nearly circular operculum with summit located two fifths of the distance from the dorsal edge; surface of exterior with apparent faint radial lines at least in some places; interior has large, rod−like, straight and widely diverging clavicles joined at base, that separate from each other at angle of nearly 60°; near proximal end nearest cardinal processes, processes diverge at angle of approximately 45°; clavicles are low, but the upper surface curves steeply upward to terminate in blunt point oriented toward ventral edge; beyond termination the edge of clavicles drops vertically toward interior surface of operculum. When viewed from above, clavicles have a massive compacted appearance with widest portion at proximal end, progressively thinning toward distal termination; surfaces of clavicles that face each other dip more steeply than those facing away; clavicles form slightly elevated ridge that connects the two at the distal termination.

Remarks.—This taxon is known from 18 specimens, which are here left in open nomenclature because the conch is unknown. The morphology of this species blurs the distinction between Hyolithida and Orthothecida owing to the lack of dorso−ventral differentiation of the exterior, as is seen among many orthothecids, yet it possesses clavicles on the interior which normally characterize hyolithids (but see Hyptiotheca ). A nearly circular operculum such as this would ordinarily be an ideal candidate for inclusion under one of the many supposed orthothecid hyolith taxa based on tubular conchs from the Lower Cambrian, such as Turcutheca Missarzhevsky, 1969 from Siberia. To ascribe this operculum to a taxon such as that implies a seemingly unusual functional morphology for this animal. Perhaps it was from such an animal that hyolithids and orthothecids differentiated into the two better defined groups seen later in the Palaeozoic. However, given the absence of a direct association of conch and operculum, it is unwise at this time to place this operculum in any established taxon, although it is provisionally included under order Hyolithida .

Stratigraphic range and distribution.—Lower Cambrian, North−East Greenland.

“ Hyolithus (Hyolithus) americanus Billings, 1872 ” Fig. 8A–D View Fig .

Hyolithus (Hyolithus) americanus Billings ; Poulsen 1932: 21, pl. 3:

7–11.

Material.— MGUH 3531 ( Fig. 8A View Fig = Poulsen 1932: pl. 3: 9); MGUH 3530 ( Fig. 8B View Fig = Poulsen 1932: pl. 3: 8), MGUH

3526 ( Fig. 8C View Fig = Poulsen 1932: pl. 3: 10), MGUH 3529 ( Fig. 8D View Fig = Poulsen 1932: pl. 3: 7).

Description.—Conch seemingly orthoconic with small apical angle; venter flat or slightly raised along mid−line; ligula short and rounded at anterior edge with gently sloping sides; all else unknown.

Remarks.—This species was listed and illustrated by Poulsen (1932) but not described, other than to say that some specimens show traces of longitudinal lines on the venter. Specimens MGUH 3531 ( Fig. 8A View Fig ) and MGUH 3530 ( Fig. 8B View Fig ) retain faint, low longitudinal ribs on the venter which are probably the lines to which Poulsen (1932) referred. The original designation of Hyolithes americanus is Theca triangularis Hall, 1847 from the Lower Cambrian of New York. It was reassigned when the name Theca fell out of favour in the 19th century, abetted by Barrande’s (1867) promotion of the name Hyolithes . The species’ name americanus was substituted when Billings (1872) thought that the name triangularis was preoccupied by another hyolith species, Theca triangulare Portlock, 1843 from Ireland, but the Irish species subsequently was recognized as a cephalopod, and reassigned to Orthoceras . Hyolithes americanus was revised by Malinky (1989) and shown to be an unrecognizable taxon, because preservation of the surviving specimens, as casts in sandstone, is incomplete. The Greenland species cannot be compared in detail to the much better preserved specimens assigned to H. americanus by Shaw (1955) from the Lower Cambrian of Vermont. There is limited resemblance in conch form between the Greenland specimens and two species of the Siberian genus Burithes Missarzhevsky, 1969 . Fig. 8A and B View Fig resemble B. erum Missarzhevsky, 1969 , and the others are somewhat similar to B. elongatus Missarzhevsky, 1969 . Burithes is based on internal moulds (Malinky unpublished), thus comparison between the two is limited.

Stratigraphic range and distribution.—Lower Cambrian, North−East Greenland.

“ Hyolithus (Hyolithus) mutatus Poulsen, 1932 ”

Fig. 8E View Fig .

Hyolithus (Hyolithus) mutatus Poulsen, 1932: 23 , pl. 3: 12–17.

Material.— MGUH 3534.

Description.—Cyrtoconic conch with apical curvature toward dorsum; conch has narrow tapering appearance due to low apical angle; venter seemingly flat, grading into sharp lateral edges; flanks of dorsum inflated and rounded, intersecting to create rounded central ridge; ligula apparently short and rounded on anterior edge with gently dipping sides; cross−section triangular; shell on dorsum and venter with faint, closely spaced transverse lines; internal mould of dorsum with widely and irregularly spaced transverse rugae.

Remarks.—This species is known from one internal mould of a large conch with a minute fragment of shell adhering to the dorsum, and shell on the venter, but only the apertural region of the venter is free of matrix. This individual is distinctive because it is much larger than Poulsen’s other material and the new material documented herein. It also has a strong apical curvature toward the dorsum, a feature widely shared among Ordovician hyoliths of Baltica but rarely seen among other individuals.

The few longitudinal elements of sculpture seen on the shell suggest resemblance to Lovenedolithes araneus ( Holm, 1893; see Berg−Madsen and Malinky 1999) from the Middle Cambrian of Sweden, and to L. groenwalli ( Poulsen, 1967) from the Lower Cambrian of Denmark. Both species of Lovenedolithes possess much smaller conchs than does H. (H.) mutatus Poulsen, 1932 , assuming that Lovenedolithes is represented by normal−sized adults. Doescherina clarkei Malinky, 1989 from the Middle Cambrian of Montana likewise has longitudinal elements of sculpture, but it too is smaller and as with both species of Lovenedolithes , the apical angles of the conchs are much larger. Owing to generally incomplete preservation, H. (H.) mutatus Poulsen, 1932 cannot be further assigned to genus.

Stratigraphic range and distribution.—Lower Cambrian, North−East Greenland.

“ Hyolithus (Hyolithus) sp.”

Fig. 8F View Fig .

Hyolithus (Hyolithus) sp.; Poulsen 1932: 23, pl. 4: 2–5.

Material.— MGUH 3536.

Description.—Orthoconic conch with small apical angle and therefore narrow appearance; internal mould of venter seemingly flat, grading in tightly rounded, blunt lateral edges; dorsum high with tightly rounded median ridge; flanks adjacent to it also inflated to create high triangular cross−section. Surface of internal mold smooth; all else unknown.

Remarks.—This taxon is known from one incompletely preserved internal mould which lacks any features that could reasonably be used to identify it to genus or species. Even its affinity to the Hyolithida is open to question because no trace of a ligula or any other apertural features remain. The triangular cross−section seen in this species is a common feature among hyolithids, but it is seen even among some orthothecids, such as Novitatus tarynicus Syssoiev, 1972 . However, no good purpose is served in transferring this form to Orthothecida owing to incomplete preservation, and the species is retained under its original designation.

Stratigraphic range and distribution.—Lower Cambrian, North−East Greenland.