Hyptiotheca karraculum Bengtson, 1990a

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 : 560-562

publication ID	https://doi.org/ 10.5281/zenodo.13522197
persistent identifier	https://treatment.plazi.org/id/03F8878E-FF9B-FF96-FF94-FA9DFD51FEA9
treatment provided by	Felipe
scientific name	Hyptiotheca karraculum Bengtson, 1990a
status

Treatment

Figs. 4E–I View Fig , 6A, B View Fig .

Hyptiotheca karraculum Bengtson, 1990a: 228 , figs. 152–155. part. Insolitotheca communis (Billings, 1972) ; Landing and Bartowski

1996: 758, figs. 7.18, 7.19, 7.21, 7.22, non 7.14–7.17, 7.20. Hyptiotheca karraculum Bengtson, 1990a ; Demidenko 2001: 102, pl.

10: 3–6. Hyptiotheca karraculum Bengtson, 1990a ; Wrona 2003: 197, figs. 9A,

B, 10A–E.

Holotype: South Australian Museum , Adelaide SAMP30902 .

Material.—MGUH 3522 ( Fig. 6A View Fig = Poulsen 1932: pl. 2: 12), MGUH 27096–27100, MGUH 27102 and 115 additional specimens from GGU samples 314804, 314835, 314918 and 314919.

Diagnosis.—As for genus.

Description.—Operculum of generally elliptical shape with conical shield only slightly more curved than cardinal shield along the edge; conical shield gently curved with low profile grading into indistinct summit; furrows and rooflets well defined though furrows shallow; cardinal shield may intersect conical shield at angle of about 20°, or it may grade smoothly without any change in angle into cardinal shield, such that operculum when viewed from lateral edge has profile of gently curving line; surface of operculum with widely spaced concentric grooves.

Interior of operculum has thickened central region that has a distinct sharp boundary with edge of the operculum; shallow depressions that mark the sites of the rooflets on the exterior may be present only along the edges of the operculum in the thinner region, or they may be present only in the thicker region, or they may be lacking entirely; interior of operculum smooth and apparently lacking any evidence of clavicles.

Remarks.—This taxon is known from the Bastion Formation only from the operculum, but the generally fine preservation of most individuals confirms their affinity to Hyptiotheca . This taxon is highly unusual for a hyolithid in two ways. First, the transition between cardinal and conical shields is smooth without any change in angle on some individuals ( Fig. 4E View Fig 2 View Fig ), whereas on others there may be an abrupt boundary between shields ( Fig. 4G View Fig 2 View Fig , H 1 View Fig ). The latter type of operculum with an angular break between shields is normal for hyolithids. Second, all opercula of this species from Greenland and Australia ( Bengtson 1990a: figs. 5I, 8B 1 –B 3) lack clavicles on the interior. The presence of these structures is one of the defining traits of the order Hyolithida ( Marek 1963) , and their absence certainly raises questions about the concept of that group, as well as the functional morphology and palaeobiology of this particular species. Perhaps the clavicles were not mineralized in Hyptiotheca , or their role in this taxon was assumed by some other structure. Possibly the mode of life of this taxon was different such that clavicles were not needed. This is the only species identified thus far in which clavicles are lacking, but given that opercula are far less common in the fossil record than hyolith conchs, the absence of clavicles may be more widespread than is generally realized. A final possibility is that Hyptiotheca karraculum may represent a short−lived experiment in the evolution of hyoliths, in which differences in mode of life and/or functional morphology ultimately proved to be only of limited adaptive value, and the animal quickly became extinct. A similar short−lived experiment in hyolith evolution may have been represented by Polylopia Clark, 1925 from the Middle Ordovician of Tennessee, which Yochelson (1968) suggested was a hyolith that adopted a vagrant mode of life, carrying its shell in the manner of a gastropod.

Conchs of H. karraculum are not known from Greenland but Bengtson (1990a) reported 15 conchs and nine opercula from Australia. The conch possesses a short ligula which is a diagnostic trait of the Hyolithida , but the strongly amblygonal nature of the aperture causes the dorsal and ventral rims of the aperture to be nearly the same height. Patholites Marek, Malinky, and Geyer, 1997 from the Middle Cambrian of Morocco is similar in terms of apertural morphology to Hyptiotheca , but its operculum remains unknown.

Conchs of H. karraculum have a distinct apical curvature toward the venter, which Bengtson (1990a) noted was unusual for a hyolith. He suggested that the animal lived with dorsum down against the sediment. While the function of ventral curvature is not well understood, it seems unlikely that the animal could survive in that orientation. The most stable orientation would have been with the flattened venter down, whereas with the dorsum against the sediment the animal probably would have tilted on its side or sank into the sediment, thereby causing suffocation. The ligula has been interpreted as a platform on which the animal could extend its soft tissue without fouling the inside of the conch with sediment. With the dorsum down, the ligula would have served as a roof over the animal’s head instead of as a platform under the head. Malinky and Sixt (1990) documented an assemblage of 76 hyoliths from the Lower Mississippian of Iowa in which some individuals curved dorsally, others ventrally and others either to the left or right when viewed from the dorsum. The function of such curvature is not understood, although it seems unlikely that within one species at one locality there would be such variability in mode of life. Probably apical curvature represents an adaptation to highly localized conditions in which the shell curved so that the animal could shift its center of buoyancy without sinking into the sediment.

Landing and Bartowski (1996) illustrated three opercula assigned to the ill−founded taxonomic wastebasket “ Insolitotheca ” communis (Billings, 1972) from the Browns Pond Formation of the Taconic Allochthon in New York State ( Landing and Bartowski 1996: figs. 7.18, 7.19, 7.21, 7.22). The poor preservation of the illustrated material makes the relationship between the conchs of “ I. ” communis and the alleged opercula difficult to address. The lack of a depressed marginal rim on the opercula was used to separate them from Hyptiotheca , but this feature appears to be the consequence of abrasion, and the specimens are here included in H. karraculum . Landing (1995) also reported an unequivocal occurrence of Hyptiotheca from Nova Scotia. Opercula that bear some resemblance to Hyptiotheca were illustrated by Poulsen (1932: pl. 2:12, as Hyolithus ( Orthotheca ?) communis Billings, 1872 ); by Missarzhevsky (1969: pl. 11: 4), as Allatheca sp. , and later one under Allatheca? concinna ( Missarzhevsky, 1989: pl. 3: 12). Sokolov and Zhuravleva (1983: pl. 22: 7b) also illustrated a probable specimen of Hyptiotheca that they referred to Majatheca tumefacta Missarzhevsky, 1969 while Qian (1989) illustrated an operculum (Qian 1989: pl. 19: 1) that bears limited resemblance to Hyptiotheca . These specimens are not formally re−assigned until their morphology becomes better known. These possible occurrences would extend the geographic range of this taxon to Siberia and the stratigraphic range to the D. lenaicus – M. tumfacta Zone. The large size of the operculum recorded by Poulsen (1932) suggests that it represents an adult or at least a much later ontogenetic stage than the smaller opercula reported herein. Poulsen’s operculum strongly supports the notion that the amount of ontogenetic change in this species is minimal.