Camenella reticulosa Conway Morris, 1990

Skovsted, Christian B., Balthasar, Uwe, Brock, Glenn A. & Paterson, John R., 2009, The tommotiid Camenella reticulosa from the early Cambrian of South Australia: Morphology, scleritome reconstruction, and phylogeny, Acta Palaeontologica Polonica 54 (3), pp. 525-540 : 529-533

publication ID	https://doi.org/ 10.4202/app.2008.0082
persistent identifier	https://treatment.plazi.org/id/C57B87C0-FFE9-FFC6-FCA2-F8F9E970F055
treatment provided by	Felipe
scientific name	Camenella reticulosa Conway Morris, 1990
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Treatment

Figs. 2–8 View Fig View Fig View Fig View Fig View Fig View Fig .

1990 Camenella reticulosa Conway Morris sp. nov.; Bengtson et al. 1990: 131, fig. 81, non. fig. 82. non 1995 Camenella cf. reticulosa Conway Morris, 1990 ; Landing

1995: fig. 7.24. non 2001 Camenella cf. reticulosa Conway Morris, 1990 ; Gravestock et al. 2001: pl. 8: 9.

Holotype: Sellate sclerite SAMP30666 ( Bengtson et al. 1990: fig. 81) from UNEL 1856 , Parara Limestone, Horse Gully, Stansbury Basin , South Australia.

Material.—30 mitral (16 sinistral, 14 dextral) and 31 sellate (16 sinistral, 15 dextral) sclerites from the Wilkawillina Limestone at Bunyeroo Gorge (Bun 1*, 3, 3*, 4, 4b), Wilkawillina Gorge (Wilk A, M, O, P, Q, R, S) and the MMF section ( MMF /0.0), 1 km south of Balcoracana Creek.

Emended diagnosis.—Species of Camenella with sclerites characterised by strong co−marginal ribs ornamented by nodes and a superimposed reticulate pattern. Inter rib areas smooth or with fine transverse growth lines. Sellate sclerite strongly asymmetrical with prominent sella separating large and small lobes. Large lobe with one or two poorly defined subsidiary folds and a weakly developed rib separating the lobe from the sella. Apex tightly coiled (up to 1.5 whorls) with duplicature adpressed to the inside of the sella. Mitral sclerite acutely pyramidal with obplicate and accrescent sides strongly developed. Both plicate and obplicate sides with three pronounced radial ridges separated by two deep folds.

Description.— Camenella reticulosa has two types of sclerites ( Fig. 2 View Fig ), one cone−shaped mitral ( Figs. 3 View Fig , 4 View Fig ) and one narrow, coiled sellate sclerite ( Figs. 5 View Fig , 6 View Fig ). Both sclerite types exhibit broad, strongly raised co−marginal ribs with a micro−ornament of nodes and a superimposed reticulate pattern ( Figs. 3E View Fig , 5A View Fig 3 View Fig , 6C View Fig 2 View Fig ; see also Bengtson et al. 1990: fig. 81e). Areas between the ribs are smooth or have faint co−marginal growth lines and occasional radial striae ( Fig. 6B View Fig 2, C 2 View Fig ). In the current collections, mitral and sellate sclerites are almost always found in direct association and out of the 61 sclerites in the total collection, 31 are sellates (51%) and 30 mitrals (49%). Both sclerite types are asymmetrical with respect to a longitudinal median plane, but occur as dextral and sinistral symmetry variants (D− and L−forms sensu Bengtson 1970), which are represented in approximately equal numbers (48% of sellates and 47% of mitrals are of the dextral variant).

Mitral sclerites.—Mitral sclerites are strongly asymmetrical cones, up to 2.5 mm long (measured from apex to the margin of the longest [obplicate] side), with a subquadrate cross−section and a slight to moderate helical twist (e.g., Figs. 3 View Fig , 4 View Fig ). Following Bengtson (1970: 367), the four sides of the cone are termed plicate, obplicate, accrescent and decrescent with the obplicate and accrescent sides more strongly developed than the opposing plicate and decrescent sides. The obplicate and plicate sides each bear three well defined radial ribs separated by deep, narrow folds ( Fig. 3A, B, C View Fig 1, D 1 View Fig , D 2 View Fig , E). On the obplicate side, the ribs are continuous from the apical region to the growing margin of the sclerite; they tend to be straight or gently curved, and the central rib is slightly lower than the marginal ribs ( Fig. 3A, B, C View Fig 1, D 1, E, F 1 View Fig ). The co−marginal ribs are continuous across the obplicate side, but the ribs are narrower and more widely separated in this region. The accrescent ( Fig. 3D View Fig 1 View Fig , E) and decrescent ( Fig. 3C View Fig 2, F 2 View Fig ) sides are uniformly curved and exhibit well developed, evenly spaced co−marginal ribs. The radial ribs and folds of the plicate side are initially identical to their counterparts on the obplicate side, but all larger specimens exhibit a marked shift in the growth vector of the ribs which occurred when the plicate side had attained a length of 0.4 to 0.8 mm ( Figs. 3F View Fig 2 View Fig , 4A View Fig 1, B 1 View Fig ). The ribs are deflected in the opposite sense of the curvature of the sclerite ( Figs. 3F View Fig 2 View Fig , 4A View Fig 1, B 1 View Fig ). This results in wavy ribs and folds on the plicate side, which forms a protruding lip extending beyond the adjacent accrescent side of the sclerite ( Fig. 4A View Fig 1 View Fig , A 3 View Fig ).

The aperture of the mitral sclerites covers about 50–75% of sclerite length (in apertural view) and is wide, deep and subquadrate in outline ( Fig. 4A, B View Fig 1 View Fig ). The internal surface mimics the various folds and ribs of the exterior surface, but is otherwise smooth or with faint co−marginal striae ( Fig. 4A View Fig 3 View Fig ). The apex tapers to a subcircular cap approximately 100 µm in diameter. A few specimens display a central circular perforation of the apical cap ( Figs. 3F View Fig , 4A 4 View Fig ), but in other specimens the cap is a simple, smooth surface ( Fig. 4B View Fig 3 View Fig ).

Sellate sclerites.—Sellate sclerites are strongly asymmetrical, up to 1.8 mm long, with a well defined sella dividing the sclerite into large and small lobes ( Fig. 5 View Fig ). The sella occupies about 50% of the sclerite width and its margin is marked by a weakly developed ridge on the large lobe that may slightly overhang the floor of the sella ( Fig. 5B View Fig 2 View Fig , E). The large lobe is approximately twice as large as the small lobe in length and width, and is substantially higher ( Fig. 5A View Fig 2, B 2, D 2 View Fig ). The long axis of the lobes may be parallel ( Fig. 5B View Fig 2 View Fig ) or form an angle of up to 45 ° ( Fig. 5A View Fig 2 View Fig , C). The large lobe often has one, occasionally two, poorly defined radial folds, and in one specimen the small lobe has an additional fold ( Fig. 5B View Fig 2 View Fig ). In some specimens, these folds are relatively deep, but are more often only visible as a slight deflection of the co−marginal ribs. The co−marginal ribs are expressed over the entire dorsal surface of most sellate sclerites, but width and height of the ribs are reduced on the sella ( Fig. 5A View Fig 1 View Fig , A 2 View Fig ). Two large specimens seem to have only very weakly developed ribs in the central portion of the sella ( Figs. 5E View Fig , 7A View Fig ). The ventral surface of the sclerite is strongly adpressed to the opposing wall and forms a smooth or weakly ornamented duplicature ( Fig. 6A, B View Fig 1 View Fig , D). A narrow opening separates the duplicature from the underlying wall under the large lobe, but the duplicature appears to be completely adpressed to the opposing shell surface over the entire width of the sella. The duplicature extends across most of the ventral surface except for a narrow region along the growing margin of the sclerite ( Fig. 6B View Fig 1 View Fig ). This region is slightly wider on the ventral surface of the lobes, but never exceeds 0.4 mm in width. The duplicature bears only vestiges of co−marginal ribs, but these occasionally preserve a micro−ornament identical to the ribs of the dorsal surface ( Fig. 6B View Fig 2 View Fig ). The apex is coiled through up to 1.5 whorls and the initial growth stages are consequently difficult to examine in detail ( Figs. 5B View Fig 1, D 1 View Fig , 6C View Fig 1 View Fig ). Some specimens have lost the apical region by fracturing parallel to the co−marginal ribs of the dorsal shell surface.

Discussion.—The sellate and mitral sclerites from the Arrowie Basin are considered conspecific based on the identical surface ornament of coarse co−marginal ridges with nodes and superimposed reticulation, a feature which is shared with the holotype of the species. As in other species of Camenella , the distribution of the two sclerite types in the Arrowie Basin is virtually identical and whenever one type of sclerite occurs, the other is also present. The size range and total number of sclerites is also similar (31 sellates [51%] and 30 mitrals [49%]).

Camenella reticulosa was originally described by Conway Morris (in Bengtson et al. 1990) on the basis of six specimens from Parara Limestone at Horse Gully and Kulpara in the Stansbury Basin. One sellate (holotype; Bengtson et al. 1990: fig. 81) and two supposedly mitral ( Bengtson et al. 1990: fig. 82) sclerites were illustrated, but the latter show little resemblance to the well preserved mitral sclerites documented herein, and probably represent poorly preserved sclerites of Dailyatia . Similarly, the single specimen from Horse Gully referred to C. cf. reticulosa by Demidenko (in Gravestock et al. 2001: pl. 8: 9) also appears to represent an incomplete sclerite of Dailyatia . Landing (1995: table 2) referred four specimens from the Placentian Series of Nova Scotia to Camenella cf. reticulosa . These specimens were not described and only a single specimen was illustrated ( Landing 1995: fig. 7.24), but its relationship to Camenella is not clear.

Camenella reticulosa can be distinguished from all other species in the genus by the characteristic ornament of strong co−marginal ribs with nodes and superimposed reticulation. In addition, the coiled apex of the sellate sclerite and the presence of three radial ribs and intermittent folds on both plicate and obplicate sides of the mitral sclerite are unique characters of this species. Further, the mitral sclerites are distinguished from those of C. baltica and C. parilobata by the helical twist (compare Figs. 3 View Fig and 4 View Fig to Bengtson 1970: figs. 1, 7, 8 and Bengtson 1986: figs. 2–4) and from “ Tommotia ” kozlowski and “ Tommotia ” admiranda by the lack of radial elements of ornamentation beyond the three plicate and obplicate ribs (compare Figs. 3 View Fig and 4 View Fig to Rozanov et al. 1969: pl. 5: 2, 3, 9, 17, and Matthews and Missarzhevsky 1973: pl. 3: 14, and Missarzhevsky 1989: pl. 16: figs. 1–8). The sellate sclerites differ from their counterparts in C. parilobata by the larger size difference between large and small lobes (compare Figs. 5 View Fig and 6 View Fig to Bengtson 1986: fig. 1) and from C. garbovskae and C. baltica by the deeper and wider sella separating the lobes (compare Figs. 5 View Fig and 6 View Fig to Rozanov et al. 1969: pl. 5: 1, 6, 8, and Bengtson 1970: 2, 10). Camenella reticulosa also differ from Tommotiid gen. et sp. nov. described by Skovsted and Brock (in Paterson et al. 2007) by the restriction of the nodose ornament to the ribs and by the well developed asymmetry and radial folds of the mitral sclerite.

The available collection of sellate and mitral sclerites suggests that both sclerite types of C. reticulosa are somewhat less variable than sclerites of the other well documented species, C. baltica and C. parilobata . The sellate sclerites differ mainly in the development of the sella, its relative width and height compared to the lateral lobes, but never to the extreme degree seen in the other two species (compare Figs. 5 View Fig and 6 View Fig to Bengtson 1986: fig. 1). The difference is even more pronounced when the mitral sclerites are compared. Both C. baltica and C. parilobata includes straight pyramidal, asymmetrical and laterally compressed mitral sclerites, but in C. reticulosa the mitral sclerites differ mainly by degrees in helical twist and development of the radial ribs and folds (compare Figs. 3 View Fig and 4 View Fig to Bengtson 1986: figs. 2–5). A single, laterally compressed mitral sclerite of C. reticulosa ( Fig. 7B View Fig ) has been obliquely compressed from the obplicate/accrescent side, but still exhibits the characteristic three−fold radial ribs of the species. The obplicate side has developed a trough that is somewhat similar to the sella in associated sellates, but the presence of a pointed apex and well developed radial ribs and folds on the plicate side are clearly mitral characteristics. This unusual mitral sclerite of C. reticulosa represents a slightly deformed variety of the normal mitral sclerite and may be comparable to the planiform mitral sclerites of “ Tommotia ” plana, C. baltica and C. parilobata . Bengtson (1986) speculated that these simplified, laterally compressed sclerites formed an integrated part of the Camenella scleritome.

Another unusual mitral sclerite from the uppermost Wilkawillina Limestone at MMF/0.0 exhibits growth deformations in the form of a wavy ridge on the internal surface of the obplicate side of the sclerite ( Fig. 4B View Fig 1 View Fig , B 2 View Fig ). Shell growth was apparently halted when the sclerite had attained about 2/3 of its final length, and when growth resumed the obplicate ribs and folds were aborted and only reformed at a later stage. The plicate surface is also deformed and exhibits a deep longitudinal split. Stratigraphic and geographic range.—Upper lower Cambrian ( Abadiella huoi to Pararaia tatei Zones ), Stansbury Basin: Parara Limestone of Horse Gully and possibly Kulpara. Arrowie Basin: Wilkawillina Limestone in Bunyeroo Gorge, Wilkawillina Gorge, near Balcoracanna Gorge, Flinders Ranges, South Australia.