Trachyplax arctica, Larsson & Peel & Högström, 2009
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publication ID |
https://doi.org/ 10.4202/app.2009.0026 |
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persistent identifier |
https://treatment.plazi.org/id/30690C5B-FFA7-5966-3C6E-F9DD6E34F9DD |
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treatment provided by |
Felipe |
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scientific name |
Trachyplax arctica |
| status |
gen. et sp. nov. |
Trachyplax arctica gen. et sp. nov.
Figs. 2–5 View Fig .
Etymology: With reference to the arctic provenance of the material.
Type material: Holotype, MGUH 29087 View Materials from GGU sample 274907, sclerite type A. Figured paratypes, MGUH 29088–29102 View Materials from the same collection as holotype.
Type locality: North side of Paralleldal , central Peary Land, North Greenland (82 ° 17' N 31 ° 9.2' W) GoogleMaps .
Type horizon: Lower part of the Paralleldal Formation, late Early Cambrian (Toyonian; Cambrian Series 2, Stage 4).
Diagnosis.—As for genus.
Descriptions.—Eight calcareous sclerite types are recognised, of which type A, the most common ( Table 1 for numbers), is considered to be the taxonomic defining element. Individual sclerites are robust, ranging in maximum dimension up to 19.3 mm; their internal surfaces are poorly known, usually matrix covered. All eight morphologies display a raised apex or apices (as in type G, Fig. 2 View Fig ), marginal accretionary growth reflected by external concentric growth lines, radial ridges originating from the apex or apices and spreading over parts of or the entire external sclerite surface with new ridges being introduced between older ones ( Fig. 2 View Fig ); a spiral component is often visible in the ornamentation of larger sclerites. Enantiomorphic symmetry pairs are present in types B–D and G–H; these are arbitrarily interpreted as right and left morphs. Differences between left and right morphs are obvious within types B–D (accounted for in Table 1), but less clear for types G–H (therefore not specified in Table 1).
Type A ( Figs. 2 View Fig : type A, 3A–D; Tables 1, 2 for all measurements) consists of bilaterally symmetrical sclerites divided into shield and rostrum by a centrally placed apex; rate of growth is highest in shield. Five exceptionally large sclerites attain a shield width (Ws) of about 18 mm ( Fig. 3D). The shield is suboval with facetted margins, plate−like, and relatively smooth, apart from facetted concentric growth lines ( Fig. 3A 1, B 1 View Fig , C, D). Internal surfaces may show a radial groove pattern ( Fig. 3A 2, B 2 View Fig ). The rostrum is elongate, subrectangular, and widest most distal to the apex (Wr); it is covered by radial ridges, but growth lines are barely visible. In lateral view the sclerite is strongly arched with the shield and rostrum forming an apical angle (A) with each other ( Figs. 2 View Fig : type A, 3A 3) of about 120 ° ( Table 2).
Types B–C ( Figs. 2 View Fig : types B and C, 4A–D; Tables 1 and 3) are asymmetrical with left and right morphs. The sclerites can be divided through the apex into a smooth and a ridged portion. In type B ( Figs. 2 View Fig : type B, 4A, B) the smoother portion of the sclerite is convex with a crescentic outline ( Fig. 4A View Fig 1 View Fig , A 2 View Fig ); the ridged portion describes an undulating angular fan forming an arch on one side ( Fig. 4A View Fig 3). In type C ( Figs. 2 View Fig : type C, 4C, D) the smoother portion of the sclerite is narrow, slightly convex with a rounded outline ( Fig. 4C, D View Fig 1 View Fig ); the ridged portion describes an undulating fan forming an arch on one side ( Fig. 4D View Fig 2 View Fig ).
Type D sclerites ( Figs. 2 View Fig : type D, 4E, F; Tables 1 and 3) are asymmetrical, with a fan−like outline with both left and right morphs and concentric growth lines; the apex is situated about halfway along the straight side ( Fig. 4E View Fig 1 View Fig , F) and ornament consists of radial ridges on an undulating fan which forms an arch on one side ( Fig. 4E View Fig 2 –E View Fig 4 View Fig ).
Type E ( Figs. 2 View Fig : type E, 4G; Tables 1, 3) is bilaterally symmetrical with a fan−like outline displaying radial ridges and concentric growth lines; the apex is situated halfway along the relatively smooth vaulted side (line of symmetry runs perpendicular to the straight side from apex to sclerite edge) ( Fig. 4G View Fig 1 View Fig , G 2 View Fig ). The sclerite is also strongly concave along the radial ridges ( Fig. 4G View Fig 3).
Type F ( Figs. 2 View Fig : type F, 5A, B; Tables 1, 3) is subpyramidal with radial ridges present on their entire surface; the ridges may be twisted in the apical region on larger sclerites ( Fig. 5B View Fig ). Some specimens have a prominent radial fold running from the apex to the sclerite margin ( Fig. 5A View Fig 1 View Fig ).
Type G ( Figs. 2 View Fig : type G, 5C, D; Tables 1, 3) is asymmetrical with left and right morphs. It is a subhemispherical, convex sclerite interpreted as a fusion of two plates ( Fig. 5C, D View Fig 1 View Fig ).
LARSSON ET AL.—CAMBRIAN MULTIPLATED FOSSIL FROM GREENLAND 517
5 mm
Two apices are placed along the steep, narrow, more or less straight side, radial ridges run from each apex to the sub−apical margin ( Fig. 5D View Fig 2 View Fig ). Concentric growth lines are only visible outside the apex interarea of the smoother convex portion of the sclerite.
Type H ( Figs. 2 View Fig : type H, 5E; Tables 1, 3) is convex, with a subtriangular outline. The apex is placed over the corner opposite the concave base, and the surface between the base and the apex is slightly depressed ( Fig. 5E View Fig 3). Radial ridges are present on the two remaining sides ( Fig. 5E View Fig 1 View Fig , E 2 View Fig ). The apex may be somewhat beaked in some sclerites ( Fig. 5E View Fig 3).
Stratigraphic and geographic range.— Trachyplax arctica is known currently only from the Paralleldal Formation (late Early Cambrian; Toyonian; Cambrian Series 2, Stage 4) of Paralleldal, Peary Land, North Greenland.
No known copyright restrictions apply. See Agosti, D., Egloff, W., 2009. Taxonomic information exchange and copyright: the Plazi approach. BMC Research Notes 2009, 2:53 for further explanation.