Hadrotreta taconica ( Walcott, 1887 )

Skovsted, C. B., Knight, I., Balthasar, U. & Boyce, W. D., 2017, Depth related brachiopod faunas from the lower Cambrian Forteau Formation of southern Labrador and western Newfoundland, Canada, Palaeontologia Electronica (Oxford, England: 1990) 17 (15), pp. 1-52 : 39-42

Hadrotreta taconica ( Walcott, 1887)

Figure 21

1887 Linnarssonia taconica Walcott , p. 189, 190, plate 1, figure 18–18d.

1912 Acrotreta sagittalis taconica (Walcott) . Walcott, p. 707, 708, plate 71, figure 1–1o (includes synonymy).

1956 Acrotreta taconica (Walcott) . Lochman, p. 1370, plate 3, figures 9–12.

2002 Linnarssonia taconica Walcott. Landing, Geyer, and Bartowski , p. 298, figure 7-1-7.9.

?2005 Vandalotreta sp. Skovsted and Holmer, p. 338, text-figure 2A-E.

Holotype. Ventral valve USNM 52163b, from lower Cambrian limestone south of North Granville, Washington Co. New York State, USA ( Walcott, 1887, plate 1, figure 18b; Walcott, 1912, plate 71, figure 1c).

Diagnosis. Species of Hadrotreta with ventribiconvex, sub-circular to sub-rectangular shell. Ventral pseudointerarea with deep intertrough. Dorsal pseudointerarea 50% of valve width, with wide triangular median groove. Ventral interior with apical pits lateral and anterior of pedicle tube, posterior of apical process. Dorsal interior with very weak, forked median septum, almost lacking median buttress. Differ from Hadrotreta primaea Walcott, 1902 by the ventral apical pits anterior to the pedicle foramen and lack of boss-like termination of dorsal median ridge. Differ from H. trichristiorum Popov et al., 2015 by the larger dorsal pseudointerarea and deeper ventral intertrough.

Material. 589 ventral, 712 dorsal valves and 10 conjoined shells from the Forteau Formation of southern Labrador and western Newfoundland, both west of the Long Range Mountains and in the Bonne Bay area (GM07-2-1, GM07-3-1, GM07-4- 1, GM11-1-6, GM11-1-6A, GM11-1-6C, GM11-1- 8C, GM11-1-9, GM11-1-10 T, GM11-1-14, GM11-1- 15, GM11-4-2, GM11-6, GM11-6-11, GM11-6-13, GM11-9A-9, GM11-9A-10, GM11-11-3, GM11-14, GM11-14A-5, GM11-14A-7, GM11-14-C, GM11- 14G-1, GM11-14G-2, GM11-14G-B, GM11-14G- 3B, GM11-14 G-C, ECP-1, ECP-3, MSM-1, MSM-2, MSM-3, MSM-4, MSM-7, MSM-8, FC1-5, FC1-6, FC1-7, FC2-2a, FC2-4, FC2-9, FCX, F-1, ICS 1545, ICS 1553, ICS 1560, ICS 1562, ICS 1565, ICS 1575, ICS 1576, ICS 1577, ICS 1578, ICS 1519, ICS 1421, JSP 1982−01)

Description. Ventribiconvex acrotretid shell. Ventral valve cone-shaped with rounded or sub-rectangular outline, greatest width about mid-valve ( Figure 21.1, 21.5, 21.7, 21.10). Ventral pseudointerarea catacline or weakly procline ( Figure 21.2,

21.9). Ventral pseudointerarea with deeply incised intertrough, slightly expanding towards posterior margin ( Figure 21.3, 21.7). Ventral larval shell convex and circular ( Figure 21.4). Pedicle foramen sub-circular, situated at posterior margin of larval shell ( Figure 21.8). Dorsal valve sub-circular with greatest width slightly posterior to mid valve, slight anterior sulcus ( Figure 21.14-19). Dorsal larval shell marginal, slightly overhanging posterior margin ( Figure 21.14). Narrow, orthocline dorsal pseudointerarea representing about 50% of valve width, with wide, triangular median groove and well defined propareas ( Figure 21.15-18). Larval shell ornamented by fine pits and post larval shell by fila, sometimes with nick points ( Figure 21.6, 21.8).

Ventral valve interior with boss-like apical process situated slightly anterior to internal tube of pedicle foramen ( Figure 21.10-13). Deep apical pits are situated lateral to and slightly anterior of the pedicle tube, between the pedicle tube and the apical process ( Figure 21.12-13). The apical process is flanked laterally by arcuate impressions of the vascula lateralia extending from the apical pits ( Figure 21.13). Well developed ventral cardinal muscle scars are situated lateral to the vascula lateralia, at the lateral margins of the ventral pseudointerarea ( Figure 21.10-11). Dorsal interior with strongly impressed cardinal muscle fields representing about 20% of valve length ( Figure 21.15, 21.18). Dorsal median septum poorly developed, except at its posterior termination ( Figure 21.17, 21.19). Almost no median buttress at all. The median septum is flanked by two, very weakly expressed ridges apparently diverging from the main septum ( Figure 21.18). Elongate anterocentral muscle scars present close to the terminations of the lateral ridges of the median septum ( Figure 21.18). Impressions of diverging vascula media present at the termination of the median buttress ( Figure 21.17).

Remarks. The majority of acrotretid specimens from the Forteau Formation belong to Hadrotreta taconica ( Walcott, 1887) . This species is referred to Hadrotreta based on the oval pedicle foramen situated outside the larval shell, the boss-like apical process of the ventral valve, the presence of deep apical pits lateral to the internal pedicle tube and the low, forked dorsal median ridge ( Holmer and Popov, 2000). Similar material from the Taconic allochthon at Ville Guay, Quebec is described by Landing et al. (2002) as Linnarssonia taconica Walcott, 1887 . Landing et al. (2002) appear to have based (erroneously) the reference of this species to Linnarssonia on the suggested presence of the apical foramen inside the larval shell. However, the only illustrated specimen with ventral external apical surface preserved ( Landing et al., 2002, figure 7.3) appear to show an apical foramen situated on the slope of the ventral pseudointerarea, at the margin of the larval shell. Like Hadrotreta , Linnarssonia has the apical foramen between the larval and adult shell but differ from Hadrotreta in the less pronounced apical pits on the ventral valve interior, the vestigial propareas of the dorsal pseudointerarea, the more closely arranged cardinal muscle scars and the higher and shorter dorsal median ridge ( Holmer and Popov, 2000; Peel et al., 2016). The acrotretid specimens described by Landing et al al., 2002 appear to belong to the same species of Hadrotreta as the specimens from the Forteau Formation described here.

Hadrotreta taconica differ from the type species, H. primaea Walcott, 1902 from the Pioche Shale of the Great Basin by the position of the ventral apical pits anterior to the pedicle foramen and the less pronounced, boss-like anterior termination of the dorsal median ridge ( Rowell, 1980). The species differ from H. trichristiorum Popov et al., 2015 by the larger dorsal pseudointerarea and larger dorsal muscle scars as well as the more well-defined ventral intertrough ( Popov et al., 2015). Hadrotreta taconica is a common species in the late early Cambrian (Cambrian Stage 4) of eastern Laurentia with specimens reported from multiple localities in the Taconic Allochthons ( Walcott, 1912; Lochmann, 1956; Landing et al., 2002). Poorly preserved acrotretid specimens from the Ella Island Formation of North-East Greenland, described by Skovsted and Holmer (2005) as Vandalotreta sp. , may also belong to H. taconica based on the position of apical pits lateral to the internal pedicle tube of ventral valves ( Skovsted and Holmer, 2005, text-figure 2D) and may further extend the range of the species. In the Forteau Formation, Hadrotreta taconica is one of the most numerous of all brachiopod species with over 1000 shells recovered. Hadrotreta taconica has a very broad range in the Forteau Formation and is present in large numbers, both in the vicinity of archaeocythan reefs in southern Labrador, in the Middle shale at Mount St. Margaret and in the argillaceous facies of the Bonne Bay area where it may co-occur with Acrothyra bonnia (assemblages 1 and 3).

Distribution. Assemblages 1 and 3. Cambrian Series 2, Stage 4, Devils Cove, Middle shale and Mackenzie Mill members, Forteau Formation of southern Labrador and western Newfoundland, west of the Long Range Mountains, and the Bonne Bay area, Canada. Cambrian Stage 4 of New York State and Vermont and British Columbia.