Pustulobolus triangulus, Skovsted & Knight & Balthasar & Boyce, 2017
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publication ID |
https://doi.org/ 10.26879/775 |
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publication LSID |
lsid:zoobank.org:pub:482B4F4C-E674-46BB-B4E7-2768C8E0D357 |
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persistent identifier |
https://treatment.plazi.org/id/A62287D7-2069-CC31-B405-FAB1FB0DFD1A |
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treatment provided by |
Felipe |
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scientific name |
Pustulobolus triangulus |
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Pustulobolus triangulus n. gen et. sp.
Figures 16-17 View FIGURE 16 View FIGURE 17
zoobank.org/ A65C0BC6-B661-4F6C-9112-39273ED65D0D
1980 Lingulella cf. prisca ( Poulsen, 1932) ; Spencer, p. 111, plate 3, figures 4-7.
Holotype. Ventral valve NFM F-2531 from sample MSM-11, Middle shale member, Forteau Formation , Mount St. Margaret, western Newfoundland ( Figure 17.2 View FIGURE 17 ) showing the characteristic triangular morphology and pustulose ornament of the ventral pseudointerarea.
Material. 36 ventral, 15 dorsal valves and a single specimen with conjoined shells from the lower shale interval of the Forteau Formation at Mount St. Margaret, western Newfoundland (MSM-4, MSM-5, MSM-7, MSM-8, MSM-11).
Diagnosis. As for genus.
Etymology. The specific name refers to the sharply pointed triangular shape of the ventral pseudointerarea.
Description. Shell dorsibiconvex and strongly inequivalved ( Figure 16.11 View FIGURE 16 ). The ventral valve has a rounded anterior margin and a sharply pointed triangular posterior margin. The umbonal angle varies from about 45° to 60°, but the umbo is gently rounded and small specimens typically have a larger umbonal angle (up to 75°). The dorsal valve is sub-circular and the commissure is roughly planar. Larval shells sub-circular ( Figure 16.4, 16.10 View FIGURE 16 ) and ornamented by two sets of fine circular pits ( Figure 16.5 View FIGURE 16 ). Larger pits have an average diameter of 0.75-1 µm and smaller pits in the spaces between the larger ones have an average diameter of 0.25-0.5 µm. The ventral larval shell has a single low and marginal tubercle ( Figure 16.4 View FIGURE 16 ), and the dorsal larval shell has two low tubercles ( Figure 16.10 View FIGURE 16 ). The adult shell of both valves is ornamented by pustules (diameter 4-8 µm, height up to 12 µm) that tend to increase in size toward the anterior ( Figure 16.2, 16.6-10 View FIGURE 16 ). In some areas the pustules are developed as small spines, arranged in gently curved rows toward the anterior ( Figure 16.8 View FIGURE 16 ).
The ventral pseudointerarea is large, orthocline and triangular, representing an estimated 40% of valve length and 75% of valve width ( Figure 17.1-2, 17.6-7 View FIGURE 17 ) with a narrow, almost parallel-sided pedicle groove (width up to 100 µm, representing about 10% of the pseudointerarea in larger specimens). The wide propareas are divided by well-developed flexure lines and areas lateral to flexure lines are characterized by the adult pustulose ornament ( Figure 17.3 View FIGURE 17 ). The ventral pseudointerarea is detached from the valve floor along most of its width, but the floor of the pedicle groove remains attached, leaving a narrow, slit-like cavity under the propareas on either side of the groove ( Figure 17.4 View FIGURE 17 ). Damaged specimens show that these cavities remained open throughout ontogeny ( Figure 17.5 View FIGURE 17 ). Dorsal pseudointerarea narrow, arcuate and with a poorly defined triangular median groove ( Figure 17.8-10 View FIGURE 17 ). The propareas are wider than the median groove by about 50%. In a single articulated specimen the posterior margin of the dorsal valve abuts the anterior end of the ventral pseudointerarea which projects substantially beyond the posterior margin of the dorsal valve ( Figure 16.11 View FIGURE 16 ).
Internal morphology poorly known due to valve fragmentation. Ventral valve interior with triangular lateral muscle scars inserted just under propareas ( Figure 17.2, 17.6-7 View FIGURE 17 ). Ventral visceral field slightly thickened and anteriorly delineated by arcuate impression of anterior muscle fields ( Figure 17.6-7 View FIGURE 17 ). Dorsal valve interior with very weakly developed median ridge and large, triangular lateral muscle fields close to the valve margin and inserted anterior to propareas ( Figure 17.8 View FIGURE 17 ).
Remarks. In her Masters thesis, Spencer (1980) compared lingulid specimens from sediments surrounding archaeocyathid patch reefs in the lower Forteau Formation (Middle shale) of southern Labrador to specimens described by Poulsen (1932) from North-East Greenland under the name Lingulella (Lingulepis) prisca Poulsen, 1932 . The Greenland specimens were later referred to Eoobolus by Skovsted and Holmer (2005), who noted that the Labrador specimens illustrated by Spencer differed from the Greenland species in a number of characters, most notably the relative sizes of the pseudointerareas. Without having studied Spencer’s material, Skovsted and Holmer (2005) suggested that the illustrated ventral and dorsal valves belonged to different taxa; the dorsal valves to Botsfordia caelata and the ventral valves to a new lingulid species. However, the close association of dorsal and ventral valves identical to those illustrated by Spencer in our material from the Middle shale interval at Mount St. Margaret in western Newfoundland as well as the identical ornament, and the occasional preservation of conjoined valves has led us to conclude that Spencer was correct in associating the two valve types. The resulting new species differ substantially from Eoobolus priscus , Botsfordia caelata , and all other known brachiopods and is here described as a new taxon. Unfortunately, our own collections from southern Labrador contain no specimens unequivocally referable to Pustulobolus triangulus , but the close comparison of Spencers illustrated specimens to our material leave little doubt about their identity.
Despite the distinct morphology of the ventral valves of Pustulobolus triangulus , co-occurring sub-circular dorsal valves with narrow pseudointerareas are very similar to the dorsal valves of both Botsfordia caelata and Eothele tubulus , which also occur in the Forteau Formation. The greatest difference to the dorsal valve of these species is the almost horizontal commissural plane in P. triangulus , which makes the dorsal valves almost flat compared to the ventrally flexed commissural plane of dorsal valves with the apex strongly depressed below the posterolateral margins in Botsfordia caelata and E. tubulus . Regardless, the similarities are so great that individual specimens may be difficult to identify, in particular as the available specimens of all three species are often fragmentary, with poorly preserved posterior margins (especially true for E. tubulus ). However, the very distinct ventral valves of all three species are generally more common than the dorsal valves and have never been found together anywhere in the formation (although B. caelata and P. triangulus both occur at different stratigraphic intervals in the same section at Mount St. Margaret). For this reason we feel justified in associating all co-occurring dorsal and ventral valves of P. triangulus .
Distribution. Assemblage 1. Dyeran Stage (Unnamed Series 2, Stage 3-4), Middle shale, Forteau Formation of western Newfoundland, and possibly southern Labrador, Canada.
| NFM |
The Rooms Corporation of Newfoundland and Labrador, Provincial Museum Division |
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.