Paroistodus Lindström, 1971

Zhen, Yong Yi & Nicoll, Robert S., 2009, Biogeographic and Biostratigraphic Implications of the Serratognathus bilobatus Fauna (Conodonta) from the Emanuel Formation (Early Ordovician) of the Canning Basin, Western Australia, Records of the Australian Museum 61 (1), pp. 1-30 : 13-15

Paroistodus Lindström, 1971

Type species. Oistodus parallelus Pander, 1856 .

Paroistodus parallelus ( Pander, 1856) emend. Löfgren, 1997

Fig. 7A–S

Oistodus parallelus Pander, 1856: 27 , pl. 2, fig. 40.

Paroistodus parallelus (Pander) .— Lindström, 1971: 47, fig. 8; Löfgren, 1997: 923–926, pl. 1, figs 1–12, 17, 21, text-fig. 5A–G (cum syn.); Albanesi in Albanesi et al., 1998: 144, pl. 8, figs 27–30; Johnston & Barnes, 2000: 31–32, pl. 10, figs 10,11,15–17, 20; Tolmacheva et al., 2001: fig. 4.12–4.13; Viira et al., 2006: pl. 1, fig. 5.

Material. 457 specimens from three samples ( Table 1).

Remarks. The species as revised by Löfgren (1997) possesses a septimembrate apparatus (including makellate M and drepanodiform or paroistodiform S and P elements), which can be distinguished from the other species of Paroistodus by having prominent lateral costae on the sides of its constituent elements. Zhen et al. (in press b) preferred to describe the S and P elements as paroistodiform in the Paroistodus species which show a sharp anterior costa extending basally into the basal cavity and forming a ridgelike structure ( Zhen et al., 2007b, p. 137) at the anterior end of the basal cavity. However this character is not shown in the material of the two Paroistodus species (similar to Paroistodus sp. recently documented from the Honghuayuan Formation in South China) reported herein from the Emanuel Formation, although the anterior part of base often exhibits a zone of recessive basal margin ( Figs 7A–C , 8H ). Paroistodus parallelus occurs abundantly in the Emanuel Formation, where it is found in association with P. proteus although the latter is rare ( Table 1).

Paroistodus proteus ( Lindström, 1955) emend. Löfgren, 1997

Fig. 8A–K

Drepanodus proteus Lindström, 1955: 566, pl. 3, figs 18–21, text-fig. 2a–f, j.

Paroistodus proteus (Lindström) .— Löfgren, 1997: 922–923, text-figs 3H–N, 4L–AB (cum syn.); Zhen et al., 2007b: 136, 137, pl. 5, figs 1–11 (cum syn.).

Material. 30 specimens from three samples ( Table 1).

Remarks. Paroistodus parallelus is relatively rare in the Emanuel samples, and can be easily differentiated from associated P. parallelus mainly by lacking a prominent costa on the lateral faces. Paroistodus proteus was revised by Löfgren (1997) as having a septimembrate apparatus. After a review of previously reported occurrences of this species in the Honghuayuan Formation and other coeval stratigraphic units in South China and comparison with Baltic material of P. proteus, Zhen et al. (2007b) concluded that Paroistodus is represented in the Honghuayuan Formation by a rare form that they identified as Paroistodus sp. It differs from P. proteus in having a smooth lateral face without a carina and having a more open basal cavity which lacks the distinctive so-called paroistodiform character. The material from the Emanuel Formation is transitional between typical P. proteus from the late Tremadocian to Floian of Balto-Scandia and Paroistodus sp. from the Honghuayuan Formation of South China. It is comparable with P. proteus in having a prominent carina ( Fig. 8A–E ) or even a weak costa ( Fig. 8J ) on the lateral faces, but similar to Paroistodus sp. from the Honghuayuan Formation in lack of the paroistodiform feature. As mentioned above, absence of the paroistodiform character in species of Paroistodus co-occurring in the Emanuel Formation may indicate that this feature is caused by ecological adaption rather than as a phylogenetically significant trait for Paroistodus . Interestingly, P. proteus from the Emanuel Formation of Western Australia and from the Latorp Limestone and Tøyen Shale of Sweden and Paroistodus sp. from the Honghuayuan Formation of South China come from three contrasting ecological/sedimentological settings; they may represent three populations or subspecies of P. proteus . The typical latest Tremadocian and early Floian P. proteus -bearing successions in Sweden (e.g., Diabasbrottet area) were deposited in the outer shelf settings of the Cold Domain ( Bergström et al., 2004), and the mid-upper part of the Emanuel Formation might be largely deposited in deep subtidal settings, while the Honghuayuan Formation with Paroistodus sp. apparently represents typical shallow subtidal environments. Therefore, in consideration of the relationship between their morphological variation and their ecological/geographical distributions, the material from the Emanuel Formation is considered as conspecific with type material of P. proteus , and Paroistodus sp. from the Honghuayuan Formation ( Zhen et al., 2007b) might be better treated as a separate subspecies of P. proteus .

Figures

Fig.7. Paroistodus parallelus (Pander, 1856).All from sample WCB705/133.A–D, M element; (A), CPC39842, anterior view (IY117-002); B,C, CPC39843, (B), anterior view (IY117-005); (C), basal view (IY117-006); (D), CPC39844, posterior view (IY117-008). E–G, Sa element; E,F, CPC39845, (E), lateral view (IY117-010), (F), basal view (IY117-011); (G), CPC39846, lateral view (IY117-009). H–J, Sb element; (H), CPC39847, outer lateral view (IY117-017); I,J, CPC39848, (I), inner lateral view (IY117-016), (J), basal view (IY117-018). K–M, Sc element; K,L, CPC39849, (K), basal view (IY117-014), (L), inner lateral view (IY117-013); (M), CPC39850, outer lateral view (IY117-012). N,O, Sd element; (N), CPC39851, inner-basal view (IY117-025); (O), CPC39852, outer lateral view (IY117-029). (P), Pb element; CPC39853, outer lateral view (IY132-020). Q–S,?Pa element, (Q), CPC39854, basal view (IY117-036), (R), CPC39855, outer lateral view (IY117-030); (S), CPC39856, inner lateral view (IY117-034). Scale bars 100 µm.

Fig. 8. A–K, Paroistodus proteus (Lindström, 1955). (A), M element, CPC39857, WCB705/133, posterior view (IY132-023). B,C, Sb element; (B), CPC39858, WCB705/133, inner view (IY132-022); (C), CPC39859, WCB705/133, outer lateral view (IY130-040). D,E, Sc element; (D), CPC39860, 161–166m, outer lateral view (IY130-025); (E), CPC39861, WCB705/133, inner lateral view (IY132-029). (F), Sd element, CPC39862, 161–166m, innrt lateral view (IY130-026). G–J, Pb element; (G), CPC39863, WCB705/133, outer lateral view (IY117-020); H,I, CPC39864, WCB705/133, (H), basal view (IY117-021), (I), inner lateral view (IY117-022); (J), CPC39865, WCB705/133, inner lateral view (IY130-043). (K), Sa element, CPC39866, WCB705/133, lateral view (IY130-41). L–O, gen. et sp. indet. A. All from sample WCB705/133. L,M, symmetrical element, CPC39867, (L), basal-posterior view (IY132-024), (M), lateral view (IY132-025). N,O, asymmetrical element, CPC39868, (N), outer lateral view (IY132-028), (O), anterior view (IY132-027). Scale bars 100 µm.