Copper and Jin—Ordovidian–Silurian brachiopod evolution, extinction, and recovery


re-dated the Leemon and Girardeau limestones of the Edgewood Group as Hirnantian.


Remarks.—There has been considerable confusion between Hindella and other homeomorphic athyrides that occur in the Ordovician-Silurian boundary interval. The deeply incised ventral muscle scars have been used as one criterion for Hindella, but these are similar in other early athyrides, and are also quite variable. Sheehan (1977) distinguished Hindella from Cryptothyrella mostly on external morphology: Hindella was noted to have a prominent beak with commonly well-developed growth lines and a transapical foramen. We note that these features occur in most hindellines. Hindella with prominent concentric growth lines are rare amongst Anticosti shells. Sheehan (1977, p. 25) also remarked that the muscle fields were “more divergent” in Cryptothyrella, and the “cardinalia more robust.” Sheehan’s (1977) diagnosis, however, was based on different species assignments compared to what we propose in this study. For example, we assign the Hirnantian species Anomites terebratulina to Hindella, whereas he assigned it to Cryptothyrella. In our re-assessment of the type species of Aeronian Cryptothyrella, we show radial capillae on the shell surface (see description under that genus), which are also observed in some species of Hindella. Herein, the internal architecture of the brachidia and dental


apparatus are given primary taxonomic importance. Detailed serial sections of both Hindella and Cryptothyrella, demonstrate that Hindella differs from Cryptothyrella in its straight, almost vertically aligned dental plates, much less prismatic callus in the ventral apex, short and blunt teeth, a distinctive hinge plate, and median septum reaching to the hinge plate, forming the appearance of a “septalium” in globose, adult shells. In Hindella, the umbonal blades are short and weakly hooked close to the short crura (in contrast to the long crura and “walking-stick-shaped” umbonal blades in Cryptothyrella), the jugum is arched towards the posterior. Davidson (1881, 1882), who described the genus Hindella


based on specimens sent by Billings from the Junction Cliff locality on Anticosti, named it after the British geologist, George Hinde. Reconstruction of the shell spiralia and jugum was carried out by Norman Glass (Davidson, 1882, p. 130), and showed the lateral orientation of the spiral lophophore, and a single continuous brachidium starting with the crura. Hall and Clarke (1894, p. 64, figs. 46–51) copied, with sketches of the jugum and the internal umbonal area, and assumed the brachidia to continue, albeit at right angles from the crura. As shown in our serial sections, the curved umbonal blades of the brachidium are not connected to the crura, but approach as curved hooks close to the crura. During life, there must have been some connecting tissue that suspended the spiralia and jugum within the shell cavity, or else the spiralia would have been loose. The soft tissue endured long enough for the lophophore supports to be left more or less in their life orientation as mud infilled the shells, with spirals pointing to the sides of the shells. In atrypides there is no such crural-brachidial structure, as the laterally positioned crura continue into the spiralial lamellae, with no sharp angle of closure. This is, de facto, a fundamental distinction from the atrypides, as shown in Copper (1986).


1129 Alvarez and Rong (2002) regarded Hindella and the junior


genus Cryptothyrella as indistinguishable, and subsumed Hindella in the subfamily Meristellinae, thus combining forms with a complex and simple jugum. Cocks (2008) adopted the 2002 Treatise synonymy of Hindella and Cryptothyrella, referring them back to the subfamily Meristellinae. At the same time Cocks assigned a Llandovery age to Hindella angustifrons (Salter, 1851), H. crassa (Sowerby, 1839), and H. furcata (Sowerby, 1839), although labelling only crassa as Hindella. The only taxon remaining in the Hirnantian was Hindella incipiens (Williams, 1951). Sheehan (1977), and Modzalevs- kaya (1985) labelled crassa and cassidea as Hindella, but Hiller (1980) referred the species to Cryptothyrella. More recently, Niemeyer et al. (2010) assigned some Llandovery specimens (mostly steinkerns) from Chile to “Hindella crassa incipiens”, but the Chilean shells appear to have a somewhat more complex jugum, as shown in the serial sections by these authors, than the typical Hirnantian Hindella from Anticosti Island. Thus, the species crassa has zigzagged between two generic names. Specimens of such athyrides in the UK are rare, and poorly preserved as siliciclastic molds and casts, without brachidia, and thus muscle scar and hinge identifications are debatable. This leaves the Anticosti record of pristinely preserved shells with full brachidia, and Hindella cassidea (Dalman, 1828), as some of the few species that are true Hirnantian Hindella.


Hindella umbonata (Billings, 1862) Figure 2.1–2.7


1862 Athyris umbonata Billings, p. 144, figs. 121a, b. 1863 Athyris umbonata; Logan, p. 317, figs. 331a, b. 1865 Athyris umbonata; Shaler, p. 69. 1866 Athyris umbonata; Billings, p. 46. 1882 Hindella umbonata (Billings); Davidson, p. 130.


1894 Hindella umbonata; Hall and Clarke, pl. 41, figs. 26, 27, 29, 30.


1928 Hindella umbonata (Billings); Twenhofel, p. 221, pl. 20, figs. 21–23.


non 1977 Hindella umbonata; Sheehan, pl. 1, figs. 26–28.


Types.—Billings (1862, p. 144) established the species based on specimens from “Junction Cliff, Anticosti, Division 1.” In modern stratigraphy, this locality at western Anticosti Island exposes the Juncliff Member, Ellis Bay Formation, as well as the underlying the recessive Fraise Member (see Copper et al., 2013, fig. 4c, d for the type locality; Jin and Copper, 1997 for a map), Hirnantian, latest Ordovician. Junction Cliff is readily accessible, and shows a 10mthick upper unit of resistant, partly nodular micrite with shaly partings, with H. umbonata (Juncliff Member) and an underlying recessive lower unit of shales and limestones (Fraise Member, with H. prinstana). Here, the distinctive large elongate shell of H. umbonata can be easily distinguished from the smaller, rounded, globose shell of the older species, H. prinstana. The restricted type locality (C718) is defined here as the east end of Junction Cliff (UTM 20,


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