Copper and Jin—Ordovidian–Silurian brachiopod evolution, extinction, and recovery
Occurrence.—Late Ordovician (late Katian) and early Silurian (Llandovery), Laurentia, Baltica, Siberia, Tadzhikistan, ?Kazakhstan, and ?North China. Katian hindellide genera have not been reported from Laurentia. Some primitive or ancestral athyrides from the mid–upper Katian of Kazakhstan and North China, such as Nikolaispira and Kellerella, are regarded as possible ancestral hindellides. During the Hirnantian, hindel- lides diversified to become a group of prominent and abundant brachiopods in tropical environments worldwide, forming extensive shell beds. The family may have survived into the late Silurian as Argella. An Early Devonian record is uncertain.
Remarks.—The order Athyrida is characterized by (1) medial crura oriented along the plane of symmetry, (2) laterally directed spiralia (some with double spiralial lamellae), and (3) a simple or complex jugum connecting the spiralia. The earliest forms, as represented by the Late Ordovician and early Silurian Hindel- linae, have a smooth or finely capillate, impunctate shell, a simple jugum, and single, flat spiralial lamellae. Another con- sistent early feature is that the crura and brachidia approach each other at a sharp angle, but do not fuse. During the late Silurian and Devonian, the spiralium evolved double parallel whorls, developed from jugal extensions, or by transforming from U-shaped, trough-like spiral lamellae to double lamellae, as in the Anoplothecidae. Evolution of the brachidia demonstrates that by Aeronian–Telychian (mid-Llandovery) time, some athyrides developed complex jugal stems or extensions, such as in the Meristellinae and Whitfieldellinae. The lateral projection of spiralia in athyrides and spirifer-
ides suggests that they had a different feeding strategy, with feeding currents (from the spiral base inwards), opposite to that in atrypides (with medially or dorsally directed spiralia, and
feeding current from the base outwards; see Copper, 1986, figs. 8, 9). Their Ordovician stratigraphic record shows that the original single spiral whorl stood in the central plane of symmetry, as seen in the Katian protozyginids (Copper, 1977). Nikiforova and Rzhonsnitskaya (1960) combined the super- family “Athyracea” under Incerti Ordinis, somewhere between spiriferides and terebratulides (in that sense, they should be accredited with raising the athyrides to ordinal status because nearly all of the families they assigned are recognized as true athyrides today). Under the suborder Athyridoidea, Boucot et al. (1964;= suborder Athyrididina in Boucot et al., 1965), how- ever, excluded many groups that are recognized as athyrides today, such as the Athyrisinoidea, Retzioidea, Dayioidea, Anoplothecidae, and Kayseriidae. Thus the “Incerti Ordinis” of Nikiforova and Rzhonsnitskaya (1960) matches more closely the order Athyrida as defined herein. Externally, it is difficult to distinguish many athyride taxa
with smooth or capillate shells, due to their strong home- omorphy. Copper (1986) reconstructed the spiralia and jugum of Hindella for the first time, based on topotype material, and demonstrated that there was no skeletal connection between the crura and spiralium in either Hindella or Hyattidina, although there should have been soft tissue to hold them together in vivo. It is primarily the Russian workers (e.g., Modzalevskaya, 1985, 1996; Grunt, 1989) who have clarified the nature of the lophophore-supporting skeletal structures, and established evolutionary relationships between genera and subfamilies.
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A key to their understanding lies in the earliest subfamily, the Hindellinae. Schuchert (1894, 1897), who named the Hindellinae,
visualized them as encompassing Early Ordovician and Silurian athyrides possessing a simple jugum, although he unwittingly included later genera that are now known to be well outside that group, such as Anoplotheca and Coelospira, with double spiralial lamellae and a very complex jugum. Later, Schuchert (1928) revised his classification, and confined the Hindellinae to four smooth-shelled genera: Hindella, Hyattidina, Greenfieldia, and Whitfieldella (the latter two were moved to other subfamilies later). At that time, he also assigned the Hindellinae to the family Meristellidae of Waagen (1883). Nikiforova and Rzhonsnitskaya (1960) and Menakova
(1964) accepted the Hindellinae as a subfamily, and included it, besides Hindella, Whitfieldella, and Hyattidina, in the family Nucleospiridae. This is close to the Hindellinae defined in this study, except that we exclude the hindellines from the later Wenlock nucleospirids. An early origin for the Nucleospiridae is uncertain, although there are relatively flat, smooth athyrides, such as “Athyris” lara Billings (1866) in the Merrimack Formation of late Rhuddanian age. “Athyris” solitaria Billings 1866 from the same strata belongs to the smooth atrypide genus Cerasinella Copper, 1995. Sheehan (1977) abandoned the subfamily Hindellinae
altogether, and allocated Hindella to the Meristellinae, and Hyattidina to a new subfamily, the Hyattidininae, both within the Meristellidae. Modzalevskaya (1985, fig. 29; 1996) proposed a compre-
hensive evolutionary scenario for the early athyrides of latest Ordovician–early Silurian age. She showed only Hindella in the Ordovician, but extended it into the Rhuddanian where Cryptothyrella was treated as a synonym. For the Rhuddanian, Modzalevskaya listed three genera: Koigia, Hyattidina, and Tschatkalia, and grouped them into the Hyattidinae. She did not use the subfamily Hindellinae Schuchert, 1894, but assigned Greenfieldia to the younger Didymothyrinae, and Hindella to the Meristellinae. Notably, Modzalevskaya (1985) showed that the Meristellinae, Meristinae, and the genus Whitfieldella (and thus Whitfieldellinae) appear first in the Wenlock, characterized by the presence of a more complex jugum. In a series of elaborate diagrams, Modzalevskaya (1985, figs. 7–19) made detailed comparisons of the jugum in a range of genera for the first time, demonstrating that complex juga evolved later, and first appeared in such late Telychian–Wenlock genera as Meristella, Meristina, Didymothyris, and Collarothyris.On Anticosti Island, such complex juga first appeared in the mid-Aeronian athyrides (work in progress). Grunt (1986, 1989) adopted the Hindellinae of Schuchert
(1894), and included in it nine genera, confining Cryptothyrella to the early Silurian. She employed Sheehan’s 1977 partial serial sections for “Hindella umbonata” from Junction Cliff. Grunt (1986) followed Schuchert (1928) in assigning the Hindellinae to the family Meristellidae. She elevated the Didymothyrinae to family status and, on the basis of their complex jugum, placed it under the superfamily Athyridoidea. Dagys (1996) reclassified the Order Athyrida (no author
assigned) into three suborders, theRetziidina,Koninckinidina and Athyrididina, and did not recognize the subfamily Hindellinae.
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