The collection closely resembles one described by Schuchert and Cooper (1930) and Cooper and Kindle (1936) from Perce, Quebec, Canada. A small assemblage of silicified Ordovician brachiopods from central eastern Alaska highlights a promising area of paleontological study along the Alaska-Canada border. Mayo collected Cambrian, Ordovician, and Devonian fossils from the north end of Jones Ridge along the valley of Hard Luck Creek, Alaska.
The general area is north of the Tatonduk River, a tributary of the Yukon, in sec. 33 E., Charley River (A-1) quadrangle, Alaska; from a thin cherty bed about 250 feet above the base of the Ordovician limestone, the southern end of Jones Ridge. Overlying limestones are highly weathered Devonian black argillites and cherts, which form much of the valley between Jones Ridge to the north and Squaw Mountain 3 miles to the south.
The Jones Ridge localization is significant because they are essentially correlative Ordovician strata along the Tatonduk River, 7 miles to the southwest (d. This collection of Ordovician silicified brachiopods and corals is very similar to the Perce group at the eastern end of the Gaspe Peninsula in eastern Canada (Schuchert and Cooper, 1930. Cooper and Kindle, 1936. The group also includes genera and species previously found in Caradoc, Girvan County, Scotland (Williams, 1962).
A review of Schuchert and Cooper's original paper shows that the Late Ordovician era was primarily based on the occurrence of Dicoelosia.
TAXONOMIC DESCRIPTIONS
Description.-Shell concavo-convex, deeply emarginate, giving externally the general shape of a maple seed (double samara); length about equal to width, greatest near antero-lateral tips of elongate wings; hinged card, approx. half the width of the shell. Brachial valve deeply concave; anterior crenulation separated from the visceral cavity by flanges; cardinal process arises from floor of simple notothyrial cavity and has a small expanded knob-like myophore; brachiophores are slender rods that lie along the edges of the internal cavity and are apparently attached to the shell for the greatest length with only the tips free. In addition, the sides of the Silurian shell are more nearly parallel and are not very concave-convex in lateral profile.
NO. 7 SILICIFIED ORDOVICIAN BRACHIOPODS ROSS AND DUTRO 7
8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I49 to Rhynchotrema. More material than the single specimen available
NO. 7 SILICIFIED ORDOVICIAN BRACHIOPODS ROSS AND DUTRO 9
The only large example is a fractured brachial valve in which there is a very strong medial septum originating under a split-mint plate with hard crustal bases. What appears to be a subcircular visceral area is actually associated with shell growth, as shown by comparison with the exterior of the valve. In the collections from Hard Luck Creek there are several specimens that are apparently closely related to Leptella?grayae, a species originally illustrated by Davidson (1883, p. 171) in part under two names, Leptaenagrayae and Lcptaenanallandeiloensis.
Ulrich and Cooper (1938, pp. 187-191) erected the genus Leptel-Ulrich and Cooper (1938, pp. 187-191) erected the genus Leptellina and presented the first modern description of Leptella, with illustrations of Leptella sordida (Billings), the type species of Leptella. These illustrations show what appears to be two lobes of a bilobed cardinal process in one specimen. They make no comment on the assignment of any European species to Leptella, except to question its accuracy.
Sampo, who noted that it lacked a denticulated hinge and suggested that this important feature is of no importance (1957, pp. 20–22). They have a rectangular platform in the notothyrium, much like that in Leptella sordida, with only a faint central groove in the posterior surface of the platform. The median septum and circumvisceral flange (brachial lamellae of 5 pike) are high, narrow, and nearly perpendicular to the plane of the brachial valve, lacking the lamellar appearance found in the other genera above.
The genus Anoptambonites was described by Williams (1962) and includes this very interesting group of bivalves and the Alaskan species. He also included RajinesquinacarinataHoltedahl (1916, p.25,pi.. 5), the interior of which was never described or illustrated. Description.—The valves are very gently concave-convex with a fine costellate exterior; costella increase with implantation and are uniform in size; the interspaces are narrower than the costae, which are 10 in 5 mm on the anterior margin of the 13 mm long shell; the greatest concavity of the brachial valve is along the midline, causing a sulcate; posterolateral flanks straight or very slightly convex.
Brachialis interior with strong median septum, terminating against strongly developed, nearly vertical edge of visceral disc; cardinal process, placed on a raised quadrate platform between the brachiophores, consists of a pair of exceedingly small linear ridges, chilidis plate covers posterior end; brachiophores slender rods as in typical Leptellina; pallial markings strongly incised; two pairs.
NO. 7 SILICIFIED ORDOVICIAN BRACHIOPODS ROSS AND DUTRO 1
I49distance of 3 mm from the beak, are about 1 mm apart; distance of 3 mm from the beak, are about 1 mm apart;. The interior of the stick with the middle septum shaped like a strong rod resting on a thin blade; there is a suggestion that the supporting blade is subject to resorption. Although ornamentation of the Scandinavian species is not illustrated, Spjeldnaes (1957, pp.. 8, text fig. IIR) states that it possesses 5-7 radial costellae.
It is not clear whether the number is related to the size of the sample. The Alaskan specimens are close in size to Diambonia gibbosa (Winchell and Schuchert) (Cooper and Kindle, 1936, pi. However, the Minnesota species is more nasal at the anterior margin, has strong ridges defining the pedicle muscle area, and has 6-7 radials. costellae (Winchell and Schuchert, 1895, pp. 416-417, pi.32, fig. 13-17).
Diambonia septata (Cooper) (Schuchert and Cooper, 1930, p.. 13) is a relatively longer species, although it also has 5 similarly spaced radial costellae.
NO. 7 SILICIFIED ORDOVICIAN BRACHIOPODS — ROSS AND DUTRO 1
Description. Shield small, concavo-convex, wider than long, widely hinged with acute cardinal limbs; anterior margin apparently intraplicate in young stages; valves finely serrated with concentric wrinkles; sharp geniculation near anterior. Footvalve with fold having median sulcus in early growth stages; inside of the pedicle not observed. Brachial valve with shallow sulcus, divided by a narrow medial fold in the posterior part, poorly defined anteriorly; a pair of elongated pits occupies the sulcus near the hinge line; interior with tent-like structure described by Cooper (1956, p. 814), apparently formed by fusion of brachiophores and cardinal process; pallialtrunks arise beneath the notothyrial structure and run almost straight forward beneath the edges of the sulcus.
The largest must be approx. 9 mm wide (estimated) and S.7 mm long; geniculation crosses the midline at approx. 3.2 mm. The complete juvenile shell is supplemented by two broken brachial valves, where no separate pedicle valve has been found. Description.-The shell is thin, concave-convex, roughly semicircular, wider than long with greatest width in line; surface with seven primary costellae, between which are many finer capillaries; regular concentric wrinkles interrupt the capillaries but do not cross the pri-.
Concentric wrinkles are closely spaced, simple and aligned transversely from one sector to the next, a characteristic that distinguishes the specimen from many. Although one can hardly discuss a species from a single specimen, it can be noted that this shell resembles P. On the other hand, the Alaskan specimen also resembles in ornamentation an immature (?) shell of comparable size from a lichen.
Discussion.–An interesting small, very wide shell is unique to the Hard Luck Creek collection. Discussion. – A single broken brachial valve attributable to Christiania is present in the silicified material of Hard Luck Creek. Discussion. A single fragment of an arm valve shows the hinge line, general shape, and arm apparatus of a leptaenid.
Although there are no other specimens, this fragment is illustrated to show a potentially important element of the brachiopod assemblage.
NO. 7 SILICIFIED ORDOVICIAN BRACHIOPODS ROSS AND DUTRO I9
LITERATURE CITED
The lower Barr and Ardmillan Series (Caradoc) of Girvandistrict, south-west Ayrshire, with descriptions of Brachiopoda.
NO. 7 SILICIFIED ORDOVICIAN BRACHIOPODS ROSS AND DUTRO 21
EXPLANATION OF PLATES
22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49
X4, USNM 145346
X5, USNM 145353
See explanation of Plate at end of text.). See explanation of Plate at end of text.).
