On the discovery of a trilobite of the genus Fordaspis Lochman in the Torgashino Limestone
Abstract Generated abstract
This paper reports the discovery of a trilobite assigned to Fordaspis cf. nana in the basal red-colored member of the Torgashino Limestone of the Eastern Sayan, a region whose Cambrian faunas are noted for strong endemism and difficult interregional correlation. The author describes a well-preserved cranidium and associated carapace fragments, compares their morphology with Lochman’s North American Fordaspis nana, and notes minor differences in the anterior glabella and marginal border. The occurrence, together with other trilobite genera known from Lower Cambrian assemblages, supports correlation of the relevant Torgashino beds with upper Lower Cambrian faunas of North America and Morocco.
Full Text
On the Discovery of a Trilobite of the Genus Fordaspis Lochman in the Torgashino Limestone
I. I. Koptev
(Presented by the scientific seminar of the Department of General Geology)
Many investigators [1] have repeatedly emphasized the extraordinary endemism of the Cambrian fauna of the Sayan–Altai region, which greatly complicates correlation of the Cambrian sections of this region not only with the Cambrian of other countries, but also with the Cambrian sections of other regions of the Soviet Union. In this connection, any finds in the Cambrian deposits of the Sayan–Altai region of fossils known beyond its limits are of considerable interest; all the more important are finds of fauna known from the Cambrian of North America—one of the most completely studied regions of the wide distribution of Cambrian deposits.
In the summer of 1960 we succeeded in finding, in the upper part of the basal red-colored subformation of the Torgashino limestone, a trilobite of the genus Fordaspis Lochtm., known from beds with Elliptocephala asaphoides in the state of New York, North America. In this region, various-facies deposits of the Lower Cambrian, assigned to the upper Olenellus subzone of the biostratigraphic scale of the Lower Cambrian of North America, have been studied in detail [2]. In the Shodack facies numerous fossils have been encountered, composing the so-called Elliptocephala asaphoides fauna. This fauna is subdivided into two parts—the lower and the upper. In both of them the species Fordaspis nana occurs; in addition, in the upper part another species is present—Fordaspis tumida (Walcott). Moreover, in the Elliptocephala asaphoides assemblage, many other trilobite genera are abundant, representatives of which are known to us in Siberia—Calodiscus Howell, Serrodiscus R. et E. Richter, Kootenia Walcott, Pagetia Walcott, Pagetides Rasetti, Bonnia Walcott, Labradoria Resser, olenellids, and others. Lochman correlates the Elliptocephala asaphoides fauna with zones VI (in part), VII, and VIII of Hupé in the Lower Cambrian of Morocco.
Below is given a description of the trilobite from the Torgashino limestone.
Fam. Dorypygidae Kobayashi, 1935
Fordaspis Lochman, 1956
Lochman [2] gives the following diagnosis of the genus: cephalic shield oval, convex; glabella bluntly tapering toward the rounded anterior margin, with approximately parallel sides, cut by three pairs of short, fine furrows, visible only on the sides of the glabella; occipital furrow deep, occipital ring narrow, low; pregla-
Scheme of Correlation of Sections, Cm₁
North America, Africa (Morocco), and the northwestern part of the Eastern Sayan
| North America Lochman, 1956; Lochman, Wilson, 1958 |
Morocco Hupé, 1952 |
Northwestern part of the Eastern Sayan Koptév I. I., 1961 |
|---|---|---|
| Cm₂ Zone Bathynotus–Elrathina Zone Glossopleura Zone Albertella Zone Plagiura–Poliella |
hiatus | hiatus |
| Cm₁ Upper Olenellian Zone Antagmus–Onchocephalus Fremontia Neoredlichia, asapha Fordaspis, Calodiscus, Bonnia, Kootenia, Pagetia, Serrodiscus, archeocyathids Cobboldia, Hebediscus, Weymouthia, etc. Zone Holmia |
Zone VIII Cobboldia, Palaeolenidae, etc. Zone VII Palaeolenidae, Serrodiscus, Micmacca, etc. Zone VI Olenellids, redlichiids, protolenids, etc. Zone V Olenellids, etc. |
Torgashino Massive limestones: Edelia, Kootenella, Pagodia?, Olenoides, Hebediscus, Bonnia, Elrathia, etc. Bazaikha Bonnia, Fordaspis, Kootenia, Cobboldia, Hebediscus, Palaeolenella, Pagetiellus, archeocyathids. hiatus |
| Lower Olenellian olenellids |
Zone IV Olenellids, Hebediscus, etc. Zone III Olenellids, redlichiids, protolenids Zone II Olenellids, redlichiids, protolenids Zone I Olenellids, redlichiids, protolenids |
Kolba redlichiids, protolenids, yakutiids, etc. |
the doublure may be present or may be absent; the anterior marginal furrow is narrow, indistinct in front of the glabella; the anterior marginal border is narrow; the fixed cheeks are flat, in width amounting to two-thirds of the width of the glabella; palpebral lobes of medium size, situated closer to the anterior half of the cranidium; palpebral ridges narrow, weakly elevated; posterolateral limbs triangular, strongly convex, approximately the same width as the occipital ring. The free cheek is situated on the downward-descending angles of the cephalic shield; the ocular platform is narrow; the marginal furrow is narrow; the lateral border (rim) is of medium width, with a short, weak lateral genal spine. The pygidium is semicircular; the rachis narrows along its entire length, with five segments, on which median tubercles or spines may be present; in adult individuals there are five pleural lobes; the interpleural furrows are deep; the pleural furrows are narrow; along the margin of the pygidium there are six pairs of small flat spines; the marginal furrow is narrow, the border narrow and flat. The outer surface is covered with small tubercles.
Typical species—Solenopleura nana Ford, 1878.
Scope of the genus—two species: Fordaspis nana (Ford), F. tumida Lochn.
Fordaspis cf. nana (Ford)
Material. One cranidium in good preservation and several fragments of various parts of the carapace.
Description. Cranidium of subtrapezoidal form, small, slightly curved in the longitudinal vertical plane, with a rounded anterior margin, with a convex, bluntly rounded anteriorly, subcylindrical glabella. The glabella is divided by three pairs of very weakly expressed short lateral furrows, of which the posterior pair is
Fordaspis cf. nana (Ford); 1—cranidium, ×8, right side of the Bazaikha River valley near the mouth of the Kaltat River, basal red-colored member of the Torgashino Formation, specimen no. 6; 2—the same specimen, ×6; 3—the same specimen, lateral view, ×4.
the most distinct and obliquely directed backward. Its depth is greatest at the axial furrows; toward the middle of the glabella the furrows gradually disappear. The other two pairs are expressed as almost imperceptible pits on the sides of the glabella. The glabella has its greatest convexity in the middle part; toward the anterior margin it gradually lowers and passes smoothly into the preglabellar limb, without being separated from it by axial furrows. In width the glabella is somewhat more than half the width of the cranidium at the level of the posterior ends of the palpebral lobes. The occipital furrow is straight, not wide and deep, reaching its greatest depth in the middle
part. The occipital ring is of uniform width, convex. The dorsal furrows, distinct along the sides of the glabella, disappear at its anterior end.
The fixed cheeks are broad, slightly convex, subtrapezoidal in shape. The posterolateral lobes of the fixed cheeks are lowered downward. The posterior marginal furrow is of medium width, deep; the posterior marginal border is narrow, roll-like, with a geniculate bend in the vertical plane, passing through the posterior ends of the palpebral lobes. Owing to the geniculate bend, the posterior ends of the cranidium are somewhat bent forward.
The frontal limb is not broad, flat; in front of the glabella it passes almost imperceptibly into the marginal border, but on the sides of the cranidium it is separated from the latter by a distinct marginal furrow, emphasized by a bending of the surface; the outer (lateral) parts of the limb are strongly lowered downward. The anterior marginal border is of medium width, flat, curved circumferentially, more distinct at the margins and faintly visible in the middle.
The ocular ridges, in the form of a narrow crest, are directed almost at a right angle to the glabella and, curving smoothly parallel to the anterior margin of the cranidium, pass imperceptibly into the palpebral lobes. The palpebral lobes are of medium length and width, situated somewhat in front of the middle horizontal line of the cranidium. The posterior ends of the palpebral lobes lie almost at the level of the middle of the glabella, extending only slightly below it. The palpebral furrows are sharp, which is due to the sharp elevation of the palpebral lobes above the surface of the fixed cheeks.
The anterior branches of the facial sutures are subparallel, directed straight forward, but then turn smoothly toward the center, rounding the anterolateral angles of the cranidium. The posterior branches diverge from the posterior ends of the palpebral lobes to the sides, but, owing to the forward-bent posterior margin, they are not of considerable length.
The surface of the cranidium is covered with small tubercles.
Other parts of the dorsal shield are unknown.
Dimensions in mm
| Height of cranidium | Length of cranidium | Width of cranidium along posterior margin | Width of the last | Width at the anterior end of the glabella | Length of glabella | Width of glabella | Length of occipital ring | Length of palpebral lobe | Width of palpebral lobe | Width of preglabellar field | Width of anterior marginal border | Width of fixed cheek at the level of the posterior end of palpebral lobe |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1.3 | 3.5 | 5.5 | 4.5 | 3 | 2.5 | 1.7 | 0.4 | 1 | 0.2 | 0.4 | 0.2 | 1.2 |
Comparison. The form described is in many respects, down to small details, similar to Fordaspis nana (Ford) described by Lochman. The latter was first described in 1878 by Ford as Solenopleura nana, and then redefined by Walcott in 1886 as Kootenia. The specimen described differs only slightly from the genotype in the form of the anterior end of the glabella: in our specimen the anterior end of the glabella lies lower and is not separated by a dorsal furrow from the preglabellar field. Our form differs from the genotype also in the width of the anterior marginal border: in the latter it is narrow,
cordlike; in our specimen it is broader, although in some photographs in Lochman’s plates an even broader marginal border is visible, not differing from the border of the specimen found by us.
Distribution — North America; Eastern Sayan, Torgashino Formation.
Locality. Occurs in the basal red-colored member of the Torgashino Limestone, on the right side of the valley of the Bazaikha River near the mouth of the Kaltata River (Eastern Sayan), together with Nehediscus sp. and, in the same beds, with Pagetiellus Lerm., Neocobboldia Ras., Palaeolenella Rep., Planaspis Rep., Kootenia Walc., Bonnia Walc., and others. Upper Lower Cambrian.
REFERENCES
- Biostratigraphy of the Paleozoic of the Sayano-Altai Mountain Region. Tr. SNIIGGiMS, vol. 1, issue 19, 1960. — 2. Ch. Lochman — Stratigraphy, paleontology, and Paleogeography of Elliptocephala—asaphoides strata in Cambridge and Hoosick Quadrangles, New York. Bull. Geol. Soc. America, vol. 67, Num. 10, 1956.