Phylum Arthropoda

Class Ostracoda

Ctenobolbina ciliata (Emmons, 1855) - Description from Ulrich and Bassler, 1908

Beyrichia ciliata Emmons, 1855, p. 219, text fig. 74c; Miller, 1875, p. 351; Lesley, 1889, p. 89; Jones, 1890, p. 19, pl. 3, 4, figs. 12-16, 16-18.
Beyrichia tumifrons Hall, 1871, p. 7, pl. 4, fig. 11; Hall, 1872, p. 231, pl. 8, fig. 11; Miller, 1874, p. 119, 354; Hall and Whitfield, 1875, p. 102, pl. 4, fig. 8.
Ctenobolbina ciliata Ulrich, 1890, p. 108, pl. 7, figs. 1a, b; Andersson, 1893, p. 128; Ruedemann, 1901, p. 575, pl. 2, figs. 8, 9; Ulrich and Bassler, 1908, p. 282, text fig. 7, pl. 40, fogs. 1, 2; Grabau and Shimmer, 1910, p. 353, text fig. 1660 t, t’; Wade, 1911, p. 452; Bassler in Zittel, 1913, p. 738, fig. 1425e; Bassler, 1915, p. 297; Ulrich and Bassler, 1923, p. 310, 311, text fig. 20 (fig. 4); Foerste, 1924, p. 252, pl. 45, fig. 2; Kirk, 1928, p. 419.

Description of the genus: Carapace small, usually less than 2mm in length, subquadrate or subovate in outline, the hinge line long and straight; posterior two-fifths more or less decidedly bulbous or subglobular in the typical section of the genus, but in the C. subcrassa section the corresponding parts of the carapace are smaller and usually of lesser thickness than certain portions in front of it. In the latter section a small node (the homologue of the median lobe of Beyrichia) is sometimes distinguishable on the inner slope of the posterior lobe. One deep, long, narrow, generally curved and more or less oblique sulcus extends from the middle of the dorsal edge toward the post-ventral angle, occasionally reaching the border. Area in front of median sulcus either simply convex or divided by a shallower furrow usually paralleling the main sulcus. Valves equal, the free edges thick, the contact margins generally concealed, partly or wholly, in a lateral view, by a variously modified overhanging border. Surface granulose, smooth, or punctate.

Class Trilobita

Amphilicas conifrons Ruedemann, 1916 - Description from Ruedemann, 1916
Platynotus trentonensis Hall, 1847, p. 235, pl. 64, fig. 1c.
Amphilicas conifrons Ruedemann, 1916, p. 90, pl. 30, figs. 5-8; Brett et al., 1999, p. 302, figs. 9.6, 9.7; Whiteley et al., 2002, p. 124.

Description: Family Lichidae. The glabella is convex and broadly trapezoidal in outline. It is about twice as broad as long and projects medially. The median lobe is also trapezoidal in outline and its posterior margin is about twice as long as the frontal margin. This lobe is very prominent, ends a little forward of the middle, and bends sharply downward in front. The first pair of glabellar furrows are sigmoidal (curving in then out), narrow, but fairly deep, with the posterior portion more strongly curved and extending into the occipital furrow. The first lateral lobe is subquadrangular, wider in the front, and sloping fairly evenly and laterally with the greater portion bent forward. The second lobe is but a narrow strip, apparently broadening at the anterior extremity. The second glabellar furrow is also bent slightly sigmoidal and shallower than the first. The occipital furrow is narrow, nearly straight in the middle and curving abruptly forward at the ends. The surface of the glabella is furnished with small tubercles and bases of spines which are largest at the apex of the middle lobe. Pygidium is semielliptic in outline and a little contracted in front. The axial lobe is broad and prominent in its anterior half, but contracted, tapering, and flat in its posterior half. It bulges in the middle where the contraction of the margins begins. The pleurae are nearly flat, sloping slightly from axial furrow; they are hooklike, backwardly curving, and terminate in rather blunt lobes. The surface is ornamented with small tubercles and bears the bases of many spines which are especially large in the last pair of pleurae.

Amphilicas cornutus (Clarke, 1894) – Description from Brett et al., 1999 and Clarke 1894
Conolichas cornutus Clarke, 1894, p. 749, figs. 72, 73.
Amphilichas cornutus (Clarke, 1894), Brett et al., 1999, p. 302, figs. 9.3, 9.5; Whiteley et al., 2002, p. 124, pl. 19.

Description: Family Lichidae. Cephalic features of Amphilicas cornutus have not been described with confidence due to damage suffered by the holotype specimen during preparation. Little can be discerned from the specimen except that it has a high, conical shape and a glabella of uniform width in the latter third of its length, narrowing at the composite lateral lobe. The thorax is subquadrate and narrows posteriorly. The axial lobe is broad, comprising one-third the width of the thorax in the anterior segments and becoming narrower at the posterior end. The axial lobe is also regularly convex and bordered by axial furrows which are broad and shallow. The pleurae are flat for about one-third of their width, the outer portion being evenly deflected. The segments are broad, recurved and tapering to acute terminations. The pygidium is comparatively large and is comprised of three lobes. These lobes are pleurae like in that they are broad, gently convex, and have pleural grooves that are narrow and sharply incised. Adjacent pleurae are connected for more than half their length. The first pair are elongate leaf shapes that are posteriorly curved. The posterior pair are rhombiform and also curve slightly towards the axis. There is a very prominent axial lobe that tapers to a blunt protuberance a third of the way down the pygidium. The surface of the exoskeleton is covered with fine to coarse tubercles or pustules.

*MCZ 112863: Hypotype. Posterior view: 35mm long*

Calyptaulax callicephalus (Hall, 1847) - Description from Delo, 1940
Phacops callicephalus Hall, 1847, p. 247, pl. 65, figs. 3a-i.
Dalmania callicephala (Hall, 1859). Clarke, 1894, p. 731, figs. 51, 52.
non Pterygometopus callicephalus (Hall). Clarke, 1894, p. 731, figs. 51, 52.
Calliops callicephalus (Hall). Delo, 1940, p. 94, pl. 11, figs. 1-4; Wilson, 1947, p. 56, pl. 10, figs. 5, 6.
Calyptaulax callicephalus (Hall). Shaw, 1968, p. 86, pl. 12, fig. 2; Ludvigsen, 1979a, p. 40, fig. 24a; Brett et al., 1999, p. 301, fig. 8.8; Whiteley et al., 2002, p. 145, pl. 116, 117.

Description: Family Pterygometopidae. The cephalon of Calyptaulax callicephalus is almost twice as wide as it is long and it comes to a blunt point in front. The eyes are large and attain a glabellar surface. The frontal lobe on the cephalon is almost twice as wide as long. The third lobe rises in the middle to the glabellar surface. The occipital lobe is long and very broadly convex backward. The first lateral furrows are straight; the occipital furrow is of moderate depth, the anterior slope is the steeper and shorter. Ornamentation consists of thickly distributed tubercules on the glabellar and occipital lobes. The cheek fields are covered with tubercules between the eye and the genal angle, with a row of tuburcules continuing forward below the base of the eye. The axial lobe along the thorax is prominent, of uniform width, with each annulation thickly and coarsely tuberculated. The pleurae are finely pebbled. The pygidium is broadly triangular posteriorly, with a length that is two thirds the width. The axial lobe is prominent and narrow, tapering more sharply in the anterior half than in the posterior, where the axial furrows are almost parallel. There are six pairs of furrowed ribs and a very short single one parallel to the axial terminus.

Ceraurus pleurexanthemus Green, 1832 - Description from Raymond and Barton, 1913, Brett et al., 1999, and Whiteley et al., 2002. Ecology from Whittington, 1941
Ceraurus pleurexanthemus Green, 1832, p. 84, 5, fig. 10; Raymond and Barton, 1913, p. 528, pl. 1, fig. 1, pl. 2, figs. 1, 2, 7; Evit, 1953, p. 33, pl. 7, figs. 1, 2, pl. 8, figs. 5-7; Whittington, 1941, p. 492-522; Brett et al., 1999, p. 300, figs. 8.3, 8.5, 8.7; Whiteley et al., 2002, p. 134, pl. 76, 77, 78.
non Ceraurus pleurexanthemus Green. Whittington, 1941, p. 498, pl. 73, figs. 1-40.

Description: Family Cheiruridae. Ceraurus pleurexanthemus is a common and widely distributed trilobite of Middle Ordovician, Trenton age rocks in central New York. The distinctive cephalon with its long divergent genal spines and the two long marginal pygidial spines make it easy to identify. The cephalon is broad, roughly semi-circular in outline, and short compared to width. The glabella is very convex and expands forward, reaching the frontal margin. The front of the glabella is nearly square, being but slightly curved. There are three pairs of glabellar furrows; the first two pairs are short, while the third pair are stronger, and turn abruptly back to join the neck furrow, thus isolating a pair of small, nearly square lobes at the base of the glabella. The other furrows are deep at their junction with the axial furrows, and thus form small, node-like side lobes. The fixed cheeks are convex, highest at the eye, and bear the long flared genal spines. The free cheeks are small, sutures reaching the anterior margin at points just posterior to the eyes. The eyes are small, minutely faceted, high, and prominent. They are situated far forward on the cheeks, and a little nearer to the glabella than to the posterior margin of the cephalon. The surface of the glabella, barring the genal spines, is covered with pits and tubercles of various sizes. On the glabella, aside from those irregularly spaced, there are two rows of coarse tubercles in rough alignment. There is also a pair of coarse tubercles aligned on each cheek. The thorax contains eleven segments. The pleural lobes are divided into an inner, nodose portion, and an outer portion consisting of the free points of the pleura. The pygidium is small and short, made up of four distinct areas, the first ends in very long, curved, divergent spines. The part of the pygidium within the two spines is rounded in outline, occasionally exhibiting very short spines on the remaining segments.
Articulated specimens are uncommon in most exposures, as it was not as robust as Isotelus gigas or Flexicalymene senaria found in the same rocks. The exception is the Walcott-Rust Quarry, where large numbers of whole, articulated specimens of all sizes are found in and on a single layer (layer 3). This same layer yielded specimens with appendages, which C. D. Walcott used in his first description of trilobite appendages.

Ecology: C. pleurexanthemus is most common in deep shelf facies. The spines on the pygidium and the position of the hypostoma, as discussed by Whittington (1941), prevented it from enrolling into a close-fitting ball. Presumably their spines protected them when they clung fast to the floor of the sea. It has been suggested that the power of enrollment would presumably be valuable, if they were attacked while swimming, to enable them to close up and sink rapidly to the protection of the sea floor.

*MCZ 111708 and MCZ 111715: Hypotypes, 44mm long and 42mm long*

*Reconstruction of C. pleurexanthemus from Harrington, 1959*

Cryptolithus tessellatus (Green, 1832) – Description from Whiteley et al., 2002. Ecology from Treatise on Invertebrate Paleontology, O (Arthropoda 1 Trilobita, Revised), p. 145
Cryptolithus tessellatus Green, 1832a, p. 560, pl. 14, fig. 4; Whiteley et al., 2002, p. 163, pl. 166, 167, fig. 5.5B.
Cryptolithus lorettensis Foerste, 1924, p. 236, pl. 43, figs. 15, 16.
Trinucleus bellulus Ulrich, 1878, p. 99, pl. 4, fig. 15.

Description: Family Trinucleidae. Considered an index fossil if the Sugar River Limestone of the Trenton Group by some authors but is not uniformly distributed. Whole specimens are rare. Although small, this trilobite has a distinctive and robust cephalon. The glabella is vaulted and smooth, with only two shallow pits anterior to the occipital ring for glabellar furrows. The cephalon is rounded in anterior outline and about twice as long as wide when including the long genal spines. The occipital ring has a central spine extending backward over the first few thoracic segments. Cryptolithus is a blind trilobite, lacking eyes on the cephalon. The thorax has six thoracic segments, and the pygidium is rounded, being much broader than it is long. The most characteristic feature of the trilobite is the anterior brim or fringe of the cephalon, which has uniform rows of pits following the curvature of the anterior edge. These pits, their number, and arrangement are major factors in distinguishing the various species. Shaw and Lespérance (1994) believe the method of determining species in this genus is incorrect stating:

Museum and field restudy of Cryptolithus from all known geographic and stratigraphic occurrences in eastern North America shows that the principle variable character in this genus is the number of fringe pit arcs. Because this character varies within populations and even single individuals, it cannot be used to distinguish the earlier, typologically defined species of the genus.

Shaw and Lespérance instead choose to use morph designations for the different variants of pit placement. At this point in time it is still unclear which method is more effective. The thorax has six thoracic segments, and the pygidium is rounded, being much broader than it is long.

Ecology: Campbell (1975) suggests that the mode of life for Cryptolithus was a benthic mud-feeder, able to dig down into the substrate, and with limited powers of walking and swimming. He argued that Cryptolithus had the ability to rapidly outroll the thorax and pygidium and that such an ability might have been used in swimming and righting the animal if it were rolled over. The animal would have rested partially buried in the substrate on the outer portion of the fringe and genal spines, the innermost rows of pits exposed above the sediment. Campbell argued that the fringe probably had a sensory function and that it contained a branching system of digestive glands.

*Cephalon with reconstructed thorax and pygidium - MCZ 146480: 21.5mm wide*

*A reconstruction of C. tesselatus showing the ventral side and appendages from Raymond, 1920*

Diacanthaspis parvula (Walcott, 1877) - Description from Whittington, 1941, Brett et al., 1999, and Whiteley et al., 2002. Ecology from Whittington, 1941
Acidaspis parvula Walcott, 1877b, p. 16; Walcott, 1879, p. 69.
Odontopleura parvula (Walcott). Clarke, 1892, p.69; Clarke, 1894, p. 744, fig. 61; Wilson, 1947, p. 43, pl. 10, fig. 8.
Diacanthaspis? parvula (Walcott). Whittington, 1941, p. 502.
Diacanthaspis parvula (Walcott). Brett et al., p. 301, fig. 9.8; Whiteley et al., 2002, p. 128, pl. 132.

Description of the genus: Family Odontopleuridae. Fairly uncommon. The glabella is slightly wider than it is long. There are two pairs of suboval lateral lobes, the posterior pair being the larger. The median lobe is wider than the lateral lobes. The occipital ring is long and has a very prominent pair of marginal spines. The occipital lobes are small and faint. The eyes of Diacanthaspis are situated near the back of the cheeks and closer to the axis of the cephalon. The facial sutures are not fused. The genal spines are curved, occurring at the posterolateral angle of the cephalon as a projection of the rolled margin of the free cheeks. The cephalon is ornamented with numerous short spines in symmetrical arrangement. On the thoracic segments there is a conspicuous pair of spines on the axial rings and various other short spines in symmetrical arrangement on the axial rings and pleurae. The pleurae have short marginal spines, which become long in the posterior segments. The pygidium is composed of three segments. There are twelve marginal spines of equal length, and other short spines on the axial and pleural lobes.

Diacanthaspis parvula is found along with Meadowtownella trentonensis and both are small trilobites with similar shapes. The former can be readily distinguished from M. trentonensis because of its significantly more pustulose character and the presence of prominent pustules on the axis and pleural regions of the thoracic segments. The thoracic pleural spines are proportionally longer and less curved than those on M. trentonensis.

Ecology: In the early developmental stages the exoskeleton is spiny. These growths usually become reduced in length and may become tubercles, or may even be lost, as the trilobite grows. The spines on the pygidium, as discussed by Whittington (1941), prevented it from enrolling into a close-fitting ball. Presumably their spines protected them when they clung fast to the floor of the sea. It has been suggested that the power of enrollment would presumably be valuable, if they were attacked while swimming, to enable them to close up and sink rapidly to the protection of the sea floor.

Flexicalymene senaria (Conrad, 1841) – Description from Whittington, 1941, Brett et al., 1999, and Whiteley et al., 2002. Ecology from Whittington, 1941
Calymene senaria Conrad, 1841, pp. 38, 49; Emmons, 1842, p. 390, fig. 2; Hall, 1847, p. 238, pl. 64, figs. 3a-n.
Flexicalymene senaria (Conrad). Whittington, 1941, p.493, pl. 72, figs. 1-27, 31-34, 38-40, 42-47, text. Fig. 1; Wilson, 1947, p. 48, pl. 10, figs. 11a, 11b, 12; Evitt and Whittington, 1953, p. 49, pl. 9, figs. 1-16, pl. 10, fig. 1; Stumm and Kaufman, 1958, p. 949, pl. 123, figs. 1-11; Ross, 1967, p. B14, pl. 4, figs. 1-6; Ludvigsen, 1979a, p. 46, fig. 29; Brett et al., 1999, p. 301, figs. 9.1, 9.2; Whiteley et al., 2002, p. 133, pl. 67-70.

Description: Family Calymenidae. The cephalon of Flexicalymene senaria is subsemicircular in outline, about twice as wide as long, and very strongly convex. The glabella is subtriangular in outline and has a rounded anterior end. It is convex, standing high above the cheek lobes, and reaches a little farther forward. There are three pairs of lateral glabellar lobes that are graduated in size. The pair nearest the occipital ring is large and the second and third pairs become consecutively smaller anteriorly. Lateral furrows are also deeper and more developed around the posterior lobes. The occipital furrow is deep, shallowing and swinging forward in a gentle curve axially. The fixed cheeks are strongly convex and the free cheeks are subtriangular in outline, descending in a steep convex slope to the thick, rolled margin. The eyes are positioned just opposite the second glabellar lobes with a ridge running from the palpebral lobes to the third glabellar lobes. The hypostome is subrectangular in outline. The cephalon has an ornamentation consisting of tubercles of various sizes that are most concentrated on the glabella, occipital ring, and area anterior to the eyes. The thorax has 13 segments. On these segments the axial ring is strongly convex, the pleurae widen laterally, and the extremities are rounded and without spines. In enrollment the lateral parts of the pleurae slide underneath those of the next anterior segment where processes on the frontal edge of the pleurae limit movement. The pleurae also lack facets. The axial lobe is covered with closely spaced medium-sized tubercles. The pygidium is subtriangular in outline and very strongly convex. It is composed of six conjoined segments and is covered by medium-sized, closely spaced tubercles.

Due to its wide distribution and robust exoskeleton, F. senaria may be one of the most common trilobites found in the New York Trenton group. Ross (1967) suggested specimens found in the Walcott-Rust Quarry of F. senaria are a separate species owing to their unusually pustulose appearance and short genal spines. Ross also pointed out that there are least four different trilobites in New York called F. senaria. Revision of this group is presently in progress.

Ecology: Flexicalymene protected itself by enrollment where the margin of the pygidium and the lateral extremities of the bent-down pleurae fit inside the rolled anterior and lateral margin.

Gabriceraurus dentatus (Raymond and Barton, 1913) - Description from DeMott, 1987, Brett et al., 1999, and Whiteley et al., 2002.
Ceraurus pleurexanthemus Green. Hall, 1847, pl. 65, figs. 1d, 1h, 1i, 1m, pl. 66, fig. 1a-1g; Brett et al., 1999, p. 300, fig. 8.3, 8.5, 8.7.
Cheirurus pleurexanthemus (Green). Billings in Logan, Murray, Hunt, and Billings, 1863, p. 188, fig. 188.
Ceraurus dentatus Raymond and Barton, 1913, p. 534, pl. 1, fig. 2, pl. 2, figs. 4, 5; Raymond, 1921, pl. 10, figs. 2, 3, pl. 11, figs. 7, 8; DeMott, 1987, pl. 9, figs. 1-7, pl. 10, figs. 1-3.
Gabriceraurus cf. dentatus (Raymond and Barton, 1913). Brett et al., 1999, p. 300, fig. 8.6.
Gabriceraurus dentatus (Raymond and Barton). DeMott, 1987, p. 78, pl. 9, figs. 1-7, pl. 10, figs. 1-3; Lesperance and Desbiens, 1995, p. 15, fig. 5.1-5.3
non Ceraurus dentatus Raymond and Barton. Raymond, 1921, pl. 10, fig. 2; Sinclair, 1964, pl. 4, fig. 10; Ludvigsen, 1978, fig. 33; Ludvigsen, 1979a, fig. 26c.
Ceraurus hirsuitus Ludvigsen, 1979. Hessin, 1989, p. 1204, pl. 1, figs. 1-7, Text figs. 2, 3a.

Description: Family Cheiruridae. An unusually large species for this genus, Gabriceraurus dentatus is separated from the more common Ceraurus pleurexanthemus due to its larger size and eyes which are set further back on the cephalon. The glabella is broad, flat, moderately inflated to the rear, and very deflated in the frontal lobe. The glabella does not reach the frontal margin. Large and small pustules sprinkled with random distribution over all glabellar lobes. The occipital ring is broad and smooth. The cheeks, within smooth boarders, are covered with a random sprinkling of small pustules and pits. Pustules are thick in the area posterior and adaxial (the side nearest to the axis) from the eye, but no pair or cluster of pustules are outstanding in this region distinct from the rest of the cheek surfaces (unlike C. pleurexanthemus). The eyes are well back, opposite the back edge of the second posterior lobes and located well out on the cheeks. The eyes also stand as high or slightly higher than the glabella. The genal spines are stout, taper rapidly, and are elliptical in cross section near the base. The thorax, of about eleven segments, has pairs of very low pustules along its axis. The pygidium has four nested segments. The most anterior of which bears the flaring, lightly curved, pygidial spines. The second and third segments extend beyond the border in short, well developed, and sharp-pointed teeth. The fourth segment may bear a small tooth as well.

Ecology: See Ceraurus pleurexanthemus.

Hypodicranotus striatulus (Walcott, 1875) – Description from Whittington, 1952,Brett et al., 1999, and Whiteley et al., 2002. Ecology from Whittington, 1952.
Remopleurides striatulus Walcott, 1875b, p. 347, fig. 27A, a, b; Raymond, 1925, p. 57, pl. 3, figs. 4, 5.
Hypodicranotus striatulus (Walcott). Whittington, 1952; Ludvigsen, 1979b, p. 50, fig. 31A, 31B; Ludvigsen and Chatterton, 1991, p. 620, pl. 1, figs. 1-28, text. figs. 1a, 2 (with complete synonymy); Lesperance and Desbiens, 1995, p. 7, figs. 3.1-3.5; Brett et al., 1999, p. 299, fig. 8.1; Whiteley et al., 2002, p. 161, pl. 160, 161, 162.

Description: Family Remopleurididae. The occipital ring is longitudinally flat, transversely moderately convex, bounded laterally by the posterior branches of the facial sutures, which are straight but run diagonally back from the posterior corner of the eye lobe, then curve to run into the posterior margin at right angles. Inside and parallel to the suture is a faint depression, the axial furrow, and just inside the point where the suture cuts the posterior margin is a rounded notch, the articulating socket. The occipital furrow is shallow. Between the eye lobes the glabella is gently and evenly convex both longitudinally and transversely. The eye lobe is long, in dorsal aspect curved, most strongly in the posterior part. The eye surface is gently convex transversely, and slopes steeply. The outer margin of the eye lobe is defined by a narrow, convex border which commences at the posterior corner and runs forwards to merge anteriorly with the border of the cheek. This species has long genal spines running parallel to spine like extensions of the occipital ring, giving the appearance of a dual genal spine. The hypostome is like no other. It is shaped like a tuning fork and extends to the end of the thorax. The thorax is moderately convex transversely, the axis broad, narrowing rapidly backwards. It seems to consist of eleven segments. A short spine is present on the eighth thoracic segment and the pygidium is very small, subrectangular, and looks to be notched.

Ecology: The long hypostome of H. striatulus prevented enrollment, but did provide some protection for the ventral surface. If no movement was possible at the hypostomal suture, then the amount of possible movement of the thorax and pygidium in the vertical plane, relative to the cephalon, must have been severely limited. The mode of articulation of the thorax precludes any considerable movement relative to the cephalon in the horizontal plane. There is ample room for the appendages to project downwards and outwards between the hypostome and the thoracic pleurae. Its streamlined shape suggests this species may be pelagic (swimming) and its wide distribution implies this.

Isotelus gigas Dekay, 1824 - Description from Brett et al., 1999 and Rudkin and Tripp, 1989
Isotelus gigas Dekay 1824, p. 176, pl. 12, figs. 1, 2, pl. 13, fig. 1; Vanuxem, 1842, p. 47, fig. 1; Hall, 1847, p. 231, pl. 63, fig. 1; Raymond, 1910a, fig. 1; Raymond, 1914, p. 248, pl. 1, figs. 1, 2, pl. 2, figs. 2-5, pl. 3, fig. 3; Bassler, 1919, pl. 48, figs. 23-25; Raymond, 1920, p. 91, fig. 28; Foerste, 1924, p. 241, pl. 44, fig. 6; Whittington, 1950, pl. 73, figs. 1, 2, 4; Howell, 1951, p. 265, pl. 2, fig. 3; Jaanusson, 1959, figs. 251, 2a, b; Whittington, 1960, fig. 4b; Lu et al., 1965, p.123, pl. 28, figs. 2a, b (as Jaanusson, 1959); Darby and Stumm, 1965, pl. 1, fig. 7; Henningsmoen, 1975, fig. 13; Towe, 1973, p. 1008, figs. 1f, g, k, l; Levi-Setti, 1975, pl. 55; Teigler and Towe, 1975, pl. 1, figs. 1, 2, pl. 2, fig. 2, pl. 3, figs. 2, 5, pl. 7, figs. 1, 2; Ludvigsen, 1978, pl. 1, figs. 18, 19 [non fig. 17]; Ludvigsen, 1979a, figs. 24c, d; Ludvigsen, 1979b, figs. 6G, H, 12E-H DeMott, 1987, p. 71, pl. 3, figs. 1-11; Rudkin and Tripp, 1989, p. 5-10, figs. 1.1-1.6, 2.1-2.3; Brett et al., 1999, p. 297, fig. 7.1; Whiteley et al., 2002, p. 159, pl. 150-153.
Isotelus planus Dekay, 1824, p. 178, pl. 13, fig. 7; Green, 1832, p. 560; Green, 1832, p. 68, cast 23.
Isotelus cyclops Green, 1832, p. 560, pl., fig. 7; Green, 1832, p. 69, cast 24.
Isotelus megalops Green, 1832a, p. 70, cast 25.
Isotelus stegops Green, 1832, p. 71, casts 26, 27.
Isotelus jacobus Clarke, 1897, pt. 2, p. 706, footnote.
?Asaphus platycephalus Stokes, 1824, ser. 2, pl. 27, no description; Buckland, 1837, p. 73, pl. 45, fig. 12, 1867, pl. 63.
Asaphus platycephalus Bronn, 1835, p. 115, pl. 9, fig. 8; Bronn, 1851-1856, p. 632, pl. 9, fig. 8, pl. 9, fig. 5; Burmeister, 1843, p. 127, pl. 2, fig. 12; Burmeister, 1846, p. 110, pl. 2, fig. 12; Walcott, 1881, p. 198, pl. 2, fig. 9.
Asaphus gigas Dalman, 1826, p. 276; Dalman, 1827, p. 91; Dalman, 1828, p. 70.
Asaphus planus Dalman, 1826, p. 276; Dalman, 1827, p. 91; Dalman, 1828, p. 70.
Asaphus megistos Billings, 1863, p. 184, fig. 182.
Brongniartia isotelea Eaton, 1832, p. 33, pl. 2, fig. 12.
Isotelus sp. Raymond and Narraway, 1910, p. 56, pl. 15, fig. 3.
Isotelus Whittington, 1961, p. 17; Whittington, 1962, pl. 8.

Description: Family Asaphidae. The dorsal shield is oval in outline and has a surface characterized by conspicuous shallow circular pits of various sizes up to approximately 200 µm in diameter, which become steadily weaker towards the posterior of the exoskeleton; pitting is variable in strength among specimens. The cephalon is subtriangular to parabolic in outline, moderately convex in both directions, and the glabella is completely fused with the occipital ring. First and second lateral lobes and furrows are faintly distinguishable on some specimens. Small Isotelus gigas, particularly those under 50mm long, often have genal spines but these are lost as the trilobite became larger. The genal spines are narrow with a slight taper throughout their length. There is a distinct similarity between I. gigas, I. iowensis, and I. walcotti in relation to their spines. Raymond (1914) makes a detailed study of the ontogeny of I. gigas and the differences separating it from other Isotelids. Rudkin and Tripp (1989) also explain these differences by using mean measurements of different parts of the Isotelus exoskeleton from I. gigas and I. walcotti specimens. I. gigas differs from the other species because the genal spines are not kept into adulthood, and the cephalon and pygidium attain a more pointed appearance. The eyes of I. gigas are holochroal and situated high on the cephalon. There is a distinct axial furrow which runs along each side of the thorax and cephalon, but is not impressed into the pygidium. The thorax is composed of eight thoracic segments. The pygidium is subtriangular with twelve axial rings faintly marked by indistinct ring furrows on the surface. The specimens from the Walcott-Rust Quarry are in two populations. Numerous small holaspids, including the neotype, are from layer 4 and the larger specimens are from overlying rocks.

Ecology: Because its morphology, being a flat-lying trilobite with raised eyes and a horizontally oriented hypostome, it seems highly unlikely that Isotelus was an infaunal filter feeder. It has been suggested, however, that adult I. gigas had limited capabilities as a swimmer. It probably functioned as an epibenthic (above/on sediment) or shallow infaunal scavenger/predator, digging subhorizontal furrows and lying covered by a thin layer of sediment, with its eyes exposed. The pitting of the surface of I. gigas, being especially concentrated around the anterior portions of the body, has led some paleontologists to believe the pits acted as some sort of sensory organ. As a defensive measure I. gigas could enroll its body, with the margins of the cephalon and the pygidium fitting tightly edge to edge, so that the more vulnerable ventral anatomy would be protected.

*Isotelus growth series based on Raymond, 1914: from left to right, MCZ 100935, MCZ 100936, MCZ 100937, MCZ 100938*

Phylum Arthropoda

Class Ostracoda

Beyrichia bella expand_more

Ctenobolbina ciliata expand_more

Eoleperditia sp. expand_more

Leperditid expand_more

Tetradella subquadrans expand_more

Class Trilobita

Achatella achates expand_more

Amphilicas conifrons expand_more

Amphilicas cornutus expand_more

Amphilichas inequalis expand_more

Bumastoides holei expand_more

Bumastoides milleri expand_more

Bumastoides porrectus expand_more

Calyptaulax callicephalus expand_more

Ceraurus pleurexanthemus expand_more

Cryptolithus tessellatus expand_more

Diacanthaspis parvula expand_more

Flexicalymene senaria expand_more

Gabriceraurus dentatus expand_more

Hypodicranotus striatulus expand_more

Isotelus gigas expand_more

Isotelus iowensis expand_more

Isotelus walcotti expand_more

Kawina trentonensis expand_more

Meadowtownella trentonensis expand_more

Nanillaenus americanus expand_more

Indet. Proetid expand_more

Sphaerocoryphe robusta expand_more