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PENNSYLVANIAN INVERTEBRATES OF THE Mazon Creek Area, Illinois

TRILOBITOMORPHA ARTHROPLEURIDA

EUGENE S. RICHARDSON, JR. Curator of Fossil Invertebrates

FIELDIANA: GEOLOGY

VOLUME 12, NUMBER 4

Published by

CHICAGO NATURAL HISTORY MUSEUM

JANUARY 25, 1956

PRINTED IN THE UNITED STATES OF AMERICA BY CHICAGO NATURAL HISTORY MUSEUM PRESS

Trilobitomorpha^ Arthropleurida

The large terrestrial arthropod, Arthropleura, has hitherto been reported only from Europe, where fragments are not uncommon in some Westphalian (Middle Pennsylvanian) deposits. Unfortu- nately, no complete specimens have been found; the head is known from only one partial impression that lacks antennae or other appendages, and the tail is unknown. Thus, the classification is doubtful. Waterlot (1934) defined the subclass Archaeocrustacea to include two orders, the Trilobita and the Arthropleurida, within the class Crustacea, in which Van der Heide (1951) has followed him. Moore, Lalicker, and Fischer (1952) have retained the order Arthropleurida of Waterlot as a class, parallel to the class Trilobita and others within the subphylum Trilobitomorpha.

Phylum ARTHROPODA

Subphylum TRILOBITOMORPHA

Class ARTHROPLEURIDA

Family ARTHROPLEURIDAE

Genus Arthropleura Jordan

The first specimens of Arthropleura that have been reported from North America are described below. ^ Since they consist of portions of the leg and its supposed articulating apparatus, and since species are discriminated on the basis of the ornamentation of the dorsal tegument, these specimens can not be specifically identified. I col- lected two of the three specimens in 1952; the third and most com- plete was found by Mrs. John M. McLuckie in 1953. They are from the Middle Pennsylvanian Francis Creek shale of Will County, Illinois, and were discovered in dump heaps of strip mines. The faunal and floral associates and their occurrence have been discussed briefly (this volume, no. 1, pp. 8-10; see map, fig. 3, in that paper, for detailed locality records).

1 Though Handlirsch (1906, p. 340) is probably correct in attributing a frag- mentary "Mylacridae?" of Scudder, from Mazon Creek, Illinois, to Arthropleura.

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72 FIELDIANA: GEOLOGY, VOLUME 12

Arthropleura sp.

Figures 38-41

Seven species of Arthropleura are known from Westphalian and Stephanian deposits of western Europe. The body is composed of an unknown number of segments (at least 22), consisting, like those of trilobites, of axial and pleural lobes. There is no differ- entiation into thoracic and abdominal segments, nor any fusion of segments into a pygidium. Ventrally, each segment bears a pair of biramous appendages, the endopodite and exopodite being similar. At the base of each leg is a "rosette organ" composed of triangular bosses arranged like the petals of half a flower; this organ is re- garded by Waterlot (1934) as constituting the attachment of the leg to the body. Our specimens consist of one leg with the rosette organ attached, and two isolated rosette organs. The size of the leg implies an animal nearly twice as long as Waterlot's (1934, pi. A; see fig. 38) restoration of A. armata, while the size of the larger rosette organ before us implies an animal 3J4 times the size of armata; the largest European species, A. pruvosti Waterlot, is slightly more than twice as long as the restoration of A. armata. *rhe Arthropleura of the ancient Mazon Creek coastal plain in Illinois must have been about five feet long — by far the largest animal of this fauna.

The leg consists of ten joints, including the basis; in the specimen, the exopodite covers the endopodite, only part of which can be seen. Each joint of the leg terminates postero-distally in a slightly re- curved spine; the last joint is claw-like. Above the basis lies the rosette organ, surrounded on the side opposite the leg by a convex border. Comparing the bosses of the rosette organ with the nomen- clature adopted by Waterlot (see fig. 40, e), we find his elements C, E, L and L'. The plate I is probably represented on the McLuckie specimen as a protuberance from the basis, marked by several irregular grooves. As in other species of Arthropleura, plate C is the most heavily shagreened in texture. The convex border of the rosette organ corresponds to the sternite of Waterlot. The two detached rosette organs have a well-developed plate I and small accessory bosses distad of L. The basis extends beyond the rosette organ on the most complete specimen as a heavily granulated plate that blends posteriorly into a less papillose plate that curves into a plane at right angles to the plane of the leg and represents the K plate (epipodite?).

Fig. 38. Restoration of Arthropleura armata Jordan 1854, from Waterlot (1934, pi. A). Ruled line = ten centimeters.

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FIELDIANA: GEOLOGY, VOLUME 12

Fig. 39. Arthropleura sp., leg and rosette organ preserved as impressions in matching halves of a concretion. McLuckie collection.

Waterlot's interpretation of the significance of the various plates of the rosette organ was based on a study of a great many specimens and should perhaps not be challenged on the basis of three. He concluded that the heavily shagreened C was a precoxa, joined to the base of the leg, and that I and E, embracing it, were swellings of the sternite surrounding the precoxa, by means of which the leg was flexibly locked to the body. The drawing that embodies his

RICHARDSON: TRILOBITOMORPHA, ARTHROPLEURIDA 75

Fig. 40. Arthropleura sp., rosette organs, a, b, CNHM no. PE154, paired impressions; c, d, CNHM no. PE153, paired impressions; e, sketch of rosette organ, showing nomenclature of elements as employed by Waterlot and others. B, basis; C, precoxa; E, swelling of sternite, external to C; E2, swelling accessory to E; I, swelling of sternite, internal to C; L, accessory linguiform swelling of sternite; L', accessory linguiform swelling of sternite; S, sternite.

I c Zli

Fig. 41. Arthropleura, morphotype; section across body showing relation of leg to body, as proposed by Waterlot (1934, fig. 27). Symbols as employed in figure 40, plus En, endopodite; Ex, exopodite; K, K-plate (epipodite? or gnath- obase?).

conclusions (see our fig. 41) shows a leg wholly outside the body, set off from the body cavity by an entire sternum, with no provision for leg muscles. It seems possible that the sternite, or ventral membrane, covered only the space between the bases of right and left legs, and that the bases themselves communicated with the body cavity.

Whatever may be the function of the rosette organ, it evidently forms some sort of a unit in itself, since it may occur apart from the leg or any other part of the animal. Besides the two specimens illus- trated here, there is one figured by Pruvost (1919, fig. 20), one figured by Waterlot (1934, pi. 13, fig. 4), and two figured by Van

76 FIELDIANA: GEOLOGY, VOLUME 12

der Heide (1951, pi. 1, fig. 18; pi. 2, fig. 7). An understanding of the form and function of this organ must await further discoveries of specimens.

If more specimens of Arthropleura are found in suitable deposits in North America, they may be among the best guide fossils of the Pennsylvanian non-marine sequence. Waterlot (1934, pp. 104, 282) remarks that any species of Arthropleura is confined to one, or at the most to two, zones of the Westphalian in Europe, though Van der Heide (1951) has since extended the range of A. armata to three zones.

Specimens. — CNHM nos. PE153 and PE154, detached rosette organs, each consisting of paired impressions in concretions (coordi- nates F8.7, d8.2 on map, fig. 3, this volume, no. 1); collected in 1952 by E. S. Richardson, Jr., and George Langford. Specimen of leg with attached rosette organ in collection of Mr. and Mrs. John M. McLuckie, Coal City, Illinois (approximate coordinates H2, f2); collected in 1953 by Mrs. McLuckie. I am grateful to Mr. and Mrs. McLuckie for permission to study their specimen at the Mu- seum.

REFERENCES

Handlirsch, Anton

1906. Die fossilen Insekten. Leipzig, ix + 1430 pp.; xl pp., 51 pis., in Atlas, 1908. (Page 340 published August, 1906.)

Moore, Raymond C, Lalicker, Cecil G., and Fischer, Alfred G. 1952. Invertebrate fossils, xiii + 766 pp., illus. New York.

Pruvost, Pierre

1919. La faune continentale du terrain houiller du Nord de la France. Paris, Ministere des Travaux publics, Memoires pour servir a I'explication de la carte geologique detaillee de la France. 584 pp., 29 pis.

Van der Heide, S.

1951. Les arthropodes du terrain houiller du Limbourg meridional (excepte les scorpions et les insectes). Netherlands, Geol. Sticht., Meded., Ser. C-IV-3, no. 5, 84 pp., 10 pis.

Waterlot, Gerard

1934. Etudes des Gites Mineraux de la France. IL Faune fossile. Etude de

la faune continentale du terrain houiller sarro-lorrain, viii + 317 pp., 24 pis.,

66 text figs. 1949. Ordre des Arthropleurides. Traite de Zoologie (ed. P.-P. Grasse), Tome

6, pp. 211-216, figs. 37-39.

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