BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 41. PP. 341-350, PLS. 5-6 JUNE 30. 1930 PROCEEDINGS OF THE PALEONTOLOGICAL SOCIETY

NEW INFORMATION ON HOMALONOTUS TRENTONENSIS1

BY LAWRENCE WHITCOMB

(Read before the Paleontological Society December 26,1929) 2

CONTENTS Page Introduction...... 341 History...... 342 Taxonomy...... 344 Description...... 345 Cranidium...... 345 Free cheek...... 345 Hypostome...... 345 Thorax...... 346 Pygidium...... 346 Size...... 346 Locomotion...... 346 Geographic distribution...... 347 Stratigraphie range...... 347 Correlation...... 348 Acknowledgments...... 349 Explanation of plates...... 350

I ntroduction

While collecting from the lower “Trenton” limestone, called the Salona formation by Professor Field3 in 1919, at the type locality, Salona, Clinton County, central Pennsylvania, in the summer of 1928, I obtained a large number of specimens of the trilobite described by Simpson4 forty years ago as Homalonotus trentonensis. Among the many dissociated parts were free checks and hypostomes, which had never been described, and one complete articulated cranidium, thorax, and pygidium.

1 Manuscript received by the Secretary of the Geological Society December 26, 1929. 2 An abstract of this paper was published in the Proceedings of the Paleontological Society for 1929 (Bull. Geol. Soc. of Am., vol. 41, p. 197, 1930). 8 R. M. Field : The Middle Ordovician of Central and South-Central Pennsylvania. Amer. Jour. Sci., vol. 48, Dec., 1919, pp. 403-428. 4 G. B. Simpson : New species of fossils from the Clinton, Lower Helderberg, Che­ mung and Waverly groups, found in the Collections of the Geological Survey of Penn­ sylvania. Trans. Amer. Philos. Soc., new ser., vol. 16, 1890, p. 460, flg. 31. (341)

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Homalonotus trentonensis is of particular interest because it is the oldest Homalonotid known in North America. Its stratigraphic and geographic range is limited, and it appears to be closely related to one of the Ordovician species found in Wales. It is the purpose of this paper to review the history of the species, to give a fuller and more accurate description of it in the light of this more complete information, and to compare it with its Welsh allies.

H istory Simpson’s paper establishing the species Homalonotus trentonensis first appeared in separate form, bearing the date 1889, and was included in the volume of the Transactions of the American Philosophical Society cited. The paper included figures of the eranidia and two pygidia here considered, and contained the following statement: “The figures represent a few of the many specimens in the State collection, found by Mr. C. E. Hall in strata of the Trenton group cropping out just above the milldam at Reedsville, in Mifflin County, Pennsylvania.” The species is listed in the “Dictionary of Fossils”5 of the Second Survey of Pennsylvania as follows: "Homalonotus trentonensis, Simp­ son. New species. For figures and localities, see the Appendix.” This appendix was never published. Under the authority of an act of the legislature of April 4, 1883, the collections of the Second Survey wer placed in the custody of the Acad­ emy of Natural Sciences of Philadelphia, where they are now preserved. In going through these collections, I was able to pick out the original cotypes, which have now been marked as such. The “Museum Catalogue”6 shows that the cotypes were not all col­ lected by Mr. C. E. Hall, as was stated by Simpson, although they were all obtained at the same locality. Of the specimens figured by Simpson, numbers 1, 2, and 4 were collected by W. A. Fellows in September, 1875. The specimens collected by A. Hale and C. E. Hall in May, 1875, and numbers 1, 2, and 4 were colected by H. Hale and C. E. Hall in May, 1875, and numbers 3, 5, and 6 were collected by W. A. Fellows in Sep­ tember, 1875. The specimens collected by Hale and Hall bear the field number 201; those collected by Fellows bear the number 204. Many of the paper labels that were pasted on the specimens have come off, so that it is now impossible to tell exactly what numbers were assigned to the types. Three type specimens, however, are known: Number 1 is 201-30, num­ ber 4 is 201-35, and numbers 5 and 6 are 204-5.

5 J. P. Lesley : Dictionary of Fossils, Pennsylvania Second Geol. Survey, 1889, T-4, vol. 1, p. 289. 9 C. E. H a ll: Museum Catalogue, Pennsylvania Second Geol. Survey, 1889, 000, vol. 3, pp. 170, 179.

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According to the table in Merrill’s “Contributions to a History of American State Geological and Natural History Surveys,” 7 Fellows worked for the Pennsylvania Survey only in 1878 and 1879. His collec­ tions were probably made privately and acquired by the Survey, or a mistake has been made in the record of the date of their collection. Plaster casts of numbers 1, 4, and 5 of the cotypes are deposited in the New York State Museum at Albany. These casts bear the catalogue numbers 4495, 4496, 4497. The next reference to the species seems to have been made in 1903, when Dr. G. L. Collie 8 presented, at a meeting of the Geological Society of America, a paper on the “Ordovician Section Near Beliefonte, Penn­ sylvania.” In this paper Dr. Collie used the subgenerie name, Brong- niartia, which Salter9 had proposed in his revision of the genus Homa- lonotus. The species was figured anew, and for the first time part of the thorax was shown. The plesiotypes, a cranidium and a pygidium with ten thoracic segments attached, were collected from what he considered the basal zone of the Trenton at Bellefonte. In this paper, we get the first information about the associated fauna and a rough delimitation of the stratigraphic range of the species. Dr. Collie’s plesiotypes are now deposited in the Peabody Museum at Yale University under the number 7449. In 1919 Professor Field10 stated that he had found this species in many sections of the lower “Trenton” limestone in central Pennsylvania. He called it Brongniartella trentonensis, following the classification of F. R. C. Reed,11 and listed it as one of the characteristic fossils of his Salona formation, which he stated appeared to contain also an un­ described species of Brongniartella. I have studied Professor Field’s unpublished illustration and description12 of this species, which he called B. collianus, and believe that the specimens on which it was based are probably actually referable to B. trentonensis, and that Professor Field was misled by apparent differences which were'due solely to individual variations and to differences in preservation.

7 G. P. M errill: Bull. U. S. Nat. Mus., No. 109, 1920, p. 445. 8 G. L, Collie: Ordovician section near Bellefonte, Pennsylvania. Bull. Geol. Soe. Amer., vol. 14, 1903, pp. 407-420, figs. 1-2. 8 J. W. Salter: A monograph of tie British trilobites from the Cambrian, Silurian, and Devonian formations. The Paleontographical Society, London, 1865, pp. 104-105. “ Op. Cit. 11F. R. C. Eeed: Geol. Mag. new ser., decade 6, vol. 5, 1918, pp. 263-276 and 314- 327. 12 R. M. Field : Unpublished thesis, Harvard University Library.

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T a x o n o m y

Since 1825, when König13 chose the species hnighti as the genotype of Homalonotus, there has been a tendency to place an increasingly large number of species in that genus. By 1865, these species became so numerous and varied that Salter,14 although admitting the validity of the genus, divided it into subgenera as follows: Brongniartia Salter, 1865 (divided into two sections); Trim erus Green, 1832; Koenigia Salter, 1865; Dipleura Green, 1832; and Burmeisteria Salter, 1865. Since Salter’s time several attempts have been made to revise the classification of the Homalonotus group. In 1918, Reed15 published a comprehensive paper on the genus. This paper included a classification modified after Salter, but instead of Salter’s five or six subgenera, he proposed ten. He did not, however, mention H. trentonensis nor did he refer it to any of his subgenera. Reed shows that Brongniartia of Salter is a homonym, having been used by Leach in 1824 for a genus of Coleoptera and by Eaton16 in 1832 for a genus of trilobites based on a specimen that would now be referred to Triarthrus bechi. Reed, therefore, raised the two sub­ divisions of Salter’s Brongniartia to the rank of subgenera, giving them the names Eohomalonotus and Brongniartella. In summing up the Stratigraphie range of his subgenera, Reed showed that the first three, Eohomalonotus, Calymenella, and Brongniartella, are restricted to the Ordovician (the first being the oldest), and that none of the remaining seven subgenera is found below the Silurian. Of these three subgenera, Clymenella, unlike the species found in Pennsylvania, has a pronounced rostrum. It is represented by the species H. (Calymenalla) boisseli, (Bergeron) and II. (C.) bayani, (DeTrom. and Lebesc) and is restricted to Prance. This leaves only two known subgenera, Eohomalonotus and Brong- niartella, in which Simpson’s species can be placed. The characteristics distinguishing these subgenera are the relative width and definition of the axial lobe of the thorax and the number of segments in the pygidium. The determination of the relative width of the axial lobe is made diffi­ cult by the fact that the pleural segments are deflected downward, so that the spatulate ends are approximately parallel. As the axial lobe of its thorax is indistinct and is wider than the pleural lobes, and as its pygidium has ten axial segments, the species II.

13 König : leones Sectiles, 1825, pi. vii, fig. 80. 11 Op. cit. 15 Op. cit. 16 Amos E aton: Am. Jom*. Sei., sei*. 1, vol. 22. 1832, p. 165.

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trentonensis can not be placed in the subgenus Eohomalonotus, but ap­ pears to be referable to Brongniartella, to whose genotype H. bisulcatus, Salter, it seems to be closely related.

D e scr ipt io n

c r a n id iu m Homalonotus (Brongniartella) trentonensis (Simpson) Homalonotus trentonensis, Simpson, Trans. Am. Philos. Soc., vol. 16, 1890, new series, p. 460, fig. 31. Brongniartia trentonensis, Collie, Bull. Geol. Soc. America, vol. 14, 1903, p. 418, pi. 59, figs. 1-2. Sub trapezoidal, twice as wide as long, highly arched. Test finely punctate. Facial sutures starting at lateral extremities of the anterior margin, running posteriorly in a straight line, with slight deflection from the axis for one-half the length of the cranidium until they meet the eyes, then turning outward nearly at right angles to the axis and continuing in that direction almost to the margin, where they swing abruptly to the rear and cut the margin at or slightly in front of the genal angle. Anterior margin gently convex; anterior border broad, concave; posterior margin straight. Occipital groove faint (but very distinct in specimens which have peeled) and having a slight forward flexure at the center. Glabella urceolate; defined laterally by dorsal furrows; narrowest at a point one-third of the distance from the ante­ rior end, where it blends into the anterior border of the cranidium. Traces of one to three glabella furrows and median keel seen in some specimens, particularly those which have peeled.

FREE CHEEK Triangular with the lateral margin straight, the border concave, and the posterior corner developed into a curved process, which carries the free cheek back to or near the genal angle. Eyes elevated on a conical bulge of the free cheek. HYPOSTOME As looked at from its convex (under) side subquadrate, wider than long in the ratio of 7 to 6. Anterior margin straight, the lateral ex­ tremities curving posteriftrly to the point of greatest width, which is two-ninths of the distance to the posterior end. Anterior brim with front edge turned up, slightly depressed in the center, and lateral por­ tions deflected downward. Lateral margins sigmoidal, slightly raised at center, tapering gently to the rear. Posterior margin distinctly con­ cave, with the distance between the points about three-tenths the great­

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est width. Center distinctly bulbose, tapering to the rear, with a con­ centric ridge between it and the forked posterior margin.

THORAX Made up of 13 segments, highly arched, with the spatulate ends of the pleural segments nearly parallel, so that the cross section is approxi­ mately one-half as high as wide. The axial lobe is only slightly wider than the true width of the pleural lobes, although in the dorsal view it appears to be three times as wide as the pleural lobes, because of the ventral deflection of the pleura. Axial lobe of the thorax wider at the front than the base of the glabella, necessitating a lateral flaring of the dorsal furrows on the neck ring of the cephalon. Thorax tapering slightly in the last few segments. Test finely punctate.

PYGIDIUM Rounded, wider than long, with 8 pleural and 10 axial segments. Pos­ terior margin flattened and not segmented. Axial lobe tapering, and running back for about seven-eighths of the length of the pygidium. Segmentation faint on unpeeled specimens but very distinct on those from which the surface has been removed. The two anterior sulci, setting off the segments of the pleural lobes, are distinctly deeper than any of the succeeding ones. Highly arched. Test finely punctate. An­ terior pleural segments tapering and modified to allow the thoracic segments to fit over them when the animal is curled up. That the animal had the power of enrollment is shown by figure 4 of plate 6 in which a part of the thorax and pygidium are shown partly enrolled. The method by which the spatulate ends of the pleural segments over­ lap to the rear is also shown there.

SIZE My nearly complete specimen, which shows the effects of slight longi­ tudinal shortening, is 6.1 centimeters long; therefore, in view of the ratio of the length of the pygidia and cranidia to the length of the whole specimen, some of the larger individuals should have been over 11 centi­ meters long. L ocomotion

According to Professor Raymond17 the wide axial lobe of some species of Homalonotus would indicate that they “had very long, strong coxo- podites” which they “probably used in locomotion, and also very well-

17 P. E. Raymond : The appendages, anatomy, and relations of trilobites. Mem. Connecticut Acad, of Arts and Sci., vol. 7, 1920, p. 100.

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developed longitudinal muscles, to be used in swimming.” The broad, rounded pygidium was, according to the same author, not well adapted to burrowing, but was of assistance in swimming. The information that is now available on the cross section of the cephalon, thorax, and pygidium, of the species H. irentonensis, all indi­ cates that this animal had a well arched test and probably crawled on the ends of its endopodites.

G e o g r a p h ic D istribution

The “Trenton” limestone of central Pennsylvania is exposed in a number of anticlinal valleys, which trend in general from northeast to southwest. According to Professor Field18 these valleys make up a rather compact and isolated area of the Ordovician terranes of eastern North America. This area extends from Kishacoquillas Valley, on the southeast, to Nittany Valley (with its extensions along the strike from Lycoming County to Bedford County) on the northwest. This area measures about 120 miles along the strike and 23 miles across the strike. S . irentonensis was first collected at Reedsville, which is on the southeasern flank of the anticline of Kishacoquillas Valley, about mid­ way between its ends. It has since been found in all the valleys where the “Trenton1” is exposed, from Salona, at the northeast end of Nittany Valley, to Loysburg, 80 miles to the southwest; and from Bellefonte, on the northwest, to Reedsville on the southeast. It has therefore been found throughout the whole width and nearly the whole length of this “Trenton” district. Although a very common and characteristic fossil of the lower beds of the “Trenton” throughout this area, H. irentonensis has never been reported from any locality outside of these valleys of central Pennsyl­ vania. It is apparently an isolated species, and its nearest known contem­ poraries lived in Wales, where the remains of several species of Ordovician Homalonotids have been found.

S tratigraphic R a n g e

In his original description of the species, Simpson19 said that the fossils were collected from “strata of the Trenton group.” Dr. Collie20 in his paper already cited, says: “Horizon A-8 may be regarded as the basal zone of the Trenton. It is marked by the abundant remains of a homalonotoid trilobite, Brongniartia irentonensis (Simpson). Other

18 Op. cit., p. 405. 10 Loc. cit. 20 Op. cit., p. 415.

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fossils occurring in this zone are “Trinucleus concentrions, Isotelus platycephalus, Gytoceras camurum H all, Protowarthia tenuissima sp. nov., Rafinesquina alternata (Emmons), Dalmanella testudinaria (Dal- m an), Orthis tricenaria Conrad.” In the section of the “Trenton” at Salona, H. irentonensis persists through a zone 33 feet thick, whose base is 70 feet above the bed that has been regarded as the base of the “Trenton” in that region. Through­ out these 33 feet it is the most abundant fossil, and it is accompanied b j Cryptolithus tessalatus. Graptolites, identified by Dr. Buedemann as Diplograptus amplexi- caulis, occur 40 feet beneath the lowest bed in which H. irentonensis has been found. The lowest beds yielding specimens of Parastrophm hemiplicata are 81 feet above the highest beds yielding specimens of H. trentonensis, or 184 feet above the base of the section. As Parastrophia hemiplicata occurs at the base of the type section of the Trenton at Trenton Falls, New York, and at the base at Montmorency Falls, Quebec, as deter­ mined by Professor Field21, it is significant that we find no specimens of H. trentonensis at those localities.

C o r r é l a t i o n

When H. trentonensis was compared with the several species of Homalonotus found in Wales it was seen to be most closely related to H. bisulcatus. The one distinct difference between the two species is the variation in the number of annulations in the axial lobe of the pygidium. In H. bisulcatus there are 11 or 12 of these acculations, but in H . tren­ tonensis there are only 10. In the description of trentonensis given by Dr. Collie the number of annulations was said to be 11. I have exam­ ined Dr. Collie’s type pygidium and am unable to find more than 10 annulations. All of the many pvgidia in my collections have 10 annulations. With the exception of the difference mentioned above, Salter’s de­ scription of H. bisulcatus would be perfectly satisfactory for H. tren­ tonensis. Any slight variations that may be noted are no greater than variations shown by individuals collected from the same locality and horizon in Pennsylvania. The greatest cause for apparent variation is the difference in the lithology. The British specimens are preserved in sandstone or other clastic rock ; the Pennsylvania specimens are preserved in a dense limestone. In consequence of this difference in lithology, the

21 R. M. Field : Personal communication, 1929.

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surface markings and the test itself are often well preserved in H . tren- tonensis, whereas most of the British specimens available for use in the study made for this paper were natural casts and none of them preserve the fine details of the test. The specific name bisulcatus was given be­ cause the two anterior sulci of the pygidium are disproportionally deep, the rest being very shallow. This peculiar characteristic holds just as well for H. trentonensis as for H. bisulcatus and is thus of value in determining the relations of the species. Dr. E. 0. Ulrich and others have stated that we should expect to find a difference in one or more parts of the anatomy of organisms whose paths of migration led them to inhabit areas so widely separated as Wales and Pennsylvania, and this variation is therefore not surprising. It consequently seems best to consider H. trentonensis a distinct species, whose closest known relative was H. bisulcatus. According to Mr. B. B. Bancroft,22 of Gloucestershire, England, the type specimens of II. bisulcatus were collected from the Longville flags. Prom unpublished information, kindly supplied to Professor Field by Mr. Bancroft, it appears that in East Shropshire, the species H. bisulcatus is limited in its stratigraphic range to the Upper Longville and Marsh- brook beds of the Caradoc. The species H. bisulcatus and II. trentonensis are probably of about the same geologic age, and the beds containing H. trentonensis can per­ haps be correlated with beds of Upper Longvillian or Marshbrookian age in England. A cknowledgments

I am deeply grateful to many persons who have assisted me in the work of preparing this paper. I am especially indebted to Prof. R. M. Field, of the Department of Geology, Princeton University. It was his knowledge of the similarity between the Pennsylvania and British sec­ tions and faunas that led me to make this study. He has been my constant adviser, and he has established the personal contacts that have enabled me to obtain both material and information concerning the British species. Prof. B. F. Howell, of the same department, has given freely of his paleontological knowledge in the close examination of the specimens and has read the manuscript. To Mr. B. B. Bancroft, of Blakeney, Gloucestershire, England, I am under deep obligations. He has supplied numerous specimens of British Ordovician species of Homalonotus and has supplied valuable information as to their strati­ graphic range. Without his assistance this paper could not have been

2715. I?. Bancroft’: Personal communication, 1029.

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written. Mr. T. C. Nicholas, of Trinity College, Cambridge, kindly arranged for the loan of specimens and casts from the collections of the Sedgwick Museum, Cambridge. I am also indebted to the Academy of Natural Sciences of Phila­ delphia for the loan of Simpson’s cotypes; to Prof. C. 0. Dunbar, of Yale University, for the loan of Collie’s plesiotypes from the collections of the Peabody Museum; and to Prof. P. E. Raymond, of Harvard Uni­ versity, for the use of certain material in the Harvard collections that was obtained by Professor Field in central Pennsylvania in 1916-1S1S.

E x p l a n a t io n o f P l a t e s *

P l a t e 5.— Honialonotus trentonensis

F i g u r e s l-o.—Cotypes of Simpson. No. 5 whitened. Figures 6-7.—Plesiotypes of Collie. F i g u r e 8.—Well preserved cranidium. F i g u r e 9.—Profile of cranidium in fig. 8.

P l a t e 6 .— Homalonotus trentonensis

F ig u r e 1.—Free cheek taken perpendicular to the gre: test surface. F i g u r e 2.—Specimen complete except for the free cheeks. F ig u r e 3.—Free cheek, dorsal view. F ig u r e 4.—Enrolled portion of thorax and pygidium. F i g u r e 5.—Spatulate end of pleural segment of thorax. F i g u r e 6 .—Polished section through the thorax shown in fig. 8, showing the pleural segments deflected downward, and trace of the ventral integument. F i g u r e 7.—Weathered surface showing the true cross section of the thorax. F i g u r e 8.—Part of thorax, pygidium. cranidium, and mold of another pygi­ dium. (Light from lower right-hand corner.) F i g u r e 9 .—Hypostome revealed by cutting down through the glabella, por­ tions of the thorax and cephalon still to be seen. F i g u r e 10.—Hypostome, ventral view.

* All illustrations are natural size.

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HOMALONOTUS TRENTONENSIS

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HOMALONOTUS TRENTONENSIS

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