S!YSTiEM\ATICS ANID EVO(LUJT(ION* O)F PH-ACOPS RAN (GEEN, 1832) AN1*D PHIACOPS IOWVEN)SIS DELO, 1935 (TR.ILoBITA) FROM-1\ THE MI41DDLE -D.EVONI)*;AN O)F NO(RTHNAMERICA NILES ELDREDGE BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY VOLUME 147:ARTICLE 2 NEW YORK:1,972 SYSTEMATICS AND EVOLUTION OF PHACOPS RANA (GREEN, 1832) AND PHA COPS IOWENSIS DELO, 1935 (TRILOBITA) FROM THE MIDDLE DEVONIAN OF NORTH AMERICA NILES ELDREDGE Assistant Curator, Department ofInvertebrate Paleontology The American Museum ofNatural History BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY VOLUME 147: ARTICLE 2 NEW YORK : 1972 BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY Volume 147, article 2, pages 45-114, figures 1-28, tables 1-9 Issued February 14, 1972 Price: $2.70 a copy Printed in Great Britain by Lund Humphries CONTENTS ABSTRACT....................................49 INTRODUCTION..................................51 Abbreviations.................................52 MIDDLE DEVONIAN STRATIGRAPHY..........................5 MORPHOLOGY AND RELATIONSHIPS OF THE BiOSPECIES Plhacops rana (Green, 1832) and Phacops iowensisDelo, 1935...............................56 Systematic Paleontology.............................58 PhacopsiowensisDelo, 1935..........................58 Phacopsrana (Green, 1832)...........................59 The Affinities ofPhacops rana and Phacops iowensis...................59 Previous Work on Middle Devonian Phacopid Taxa in North America..........60 ANALYTICAL TECHNIQUES ...............61 ONTOGENY OF THE CEPHALON OF Phacops rana.....................65 INTERPOPULATIONAL VARIATION IN Phacops rana.....................69 Interpopulation Variation in Number of Dorsoventral Files...............70 Factor Analysis and Interpopulational Variability ..................71 Systematic Paleontology.............................78 Phacops rana crassituberculata Stumm, 1953....................78 Phacops rana milleri Stewart, 1927........................79 Phacops rana norwoodensis Stumnm, 1953......................79 Phacops rana paucituberculata, New Subspecies...................80 Phacops rana rana (Green, 1832).........................81 A Theory ofRelationships among Subspecies ofPhacops rana ..............81 Trends (Biostratigraphic Character Gradients) in Phacops rana..............86 Mode of'Origin of the Subspecies..........................87 Biostratigraphic Significance ofPhacops rana.....................88 N7ARIATION IN Phacops iowensis ............................89 Qualitative Analysis ofMorphology.........................89 Subtaxa ofPhacops iowensis .............................91I Systematic Paleontology.............................91 Phacops iowensi-s alpenensis (Stumm, 1953).....................91 Phacops iowensis iowensis Delo, 1935.......................92 Phacops iowensis southworthi Stumm, 1953.....................92 INTERACTIONS BETWEEN Pha-cops rana AND Pliacops iowensis.................93 Summary of the Distribution of the Two Species...................93 Phacops rana.................................93 ni Plhacops.iw. 93 Mutual Occurrence ofPhacops rana and Phacops iowensis.................95 Interactions between the Species..........................96 SUMMARY.~~~~~~~~~~~~~~~~~~~~103..... APPENDIX 1.- Glossary of Morphological Terms .106... APPENDix 2. Locality List. ..............108 REFERENCES CITED.................111 47 ABSTRACT Twvo TRILOBITE SPECIES, Phacops rana (Green, 1832) the reduction is an allopatric phenomenon, involving and Phacops iowensis Delo, 1935, from the Middle transitional populations acquiring a reduced number Devonian of North America, are analyzed in detail of dorsoventral files on the eastern margin of the from the point ofview ofgeographic and stratigraphic craton (exogeosyncline), which subsequently invade v-ariation. A purely morphological, as opposed to bio- the cratonal interior. Other evolutionary changes stratigraphic, approach is used in analyzing relation- in the P. rana lineage appear to be phyletic trends. ships among subtaxa of the two species. Subsequent The two species are nearly mutually exclusive, comparison of the relative sequence of inferred though coeval and their geographic ranges overlap evolutionary events with documented biostratigraphic considerably. Although P. iowensis occurs from Iowa distributions allows an analysis of trends (biostrati- to New York, it was confined to the Michigan Basin graphic character gradients), character displacement for the larger part of its history. Phacops rana is found between the two species, and mode of origin of the from New York south to Virginia and west as far as subtaxa. Iowa. Phacops iowensis was by far the more The stable schizochroal eye of these species is the most through time; it is invariably rare, and generally important anatomical complex in terms of both confined to purer limestones. The one case ofsympatry intrapopulational and interpopulational variation between rana and iowensis occurs in the Hungry and species discrimination. Lens number per eye Hollow Formation of Ontario, resulting in may be broken down into number mor- of dorsoventral phological changes in the two species which are best files (vertical columns of lenses) and number of explained as character displacement. lenses per dorsoventral file. The adult The P. rana population lineage as a whole converged on iowensis in number number of dorsoventral files per eye in Phacops is ofdorsoventral files and in many ornamental features; reached early in ontogeny and is stabilized; the the convergence was number of dorsoventral files is closest in the Taghanic. the most consistently The distributions of, and interactions between, reliable criterion for discrimination of the two species. rana and Throughout its P. Phacops Phacops iowensis are best explained history, iowensis had 13 dorso- if the two taxa are considered true "bio-species." ventral files in normal adults. This species belongs to a the native North American Although little change that occurred within the phacopid lineage that can iowensis lineage seems to have been phyletic in be traced back at least as far as the Gedinnian the nature, ("Phacops" logani Hall). allopatric model is necessary to account for the Phacops rana, more important evolutionary changes in P. rana. morphologically closest to P. schloth- Five subspecies of Phacops cimi (Bronn) from Europe and elsewhere, has from rana, including P. rana 15 to 18 dorsoventral files. paucituberculata, new subspecies, and three subspecies Most of the history of ofP. are species involved the reduction from 18 to 15 files; iowensis, recognized. 49 INTRODUCTION INVESTIGATIONS of evolutionary phenomena on Museum of Natural History and Columbia the species level are again coming into vogue in University devoted many hours to discussing paleontology. This renewed interest stems pre- various evolutionary implications, as did Dr. dominantly from the emphasis on populations N. D. Newell of the same institutions. I am also in the "New S,ystematics," the realization that grateful for conversations with Mr. J. C. Boylan, large samples of fossils, particularly of marine of Columbia, Dr. L. Marcus of Queens College, invertebrates, spanning broad geographic areas Dr. R. M. Finks of Queens College, and Dr. G. and thick stratigraphic sequences are available, J. Nelson of the American Museum of Natural and the relatively recent appearance ofsophisti- History. cated multivariate statistical routines per- To Dr. D. V. Manson of the American Mus- formed on computers. The present study gives eum I owe a special debt of gratitude. Dr. the results of a detailed consideration of the Manson wrote thefactoranalysisprogram MUST variation and evolution within two coeval species used extensively herein, and arranged for the of trilobites whose geographic ranges overlap. analysis of the data. He also reinitiated me into Taken together with other similar studies that the intricacies of factor analysis, thereby greatly have appeared recently, it is hoped that the increasing my knowledge of this multivariate feasibility of studying micro-evolutionary phe- procedure. nomena in the fossil record may be demon- Dr. E. N. K. Clarkson of the University of strated. Edinburgh has given me instructive advice on Quantitative techniques, especially various the nature ofphacopid eyes and has also critically forms of factor analysis, are used extensively in reviewed portions of an earlier version of the parts of the present report. By now these tech- manuscript. niques are conventional and need no further I wish to thank the following people for discussion, although the basic elements of the arranging loans of specimens from their re- multivariate models actually used are explained spective institutions: Dr. S. J. Gould, Museum of in the text. The study does stray a bit from con- Comparative Zoology, Harvard University; vention in that a biostratigraphic approach for Mr. C. Kilfoyle, New York State Museum; phylogenetic reconstruction is abandoned in Dr. Steven W. Mitchell, Wayne State Univer- favor of an analysis of relationships based sity; Dr. E. C. Stumm, University of Michigan strictly on morphological features determined Museum of Paleontology; Mr. F. J. Collier, to be "primitive" or "advanced." A purely United States National Museum, Smithsonian "cladistic" approach to the systematics of fossil Institution; Dr. R. Linsley, Colgate University; taxa need not debar application of evolutionary and Mr. H. Strimple, State University of Iowa. models to the explanation of phylogenetic Mr. Strimple