There is a general consensus among most of those familiar with these rodents that the generic distinction between Ischyromys and Titanotheriomys is valid, and it is so considered in this work. This leaves a problem when referring to the group as a whole. Some workers consider the Family Ischyromyidae to include only these two genera (Wood 1937, 1980) while others include all paramyid rodents in the family as well (Black 1968; Korth 1984). So both the terms "ischyromyid" and "Ischyromys" are ambiguous as to their inclusive meaning. This is unfortunate since the rodents under study (Ischyromys plus Titanotheriomys) form a unified group and are quite distinct from all other rodents. To avoid confusion, the subfamily name Ischyromyinae will be used to refer to both Ischyromys and Titanotheriomys.

Ischyromyine fossils are now known from Saskatchewan to Texas and from the latest Eocene (Duchesnean land mammal age) to the late Oligocene (Whitneyan land mammal age), but only in the Orellan of the Great Plains are they abundant. Titanotheriomys is restricted to the Chadronian and comprises a number of distinct forms, most of which are known only from single localities and most of which have not yet been given species names. Ischyromys, in contrast, is best known in the Orellan where its fossils are very abundant. In the Orellan it displays little diversity, and most of its named species are invalid. Its fossils are also found in the late Duchesnean and Chadronian, and the more primitive skull pattern of Ischyromys suggests that it is the ancestor of Titanotheriomys despite the fact it became dominant later in time.

A number of works have been devoted exclusively to the Ischyromyinae (Troxell 1922; Howe 1956, 1966; Black 1968; Wood 1976; and Flynn 1977), and good taxonomic summaries have been provided by Wood (1937, 1980). A serious problem has plagued the taxonomy of the group, however, and this is the difficulty of correlating characters between skulls and lower jaws (dentaries). Complete skulls are very rare, but they display much diversity of character. The distinction between Ischyromys and Titanotheriomys has been based entirely on characters of the skull. Dentaries are by far the most abundant elements, but they reportedly lack the diversity of the skulls. Skulls and dentaries are rarely found associated, and both have been used as type specimens.

Thousands of ischyromyine dentaries have been collected, many with excellent stratigraphic data. Howe (1956, 1966) studied changes over time in Orellan Ischyromys dentaries from northwestern Nebraska and reported trends of increasing tooth size and increased incidence of accessory cusps that he considered worthy of three or four successive chronospecies. Flynn (1977) studied changes over time in Chadronian ischyromyines from Flagstaff Rim, Wyoming and found significant changes in several characters. But he was frustrated at finding a dichotomy in the skulls that was not observable in the dentaries, and consequently he could not tell if the changes were due to evolution or replacement.

These former evolutionary studies of ischyromyine dentaries used only measurements of gross tooth size (tooth lengths and widths) and total number of accessory cusps (of four possible types) to measure change over time. Taxonomic studies noted many specific peculiarities in the configuration of the cusps, valleys, and accessory features, but these were only examined on small samples (sometimes only on type specimens), so their significance for whole populations was unknown. The opportunity was clearly available for examining the large samples of ischyromyine dentaries in terms of every available character and using this increased resolution to both resolve the taxonomic difficulties and study evolutionary changes over time.

The manner in which new species arise and undergo change in their morphology has been a subject of great interest ever since Darwin (1859) published his theory of natural selection. Darwin believed that species change gradually through a sequence of finely graded intermediate steps, and he was very concerned that the fossil record failed to document such gradual transitions. Instead the characteristic pattern is for changes to occur abruptly, and Darwin resigned himself to the notion that the fossil record is hopelessly imperfect. In recent years Eldredge and Gould (1972) have challenged this view, claiming that it is natural to have long periods of stability punctuated by short periods of rapid diversification. They proposed that the fossil record is an accurate rather than an imperfect history and that this pattern of stasis and punctuation is the natural result of allopatric speciation and of evolutionary rates being an inverse function of population size.

The controversy sparked by Eldredge and Gould's (1972) theory of "Punctuated Equilibria" has lead to many careful examinations of fossil sequences including mammals. Gingerich (1974, 1976, 1979, 1980) has studied many lineages of mammals and has presented many cases of gradual anagenesis and cladogenesis. Gingerich's work has a serious flaw in that he has only used a single morphological character (log of length * width on M/1), so all he has considered is the overall size of the animals. Though easy to measure, body size alone is of trivial taxonomic importance and has zero resolution for detecting the more important changes in shape. His studies are also limited in geographical extent. Chaline and Laurin (1986) presented a good case for gradual change in the European vole Mimomys which lacked these limitations. Gould (in press) has pointed out the bias that only lineages where gradual change was thought to be seen upon initial examination are normally used as test cases to resolve the controversy. The Ischyromyinae is no exception.

To familiarize the reader with the Ischyromyinae, this report begins with a review of the taxonomic history of the group and a description of its fossil occurrences at each locality. Next is a description of methods used and results of all statistical analyses employed. Finally the results concerning taxonomic relationships and patterns of evolution are discussed.

Many mammalian lineages were considered for study in this project. Because of my interest in the evolution of beavers I came in contact with Prof. Emeritus T. Mylan Stout who suggested the study of Ischyromys and arranged for me to borrow an enormous collection from the University of Nebraska to get started. Dr. Gould and Prof. Stout have opposing views on many aspects of paleontology, and I enjoyed contrasting their opinions and experiences. Prof. Stout and Mr. Lloyd G. Tanner took me to most of the Oligocene localities in Nebraska and helped familiarize me with the somewhat unusual stratigraphic history of the terrestrial deposits of the Great Plains. Dr. C. Bertrand Schultz also met with me during my trips to Nebraska and gave helpful assistance.

I express great thanks to the many curators and collection managers who spent many hours loaning fossils to me and providing information concerning them. These include Dr. Margery C. Coombs (ACM); Dr. Richard H. Tedford, Dr. Michael J. Novacek, and Mr. John Alexander (AMNH); Dr. Charles L. Smart (ANSP); Drs. Mary R. Dawson, Leonard Krishtalka, and Richard K. Stucky (CM); Ms. Mary Carmen (FMNH); Dr. Farish A. Jenkins, Jr. and Mr. Charles R. Schaff (MCZ); Dr. Robert W. Fields and Mr. Daniel Garcia (MUPM); Drs. Phillip M. Youngman and C. R. Harington (NMC); Dr. Gordon Edmund and Mr. Kevin Seymour (ROM); Ms. Melissa C. Winans (TMM); Dr. John E. Storer and Mr. Tim T. Tokaryk (SMNH); Dr. Peter Robinson, Mr. Donald G. Kron, and Mr. Logan Ivy (UCM); Prof. T. Mylan Stout and Dr. Richard G. Corner (UNSM); Dr. Robert J. Emry and Mr. Robert Purdy (USNM); Dr. Brent H. Breithaupt (UW); and Dr. John H. Ostrom and Ms. Mary Ann Turner (YPM).

Dr. Donald R. Prothero and Mr. Donald G. Kron sent me long letters with invaluable information about many localities, particularly in eastern Wyoming. I had the privilege of doing a ten week fellowship at the Smithsonian Institution with Dr. Robert J. Emry as my advisor. Dr. Emry provided information on many obscure Oligocene localities and allowed me to use many of his unpublished stratigraphic sections. Mr. Morris F. Skinner, Frick Curator Emeritus (AMNH), provided me with valuable information about his vast collections and allowed me access to his voluminous unpublished material. I also benefited from discussions at the American Museum with Drs. Richard H. Tedford, John J. Flynn, and John H. Walhert. Since my knowledge of the Oligocene and of primitive rodents was very limited when this project began, I owe a great debt to these persons for their generous assistance.

Many decisions had to be made concerning methodology, both in obtaining measurements and analyzing them. In this area Dr. Peter G. Williamson offered enormous assistance. Without the ideas derived from our brainstorming sessions together, this project would have taken much longer than it did. I also thank Mr. Paul Morris for help with writing the computer programs. The following persons made critical reviews of the manuscript and gave many helpful suggestions: Drs. Stephen Jay Gould, Peter G. Williamson, Andrew H. Knoll, Robert J. Emry, Donald R. Prothero, and Mrs. Julia S. Heaton.

Perhaps the greatest debt of all I owe to the numerous collectors of the fossils used in this study. Many of them are mentioned above, and many of them I know only as names on specimen labels and field notes. It was their countless hours of fieldwork spanning over a century and encompassing 7 states and a province that made a project of this type possible. I am particularly indebted to those who kept detailed locality and stratigraphic data while collecting.

This project was funded by fieldwork grants and other funds from Harvard University, a fieldwork grant from the Geological Society of America, a graduate student fellowship at the Smithsonian Institution, and a collection study grant from the American Museum of Natural History. Drs. Stephen Jay Gould and Peter G. Williamson of Harvard University provided funds from their research grants to purchase supplies.

And finally I would like to express thanks to my family. My parents, Howard and Carolyn Heaton, did much to provide for the logistics of my stay in Massachusetts as well as provided constant encouragement. My wife, Julie, provided constant support and assistance in innumerable ways and also helped with the preparation of the manuscript. I also want to express my love to my three daughters, Christy, Amy, and Holly, who spent countless hours wishing that Daddy would stop sitting at his computer and play with them instead.

The controversy occurs at several levels: 1) whether the ischyromyines belong in the same family as the paramyid rodents or in a family by themselves, 2) whether Titanotheriomys is a distinct genus or merely a synonym of Ischyromys, and 3) whether few or many species should be recognized and whether they must be distinct as individuals or as populations. Only the latter two levels are addressed here.

While this study is not primarily taxonomic and does not deal with descriptions of individual fossils, a review of the taxonomic history and its controversies is important groundwork for studying the group. Whenever possible past controversies will be resolved using multivariate statistical techniques on the large data set that is now available. Since this study only deals with dentaries, diagnoses of lower jaws and teeth are given greatest treatment.

Cope (1873a:1) named Colotaxis cristatus (new genus and species) based on three specimens without any illustration and without any reference to Leidy's two articles mentioned above, but he gave five tooth measurements from one specimen. Cope (1873b:5-7) named Gymnoptychus chrysodon, G. nasutus, G. trilophus, and G. minutus (new genus and species), and several measurements were given for each species. Gymnoptychus chrysodon appears to have been described from a single skull. Gymnoptychus nasutus was said to be smaller, and measurements of skull and mandibular characters were included. Gymnoptychus trilophus included only mandibular measurements, and the teeth were said to be distinct. Gymnoptychus minutus was said to be the smallest species, and only mandibular measurements were included. In his synopsis of the new vertebrata from the tertiary of Colorado, Cope (1873c:3) listed Ischyromys chrysodon, I. nasutus, I. trilophus, I. minutus, and Colotaxis cristatus as being present among the rodent fauna and cited Cope (1873a, 1873b) as sources, but he did not explain the reason for the synonymy of Gymnoptychus with Leidy's Ischyromys, nor did he give any description or list of specimens. Cope (1874:477, 1884:833-838) discussed Ischyromys, and he synonymized Colotaxis cristatus and Gymnoptychus chrysodon with I. typus. Cope (1874:476, 1884:819-826) retained Gymnoptychus trilophus and G. minutus (including G. nasutus) in the genus Gymnoptychus without giving any reason for applying them to Ischyromys in Cope (1873c), but they are now considered to belong to the eomyid genera Paradjidaumo and Adjidaumo respectively (Wood 1937, 1980; except the G. nasutus material is assigned to P. trilophus). Cope (1884, pl. 66, fig. 30, pl. 67, figs. 1-12) illustrated some Ischyromys typus material including a skull and five dentaries. The skull and one dentary (pl. 67, figs. 1, 1a, 1b, and 5b) are also illustrated in Cope's (1888, fig. 5) article on the origin of rodent dentition. At this time the genus Ischyromys was considered to be monospecific, and I. typus was the only name assigned to the material under study. All of the fossils discussed above were found in Orellan strata of South Dakota, Nebraska, and Colorado.

Matthew (1903:211-212), in his report on Chadronian deposits of Pipestone Springs, Montana, reported a partial skull and some forty jaws of Ischyromys. He described these fossils as being substantially smaller than those from the Dakota area and having narrower teeth with higher cusps. He said they were also distinct in that the last lower molar has "a narrow heel with the last crest imperfect internally, while in all the Colorado specimens the heel is as wide as the rest of the tooth, and the third (last) crest perfectly developed." He said that "in the upper teeth a corresponding difference is to be seen in the last molar, and also the valley between the anterior and posterior inner cusps is well marked on all the teeth, distinct nearly to the base of the enamel, while in the specimens from Colorado and from South Dakota it is absolute on P4/ and on the molars does not extend so far down."

Matthew (1903) named the Pipestone Springs material Ischyromys veterior (new species), and according to American Museum of Natural History records the skull Matthew intended as the type specimen is AMNH 9647. Wood (1937:196-197, fig. 27), however, designated AMNH 9658, a left dentary with P/4-M/3, as the lectoholotype. Matthew (1910:63) erected the subgenus Titanotheriomys for his new species, I. (T.) veterior. Compared to the subgenus Ischyromys he described Titanotheriomys thus: "Incisors narrow, muzzle small, no sagittal crest but an indistinct lyrate area; zygomata slender, superior border of origin of masseter extended forward on muzzle in an indistinct ridge."

In addition to the Pipestone Springs material Matthew (1910) assigned to I. (T.) veterior a Chadronian skeleton from Beaver Divide in central Wyoming (reported in Granger 1910:240-241). Matthew (1910:62, figs. 16-18) recognized two species in the subgenus Ischyromys: I. (I.) typus from South Dakota having "teeth wider transversely with lower crowns and heavier enamel" and I. (I.) cristatus from Colorado having "teeth narrower transversely than in I. typus, with higher crowns and thinner enamel; skull and teeth usually smaller and sagittal crest sometimes incomplete anteriorly."

Miller and Gidley (1918:436), in their synopsis of the supergeneric groups of rodents, listed only the genus Ischyromys as part of the Family Ischyromyidae. But in a later paper Miller and Gidley (1920:73-74) considered the Pipestone Springs material as a separate genus (Titanotheriomys) with distinctly more slender jaws and incisors than Ischyromys. But the purpose of their paper was to describe a new smaller species of Ischyromys from the Badlands of South Dakota. Ischyromys parvidens, described from five mandibles, was said to be similar to T. veterior in size but similar to I. typus in proportions. Nothing was said about the stratigraphic relationship of I. typus and I. parvidens in the Big Badlands.

Of the larger Ischyromys Miller and Gidley (1920) considered Cope's types of Colotaxis cristatus to be I. typus jaws containing deciduous premolars. But they still recognized two large species of Ischyromys: I. typus with a smaller jaw breadth and "area of muscle attachment in front of antero-external root of P/4 an ill defined shallow depression," and I. chrysodon with a greater jaw breadth and "area of muscle attachment in front of antero-external root of P/4 a well defined, rounded pit."

Troxell (1922) made the next revision of the genus Ischyromys and speculated (quite erroneously) that it is ancestral to the living prairie dog (Cynomys). Troxell (1922:123) considered Cope's Colotaxis cristatus and Gymnoptychus chrysodon to be synonyms of I. typus, a synonymy that has not been challenged since. He also named a new species and two subspecies. Ischyromys pliacus (Troxell 1922:124, fig. 1) was named for a dentary that is much larger than typical I. typus and has many additional tubercles and pits on the cheek teeth. "The posterior cross crests," said Troxell, "do not arise directly from the external tubercle but from its union with the central longitudinal ridge, thus forming a 'Y.' This cross is made up of two distinct minute cusps...." Ischyromys typus nanus was described as being small, having a narrow M/2, and lacking secondary tubercles (Troxell 1922:124-125, figs. 2-3). Later authors synonymized this subspecies with I. parvidens. Troxell (1922:125-127, figs. 4-5, 1923, figs. 19-20) gave the subspecific name I. typus lloydi to an exceptionally well preserved set of skull and jaws from Nebraska (YPM 12521).

All of the above studies were extremely limited in scope, were based on very small collections, and did not consider the ages or relationships of the species involved in any useful way. Nothing is known of the stratigraphic level from which the type specimens came. While this early work is important for taxonomic reasons, it does more to confuse than to help the work of assigning names to ischyromyine species.

Wood (1937) recognized four species of Ischyromys and two species of Titanotheriomys, including a new species he named in each genus. He distinguished I. parvidens (small), I. typus (intermediate), and I. pliacus (large) mainly by size and by I. parvidens having more primitive teeth (Wood 1937:192-193, figs. 23-25). Ischyromys typus was said to be most distinct from Titanotheriomys and was therefore used as the basis for generic comparison (Wood 1937, fig. 21, pl. 23, figs. 6-7, pl. 25, figs. 1-10a, pl. 26). Wood (1937:191-192, pl. 23, fig. 3) described I. troxelli (new species) as being the same size as I. typus but having a narrower skull at the postorbital constriction and having the longest orbit of any species of the genus. It was also said to be distinct from I. typus in having M/1 and M/2 wider than long and having short posterolophids on all lower cheek teeth. Wood (1937:191) mentioned several specimens with skull characters that match I. troxelli in some respects and I. typus in others, and he proposed that they might represent an additional species. He said I. pliacus is most similar to I. troxelli because it has a narrow postorbital constriction and a short posterolophid on all teeth (Wood 1937:188-191, fig. 22).

Wood (1937) viewed these four species of Ischyromys as long-term, wide ranging, and coexisting lineages rather than chronospecies. To I. parvidens he assigned the small lower Orellan specimens from South Dakota and the Chadronian material from Cypress Hills, Saskatchewan. He considered I. typus to be the dominant form in the Cedar Creek Beds of Colorado and a rare form in the Big Badlands of South Dakota, thus limiting it to the middle Oligocene. Ischyromys troxelli was considered to have the same range as I. typus but to have been more common in South Dakota and rarer in Colorado. Ischyromys pliacus was considered the most long-ranging species, possibly comprising several large forms. To it Wood (1937:190) assigned a large form from the Chadronian of Pipestone Springs (less common than T. veterior), the largest Orellan specimens from South Dakota and Colorado, and a Whitneyan specimen from Colorado.

Wood (1937:196-197, figs. 26-27, pl. 27, figs. 2-4) described Titanotheriomys veterior from Pipestone Springs as having lower cheek teeth that are distinctly longer than they are wide, and he noted the presence on the holotype of a barrier forming across the median valley of some of the teeth, a condition also sometimes present in Ischyromys (see Friant 1935, Wood 1937:177, Burt and Wood 1960:957-958, fig. 1A). He also said the upper cheek teeth have deep notches in the paracones. Wood (1937:198, pl. 27, figs. 1, 1a-b) also erected a new species, Titanotheriomys wyomingensis, based on the skull and jaws collected at Beaver Divide by Granger (1910). He stated that T. wyomingensis lacks the notches in the paracones and has a deep pit in the skull anterior to P3/. Its lower teeth are as wide as they are long and lack the barrier across the median valley.

The first attempt to evaluate the stratigraphic relation of ischyromyines from a single region was done by Stout (1937:18-50) along the Pine Ridge exposures of northwestern Nebraska and adjoining parts of Wyoming. He identified Titanotheriomys sp. from the lower Oligocene Chadron Formation based on a small, slender jaw and a small isolated tooth that matched Miller and Gidley's (1920) description of T. veterior. From the lower part of the middle Oligocene Oreodon beds Stout found 125 slightly larger jaws that he called I. cf. parvidens. From the middle and upper Oreodon beds Stout found 401 distinctly larger Ischyromys jaws that he identified as I. typus and I. cf. typus lloydi, and he suggested that this latter group might be further subdivided into two or three distinct stratigraphic varieties. Stout's (1937) study extended the range of Titanotheriomys eastward, put the ischyromyine species in a stratigraphic context, and led to the suggestion that Titanotheriomys and Ischyromys comprise a lineage that increased in size through time.

Work on Ischyromys continued at the University of Nebraska following Stout's (1937) original work. Barbour and Stout (1939) and Galbreath (1953:57-58) considered the largest specimens from the upper Oreodon beds to be I. pliacus, and Schultz and Stout (1955:43) concluded that I. parvidens is restricted to the lower Orella. Howe (1956, 1966) collected 244 dentaries from Sioux County, Nebraska: 73 from the lower Orella which he considered I. parvidens, 124 from the middle Orella which he considered I. typus, and 47 from the upper Orella which he considered I. pliacus. He also examined a fragmentary skull from the basal Whitney which he proposed to be a distinct species even larger than I. pliacus. Howe considered these species to represent an anagenetic evolutionary transition in which the lineage underwent an increase in size, crown height, and incidence of accessory cusps. He recognized that there was considerable overlap between the species in these characters, especially between I. typus and I. pliacus. Because of the great variability at each level, he believed that many characters previously used for species recognition were unreliable, and he suggested that I. troxelli was a synonym of I. typus despite Burt and Wood's (1960:958, fig. 1B) recognition of that species.

The conclusions of Howe (1956, 1966) and other Nebraska workers differed from those of Wood (1937) in that they considered Ischyromys to be a single evolving lineage rather than a suite of coexisting species. Howe failed to address the problem of the large Chadronian ischyromyine at Pipestone Springs which Wood (1937) assigned to I. pliacus and from which he implied that the large Orellan form evolved. But to Howe the stately march of a single continuous size distribution through the Nebraska section was very suggestive of a single evolving lineage and made that the most logical conclusion.

Black further synonymized the group by recognizing only two valid species from those previously named: I. typus and I. veterior. He stated that the size and cheek tooth pattern of I. typus and I. troxelli are identical and that the skull characters used by Wood (1937) to distinguish them are either not preserved on the I. troxelli type or fall within the size range of a single population. He also said that the size and cusp pattern of the type of I. pliacus is identical to the mandible of the I. troxelli type. Black (1968:277-278) therefore synonymized I. pliacus and I. troxelli with I. typus. He also claimed that the characters Wood (1937) used to distinguish T. wyomingensis from T. veterior are either unavailable on the T. wyomingensis type due to wear or fall within the range of variation seen in I. veterior at Pipestone Springs. In addition he stated that the large available samples of I. veterior from Montana (Black 1965) and I. parvidens from Nebraska (Howe 1956, 1966) clearly show that the distinguishing characters recognized by Miller and Gidley (1920) represent individual variation. Black (1968:278-283, figs. 1-6, 13-15) therefore synonymized T. wyomingensis and I. parvidens (including I. typus nanus) with I. veterior. He distinguished I. veterior from I. typus by its smaller size, by the presence of conules on the upper molars, and by the frequent absence of a sagittal crest.

Black (1968:283-287, figs. 7-12, 16-17, 19-20) erected a new species, Ischyromys douglassi, based on new material from the early Oligocene of McCarty's Mountain, Montana. He said the teeth are wider in relation to their length than those of I. typus but are of the same overall size, and he listed many other minor differences in the teeth. He also plotted a number of characters from four ischyromyine populations, some of which mildly suggest a trend to more elongate teeth through time with I. douglassi being the earliest and most primitive form. Black (1971:203-204, figs. 41-45) later found five isolated teeth of ?Ischyromys sp. from late Eocene deposits of Badwater Creek, central Wyoming which he said were most similar to I. douglassi.

Wood (1969:2-5, 1976:246) initially conceded to all of Black's (1968) synonyms (see also Harris 1967:53). But in later publications he resurrected all the synonymized taxa except T. wyomingensis (Wood 1974, 1976, 1980), and some workers believe it is valid as well (Wahlert 1974:388-393; Donald G. Kron, per. comm.).

Russell (1972:27-30, figs. 7J, 8A-D) reported ten additional isolated lower cheek teeth from Saskatchewan and erected a new species, I. junctus, which he described as being intermediate in size between I. typus and I. parvidens. He said its unique features are the joining of the anterolophid and metalophid to the metaconid, forming a triangular valley, and the curving of the posterolophid around the posterior margin of the crown almost to the entoconid. Storer (1978:4-7, figs. 1A-I) reported 28 additional teeth of this species from the Calf Creek local fauna, Saskatchewan. Storer (1988:90) later reported 9 teeth from the late Eocene Lac Pelletier Lower Fauna which he considered to be a distinct, earlier species from Saskatchewan.

Russell (1972:30, figs. 8E-G) also considered there to be a smaller species of Ischyromys from Cypress Hills, but Wood (1980:21) considered the material to belong to other families. Russell's (1972:30) study of the ischyromyids led him to the conclusion "that Titanotheriomys is a valid genus, distinguished by elongate molars, in which the cingular crests, especially the posteroloph, are widely separated from the main crests."

The ischyromyine fossils found farthest from the rich localities of Nebraska and South Dakota come from two sites in western Texas. Harris (1967:51-73, figs. 9A-B, 10A-B) reported T. veterior and a new smaller species of Titanotheriomys from the Chadronian Ash Spring local fauna, but Wood (1974:27-29, figs. 11-12) considered all seven specimens from that site to be variations of T. veterior. Wood (1974:22-26, figs. 8-10) erected a new species, I. blacki, for a skull and dentary from the earliest Chadronian Porvenir local fauna. He described the skull as being similar to I. typus but the teeth having many primitive characters like T. douglassi such as prominent metaconules and incomplete metalophs. The lower molars have an incomplete metalophid and a distinct intermediate cusp in the hypolophid which is also sometimes present in T. douglassi (Black 1968:286, Wood 1974:26).

Because Titanotheriomys has a more derived jaw muscle configuration than Ischyromys, Wood (1976:268) considered Ischyromys to be the probable ancestor of the group, and he believed that both genera coexisted during the entire Chadronian but not at the same localities at the same time. He considered material from the Porvenir (earliest Chadronian, Texas), Cypress Hills (early Chadronian, Saskatchewan), lower Bates Hole (early Chadronian, Wyoming), and Douglas (late Chadronian, Wyoming) to belong to Ischyromys and material from McCarty Mountain (early Chadronian, Montana), Pipestone Springs (middle Chadronian, Montana), Beaver Divide (middle Chadronian, Wyoming), Ash Spring (middle Chadronian, Texas), and upper Bates Hole (middle Chadronian, Wyoming) to be Titanotheriomys.

Flynn's (1977:15-16) detailed study of ischyromyines from Flagstaff Rim (Bates Hole) lead him to believe that lineages of Ischyromys and Titanotheriomys coexisted there throughout much of the Chadronian. He based this conclusion on a clearly bimodal distribution in skull musculature arrangement (based on Wood 1976) with no intermediates among a group of small ischyromyines with identical cheek teeth. He also suggested that a small sample of larger specimens represent a second coexistent species of Titanotheriomys. Although the smaller lineages are indistinguishable on tooth morphology, the combined group increases in size, increases in incidence of accessory cusps, and may change slightly in tooth proportions through time. Kron (1978:94-95) reported coexisting Ischyromys and Titanotheriomys from the latest Chadronian of the nearby Douglas locality.

Flynn (1977:20), noting a marked difference in tooth proportions between the Flagstaff Rim material and equivalent age Titanotheriomys from Montana, suggested that ischyromyines from different regions may have evolved in isolation from one another. He also stated that the Chadronian ischyromyines from Flagstaff Rim are distinct from the lower Orellan I. parvidens of Nebraska in having an accessory ridge between the protoconid and hypoconid (Flynn 1977:19).

The latest taxonomic review of ischyromyids is in Wood's (1980:17-22) monograph on Oligocene rodents. It is a good summary of previous research. He recognized six species of Ischyromys (I. typus, I. pliacus, I. troxelli, I. blacki, I. parvidens, and I. junctus) and two species of Titanotheriomys (T. veterior and T. douglassi). He noted, however, that the taxonomic status of I. pliacus, I. troxelli, I. parvidens, and I. junctus is not certain.

Disagreements concerning ischyromyine taxonomy have lead to considerable confusion, and many generic and specific identifications in the literature must be regarded with extreme doubt. The problem is even worse in museum collections where the basis of taxonomic assignments is rarely stated, and most such identifications have no value whatsoever. To rectify this situation it must first be learned which fossil elements are recognizable at the genus and species level and which populations are conspecific. It is hoped that this study will make a significant step toward that goal.

Ischyromys are abundant throughout the Orella section but are particularly numerous in Orella A and D. Most of the Orella D material comes from the bench-forming layer separating Orella C and D and just above it (Diplolophus zone) in the immediate vicinity of Toadstool Park. Most of the Orella A material comes from the southern end of the linear outcrop (around Arner Ranch) and farther southeast in a large area containing only Orella A exposures (around Everson Ranch).

Schultz and Stout's (1955) section is used to correlate all the collecting localities in the Toadstool Park area. The vast majority of specimens (300 dentaries measured) were collected in the Orella by UNSM and use Schultz and Stout's (1955) stratigraphic terminology, and nearly all of them have excellent stratigraphic data. UCM has a large collection (106 dentaries measured) all from the Diplolophus zone (Hirsch Small Jaw locality). USNM has a small collection from this area (75 dentaries measured), most of which lack good data. Four Orella A dentaries were collected by the author.

Some other localities which lie west of Toadstool Park (but east of Munson Ranch) have been correlated with this section (Harrison, Meng, Coffee, and Eberspecher Ranches). Of the 161 Ischyromys dentaries measured (146 UNSM, 13 USNM, 1 AMNH, 1 UW) the UNSM collection has the best data.

In addition to the Orella material listed above, 16 ischyromyine dentaries (7 UNSM, 6 FMNH, 3 AMNH) have been found in Chadron sediments of the area, mostly east of Toadstool Park (including Chadronia Pocket; Wood 1969). Ostrander (1980:76-78) reported 80 isolated teeth of I. veterior and 4 of I. douglassi from north of Toadstool Park (Raben Ranch) which have not been examined. Hough and Alf (1956) reported finding 81 ischyromyine teeth from ant hills on Chadron sediments near Orella, but Guthrie and Allen (1974) showed that this was a mixed fauna. Hough and Alf (1956) also reported finding two dentaries of I. pliacus and one maxilla of Titanotheriomys cf. wyomingensis in place in the Chadron Formation, but no specimen numbers are given. A single UNSM dentary was found high in the Whitney.

Because the total number of zoned ischyromyines is not large, all localities in the Lusk area are zoned together although there may be discrepancies in the upper part of the section. This includes 124 AMNH, 17 USNM, 12 ROM, and 2 UW dentaries. The largest numbers come from the lower Orella, but some are from higher in the Orella and some are from the Chadron just beneath the Orella.

Measured ischyromyines from the Douglas area with good data include 52 UCM, 48 AMNH, 24 UW, 9 USNM, and 1 ACM dentaries. The UCM and UW material is zoned into local faunas. The AMNH and USNM material is zoned in distance from marker beds, some from the Persistent White Layer and some from a black ash layer 50 to 90 feet higher in the section (according to unpublished sections by Morris F. Skinner).

A composite section was constructed for all the material with the Persistent White Layer at 0 feet and the black ash at 60 feet. The Orella is about 100 feet thick according to this section. As in the Lusk and Geike Ranch sections, the bulk of the material comes from the lower Orella. A few come from the upper Orella and lower Whitney. There is also a sizable collection from the Chadron, separated by a barren zone from the lowest Orella material.

Wood (1976:272) considered all the skull material from the Douglas area, Chadronian and Orellan, to be of the Ischyromys type. Kron (1978:94-95) referred Chadronian material both to I. cf. typus and Titanotheriomys cf. veterior, and he has since found material that he considers to be T. wyomingensis because of its small size (pers. comm.). In the Orellan the material from the lower 130 feet is considered I. parvidens because of its small size, and what little material available above that is considered I. typus because of its larger size (Prothero 1982; Kron, pers. comm.).

Ischyromyine dentaries measured include 288 AMNH, 191 FMNH, 57 MCZ, 54 UCM, 15 ACM, 10 ANSP, 8 USNM, 7 SDSM, 3 TMM, and 2 UNSM. As far as stratigraphic information is available, all of these are Orellan in age except a possible early Whitneyan specimen from near Interior (AMNH) and two possible Chadronian specimens from near Scenic (MCZ). Clark and Beerbower (1967:27) also reported a single dentary from the Chadron (CM 9493), and Harksen and Macdonald (1969:15) listed Ischyromys sp. as a Chadron species. In addition to the material from the Big Badlands proper, one dentary was found at the Chadron-Orella contact near Kodoka, Jackson County; one was found in the middle Orella near Oglala, Shannon County; and two were found in the middle Chadron near Oelrichs, Fall River County (all AMNH). Two Orellan dentaries have also been recovered from the northern Black Hills, Lawrence County (USNM).

Ischyromys typus was reported from the middle Oligocene (Orellan) in the Big Badlands by Leidy (1856, 1869; type description), O'Harra (1910:87), and Harksen and Macdonald (1969:17). Ischyromys parvidens was named for five small dentaries from the Orellan of the Big Badlands (Miller and Gidley 1920). Ischyromys was reported by Wilson (1971). Harksen and Macdonald (1969:20) also reported I. typus as a late Oligocene (Whitneyan) species in the Big Badlands.

Of the Ischyromys dentaries measured, 176 FMNH and all 54 UCM dentaries measured are part of an ecological study of the Lower Nodular Zone by Clark and Kietzke (1967:124-125). They found Ischyromys to be the most abundant rodent in every environment, but it was particularly abundant in drier plains environments which lacked trees. Clark (1967:99) used the uniformity of I. typus and several other taxa throughout the Scenic Member (Orella equivalent) as evidence for its rapid deposition.

AMNH has a smaller collection (52 dentaries measured), some zoned to an unpublished section by Morris F. Skinner and some zoned only to lower, middle, or upper part of the Oligocene exposure. Where possible these were correlated to Lillegraven's (1970) section and included. Southwest of Slim Buttes (north of East Short Pine Hills) is a locality of Chadron sediments from which 6 FMNH ischyromyine dentaries were measured.

Ischyromys material from Slim Buttes and from the North Dakota localities described below tends to be quite large. For this reason the material has generally been assigned to I. typus and I. pliacus (Prothero 1982). Burt and Wood (1960) assigned a fragmentary maxilla from Slim Buttes (ACM 10604) to I. troxelli.

Despite a number of significant Oligocene exposures in western Scotts Bluff County (Schultz and Stout 1955), no ischyromyines are known except at Lyman Beaver Site. The same applies to a continuation of these exposures into Goshen County, Wyoming, where the only specimen reported is a dentary of Titanotheriomys sp. from near Lingle (Stout 1937:18) which apparently is lost from the UNSM collection.

From other collections a few ischyromyine dentaries were obtained from various localities in Weld and Logan Counties, Colorado: 25 AMNH, 9 UCM, 5 YPM, 4 FMNH, 3 ACM, 1 UNSM, and 1 ROM. Most have little data. Three additional YPM dentaries comprise the material Troxell (1922) used as types for I. pliacus, and Wood (1937:190) said these came from the Cedar Creek beds of Colorado. Galbreath (1953:58) showed that this is probably in error, and the type of I. pliacus probably comes from somewhere in southeastern Wyoming (and is therefore of uncertain age). Barbour and Stout (1939) suggested that the I. pliacus type came from the same locality as the type of Diplolophus insolens, an upper Orellan index fossil, because O. C. Marsh apparently collected them on the same day (August 22, 1870).

Wood (1976) was the first to study the material in question. He concluded that all 14 skulls available to him from the middle part of the section were of Titanotheriomys, but a single skull from the bottom of the section he identified as Ischyromys.

Flynn (1977) did a more detailed analysis and tried to determine the number of coexisting lineages and the patterns of change through time. From 16 skulls with snout features exposed he concluded that Ischyromys and Titanotheriomys coexisted through the section. Most of the upper teeth associated with these skulls could not be distinguished between the genera, but a few of the Titanotheriomys skulls had larger teeth, suggesting the possibility that two species of that genus coexisted. Flynn (1977) took length, anterior width, and posterior width measurements on all the lower cheek teeth available to him and also noted the presence or absence of accessory cusps and ridges. From lumped samples at each level Flynn noted statistically significant increases in tooth size and incidence of accessory cusps through time. He also noted a marginally significant increase in the ratio of length to average width of M/2 over time. A few additional fossils are included in my study that were not available to Flynn (1977).

Granger (1910:240-241) recovered a skull and jaws from Devil's Gap at the western end of the divide which he said was identical with Ischyromys veterior from Pipestone Springs and which Matthew (1910:63) used as a paratype for that species. Wood (1937:198) made this material the type for Titanotheriomys wyomingensis. Van Houten (1964:68-69) lists additional Titanotheriomys material collected near Green Cove, Devil's Gap, and Dishpan Butte, all in south central Fremont County. Five AMNH dentaries are included in the present study.

Lambe (1908:56) reported a single molar from Cypress Hills which he assigned to I. typus, but Russell (1934:56) reassigned it to I. typus nanus which Wood (1937) synonymized with I. parvidens. Russell (1972:28-30) erected the species I. junctus to cover all ischyromyines from Saskatchewan, and Storer (1978) concurred with this. A total of 29 lower cheek teeth were measured for this study (14 SMNH, 12 ROM, and 3 NMC). Of these, all 14 SMNH and 4 of the ROM teeth are from the Calf Creek local fauna.

Harris (1967) did a detailed paleontologic study of the Ash Spring fauna and reported three ischyromyine dentaries with teeth, all of which were measured for this study. Two of these (from the same individual) he considered to be Titanotheriomys veterior and the other he considered to be a new, smaller species of Titanotheriomys. Wood (1974:29) felt that the size difference did not warrant taxonomic distinction, and he considered T. veterior to be the only species of the group at Ash Spring.

For rodents such as Ischyromys the only elements that are usually identified are skulls and jaws, and lower jaws outnumber skulls and upper jaws by nearly ten to one. Each complete jaw half contains one ever-growing incisor and four rooted cheek teeth (three molars [M1-3] and one molariform premolar [P4]) separated from the incisor by a diastema. Upper jaws also contain a rudimentary peg-like premolar [P3]. Since cheek teeth have the most complexity and are easiest to make consistent measurements on, they form the basis of this study. I originally intended to measure both upper and lower cheek teeth, but only lower jaws are included in this study. Results of my study on skulls and upper cheek teeth will appear subsequently.

Collections of ischyromyines from many institutions were used in this study. Of 4015 dentaries (or isolated lower cheek teeth; 2042 left and 1973 right) that were photographed and digitized, 1237 were from the American Museum of Natural History (AMNH [232] and F:AMNH [1005]), 1035 from the University of Nebraska State Museum (UNSM), 652 from the U.S. National Museum (USNM), 429 from the Field Museum of Natural History (FMNH), 227 from the University of Colorado at Boulder (UCM), 113 from the South Dakota School of Mines (SDSM), 78 from the University of Montana (MUPM), 64 from Harvard University's Museum of Comparative Zoology (MCZ), 31 from the Royal Ontario Museum (ROM), 31 from the University of Wyoming (UW), 29 from the Amherst College Museum (ACM), 22 from the Academy of Natural Sciences of Philadelphia (ANSP), 21 from Carnegie Museum (CM), 19 from the Saskatchewan Museum of Natural History (SMNH), 10 from Yale University's Peabody Museum (YPM), 9 from Texas Memorial Museum (TMM), and 3 from the National Museum of Canada (NMC). In addition I collected 5 dentaries myself. Of these, the F:AMNH, UNSM, USNM, UCM, and SDSM collections have the best stratigraphic data. The TMM, CM, FMNH, SMNH, ROM, and NMC collections provided valuable fossils, especially for Chadronian localities where this rodent was very rare.

To make photography quick and consistent, I constructed a cubical wood block with 11 cm sides to hold ischyromyine dentaries for photographing. A wide slot in the top of the block was filled with foam rubber which was cut to conform to the shape of the dentaries. The dentaries were placed between the foam and a band of fine, taut fishing line around the upper edge of the block. One side of the block holds right dentaries and the other side left dentaries. Each dentary was inserted so that the occlusal surface was at the level of the top of the block and the lingual surface was even with the side of the block. Using this block an occlusal photograph could be taken, the block turned on its side, and a lingual photograph taken, all without having to adjust the fossil or the camera. A measuring grid was glued to the foam on the top and side of the block so that a scale would automatically be included in each photograph. With this system it took approximately 5 minutes per specimen to photograph it and copy the data included with it. All dentaries were photographed at Harvard University (MCZ) except the AMNH (including F:AMNH and SDSM) and USNM collections which were photographed at the American Museum and National Museum respectively using the same equipment.

For all photography a Canon FTb camera was used with a Canon FD 50 mm macro lens and extension tube and with the focus always fully extended for minimum focal distance. With this setup the image on the film is exactly the same size as the object photographed (ratio 1:1). For ease in viewing the subtle details of the teeth, Kodak Ektachrome (EPY404 Tungsten 50 ASA) slide film was used to give a positive color image. Two tungsten floodlamps were used for illumination. Film was bulk loaded from 100 foot rolls and developed in the lab but not mounted. Developed rolls of film, with photos of about 20 dentaries each, were numbered, wound, and stored in film cans. Rolls shot at MCZ are numbered 001 through 106, those shot at AMNH are numbered 193 through 261, and those shot at USNM are numbered 351 through 384.

Data files produced by DIGITIZE.BAS are quite large and sometimes contain incorrect values due to digitizer or operator error, so COMPRESS.BAS was written to check for errors and put the data into a compact format with only one index number per specimen (888.99 where 888 is the film roll number and 99 is the specimen number on the roll). The digitizer occasionally gives erroneous values (either zero or a ridiculously large number) for the area or periphery, so COMPRESS.BAS checks for unusual values for these measurements. It also checks for non-consecutive measurement numbers and for tooth wear and accessory cusp size values that are too large. This scheme easily exposes measurements that are out of sequence. When errors were found, the problematic specimens were redigitized and the data files updated.

This thesis was written using WordPerfect version 5.0 word processing software and takes advantage of this program's text integrated graphics features. Plots were imported as graphics files from PlanPerfect to WordPerfect. The thesis was printed on a Hewlett-Packard Laserjet Series II printer with Dutch typeface from Bitstream Fontware.