Figure 2 Duchesnean to Orellan Polarities of the northern Great Plains .
Peter Wetherwax, Research Assistant Professor, 473 Onyx Bridge, Department of Biological Sciences, University of Oregon 97403
Table of Contents
Recent systematic research on two of three subfamilies of the Canidae has established the main evolutionary trends among dogs. Several workers have attempted to establish various methods for quantifying these relationships. An ecological polarity method refined by G. J. Retallack attempts to identify ecological roles from dental measurements: breeder (incisor dominated), competitor (canine dominated) and tolerator (molar dominated). Breeders are characterized by high fecundity, small size and nervous temperament. Competitors have moderate fecundity, medium size and aggressive behavior. Tolerators have low fecundity, large size and withstand difficult conditions. Individual taxa are scored from functional areas of each of the principle dental elements: I = incisor, C = canine and M = molar. The measurements for incisors are converted into percentages using the equation (I/I+C+M)*100. The results are plotted on a triangular diagram as a polygonal error envelope, representing an organism’s position in ecological space with poles at 100% incisor, 100% canine and 100% molar. Dogs known from the fossil record evolve away from the competitor pole early in their history, and back towards this pole later. The competitor pole is initially occupied by the ancestral hypercarnivorous subfamily Hesperocyoninae, the tolerator role by the Borophaginae. The last subfamily, Caninae, evolved during the late Arikareean disappearance of the nimravid cat-like predators. The Caninae displaced earlier subfamilies, evolving toward the competitor pole left vacant by nimravids. Further considerations include local ecological effects, apparent at some locations where paleosols have been studied. The recovery of a skull of a hespercyoninae (Mesocyon coryphaeus) in wooded grassland paleosols and Borophaginae (Rhizocyon oregonensis and Cormocyon copei) in dry shrubland paleosols in the medial Arikareean John Day Formation of Oregon, provide examples of environmental separation of adaptively distinct species.
An ecological polarity can be described as an axis for ordination of quantitatively measured adaptive features (Retallack, 1999). Two polarities that are widely used to define an organism’s ecological role are r-K selection. However, Schad (1977) noted the three-folded nature of mammals, establishing the basis for a third polarity, defined here as ‘tolerator’; the selection for stress tolerance. The position a given organism will occupy within an ecosystem corresponds directly to its dental morphology, thereby redefining the concept of niche partitioning.
The three polarities are approximated by the measurement of incisorization, caninization or molarization. Breeders (r-selection) emphasize reproduction, small body mass and nervous temperament. Competitors (K-selection) have compromised their reproductive output for medium body size and agonistic behavior. Animals selecting a tolerator strategy represent a compromise between low fecundity, large body mass and docile temperament (Retallack, 1999; Schad, 1977).
Phylogenetic systematics of the three subfamilies of family Canidae have been exhaustively researched and published (Wang, 1994a; Wang et al, 1999), with the last treatise on the Caninae nearing completion (Tedford, 2001). The Hesperocyoninae emerged in the Duchesnean, evolving from a miacid (Miacis) ancestral stock (Flynn, 1982; Wang, 1994a; Wang, 1994b). These early canids are considered hypercarnivorous, possessing many primitive cranial and skeletal traits. The group as a whole was diverse; the widespread diversity of this subfamily is also apparent from differences in body mass and dental morphology, which indicate varied ecological roles.
The second subfamily is the Borophaginae, whose earliest Orellan members share a number of synapomorphies with the Hesperocyoninae and later members of the Caninae. While phylogenetically intermediate between the earlier and later subfamilies, the Borophaginae have numerous unique features (Wang et al, 1999). The evolution of a dentition that accommodated the crushing of bone early in the history of this group (Hunt, 2002) may have aided their adaptive radiation in North America during the Arikareean. The cranial, dental and skeletal proportions of later, more advanced borophagines earned them the moniker ‘Hyena-like’ among paleontologists, for close morphological resemblance to living hyenas (Munthe, 1989; Munthe, 1998; Werdelin, 1989; Wang et al, 1999).
The third evolutionary radiation is represented by the Caninae, which persist into the Holocene as significant members of the predator guilds they occupy. The Caninae posses characters that have been selected for time and again throughout canid evolution: variability in dentition, cranial proportions, body mass and skeletal architecture. For example, the slight doming of the frontal bone and bicuspid talonid basins found in Canis are evolutionary novelties of its borophagine ancestry. The sleek forms in this subfamily are unique, and probably selected for hunting ungulates on the spreading grasslands of the evolving North American prairie (Bakker, 1983; Retallack, 1997). Furthermore, the expression of the prorean gyrus and development of social hierarchy of pack structure are also new evolutionary innovations in canid history (Radinsky, 1969).
Fossil Canidae are well represented in many museums in North America, making them an easily accessible group to study. The extensive publications that have been produced by previous workers have aided the understanding of the evolutionary history and relationships of this diverse group (Cope, 1883; Merriam, 1906; Scott, 1937; Munthe, 1989; Munthe, 1998; Wang, 1994a; Wang et al, 1999). While there is great interspecific diversity of dentitions, they display little intraspecific sexual dimorphism (VanValkenburgh, 2002). These features make the Canidae an excellent group in which to investigate ecological polarities. An exhaustive effort to measure and evaluate each catalogued fossil belonging to each subfamily was not possible. The relatively compete skulls that were studied represent the most significant evolutionary transitions in the family Canidae, demonstrating the major trends and ecological (niche) partitioning of the group as a whole. Such trends have been compared to adaptive radiations and extinctions in other carnivorous groups, such as nimravids and felids (VanValkenburgh, 1988; VanValkenburgh, 1991). These other components of predator guilds were presumably under comparable selection pressures.
A unique component to this research was fitting published data from well-studied paleosols as a means for the interpretation of shifts in the dental morphology of this group. The recovery of a skull of a Hespercyoninae (Mesocyon coryphaeus) in wooded grassland paleosols and cranial material of the Borophaginae (Rhizocyon oregonensis and Cormocyon copei) in dry shrubland paleosols in the medial Arikareean John Day Formation of Oregon, provide examples of environmentally induced separation in ecological space (stratigraphic, pedologic and specimen data are provided in Appendix V). Although the data are limited, it offers a possible explanation for similar ecologic associations that occur time and again throughout the evolution of the Canidae in North America.
Data was collected using digital calipers, measuring the upper dentition of fossil skulls from select members from each of the three described subfamilies of the Canidae. The functional area of each tooth type were measured and converted into areas (in millimeters) using the following methods:
Incisor width was measured on the apical end of each tooth and height along the enameled labial surface. These measurements are converted into areas by approximating the tooth shape as a rectangle, and multiplying the width by the height.
Canine width was measured antero-posteriorally at the base of their enameled surfaces and height was measured along the enameled labial surfaces. Area was quantified by approximating each canine as two right triangles, whose adjacent sides meet at the apex of the labial surface.
Maximum molar widths for M1 and M2 were measured linguo-labially and lengthwise along their labial surfaces antero-posteriorally. Area was calculated by approximating the shape of a rectangle. The carnassial (P4) of each canid was included in the molar battery, as its morphology is directly related to corresponding change in the M1-M2 set. The carnassial measurements were collected in an identical manner to that of M1 and M2; however, area calculation varies by approximating a right triangle rather than a rectangle.
The individual taxa were scored from the calculated functional area of each principle dental element: I = incisor, C = canine and M = molar. For example, the measurements for incisors were converted into percentages using the equation (I/I+C+M)*100. Appendices 2 through 4 contain the measurements for each subfamily, according to published phylogenetic relationships (Wang, 1994a; Wang et al, 1999). Section 1 of each appendix lists the raw scores of the element for each taxon by institution and specimen number. Section 2 lists the statistics for each species by dental element. The third section in each appendix lists the polarity scores, estimated body masses and trenchant index for each taxon. Body mass estimates for individual taxon in this research were derived by using published regression equations and the log of the lengths of the lower first molars of each species (Legendre, 1988; VanValkenburgh, 1990).
Incisorization
Figure 1 Each corner of the triangle represents 100% for that polarity.
The 50% molarization line is a reference for tracking the relative
movement of individual species around this polarity.
The scores for each species were tabulated and plotted on triangular diagrams (Figure 1), with each corner of the triangle representing 100% for a specific polarity. The top corner is caninization, the lower right corner is incisorization, and the lower left represents molarization. These are chosen to reflect competitor, breeder and tolerater ecological roles, respectively. Each score is plotted as a percentage, relative to its distance from 100%. Error bars are then added (calculated error for each element are from section two of each appendix), joining the points of each polarity, which form the sides of a polygon. This error envelope represents the position of a given species in ecological space.
The animals from a given geographical area in a given geologic time interval are plotted together, using the “North American Land Mammal Ages” published by Wang et al, (1999). Groups of three to five animals are utilized for ease of interpretation. Occasionally, however, a given geographical area will contain several animals living in close association, as is the case for the Great Plains in the late Oligocene. In these cases, the data are divided into two or more plots and designated with a letter corresponding to the caption of each figure.
The evolution of the Canidae from the miacids (Cope, 1880; Flynn, 1982; Wang, 1994b) in the Duchesnean is represented by the appearance of Hesperocyon gregarius in the northern Great Plains of North America (Figure 2). H. gregarius is molarized, which may be interpreted as a tolerator adaptation, a trend later exemplified by the Borophaginae. The hypercarnivorous dentition of the Hesperocyoninae, especially the presence of a trenchant heel on the lower first molar (VanValkenburgh, 1991; Wang, 1994a), and skeletal adaptations for cursoriality (Wang, 1993) evolving in this subfamily are demonstrated by a trend toward canine emphasis, interpreted as a competitor.
Figure 2 Duchesnean to Orellan Polarities of the northern Great Plains .
By Oligocene (Orellan) in the northern Great Plains, the Borophaginae are represented by Otarocyon macdonaldi (Figure 2).
Figure 3 Whitneyan polarities of the northern Great Plains : South Dakota and Nebraska , A; South Dakota , Nebraska and Wyoming , B.
The evolution of Otarocyon macdonaldi in the same dental ecospace as H. gregarius but not as generalized as Mesocyon temnodon, may indicate an overlapping niche for Otarocyon and Hesperocyon, but a more competitive niche for Mesocyon.
By the Whitneyan , Otarocyon macdonaldi disappears from South Dakota and Nebraska, and was replaced by Cynodesmus thooides (Figure 3A). Osbornodon renjiei and Paraenhydrocyon josephi form an integral part of the Whitneyan canid guild, occurring as a hunting set in Wyoming; these two species also have substantially overlapping dental adaptations, which may be due in part to their similar reconstructed body mass estimates of 8 Kg (Appendix II, section 3). Here the term ‘hunting set’ refers to “species similar in hunting habits, but differing in body size” (Rosenzweig, 1966).
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Figure 4 Early Arikareean of the northern plains: South Dakota and Nebraska , A; South Dakota and Wyoming , B; South Dakota , C; Wyoming , D.
By the early Arikareean, the most diversified and successful period in canid evolution begins in the northern Great Plains. The canid hunting set in South Dakota and Nebraska (Figure 4A and C) includes Philotrox condoni, Enhydrocyon crassidens, Sunkahetanka geringensis and Caedocyon tedfordi. Despite its interpreted generalist position, C. tedfordi scores the highest incisor score at 33, and the lowest molar score of 20; neither extreme is found in any other canid investigated here.
At the close of the Whitneyan, H. gregarius became extinct, replaced by the basal borophagine Cormocyon haydeni. Despite a gap in the data for all representative species of Borophaginae in the early Arikareean, comparing figure 2 and 4 reveals a trend towards molarization from the early transitional outgroup into this basal member of the Borophagini tribe (Wang et al, 1999).
In Wyoming (Figure 4D) M. temnodon persisted as an immigrant, however the hunting set it formed with C. thooides has been replaced with Enhydrocyon pahinsintewkpa. By late early Arikareean (Figure 4B), the composition of this hunting set again shifts as M. temnodon and E. pahinsintewkpa are replaced by the migrating three-member hunting set of Philotrox condoni, Enhydrocyon crassidens and Caedocyon tedfordi.
Figure 5 Early to early medial Arikareean of Oregon: early, A; early to medial, B.
Canids of the early to medial Arikareean of Oregon demonstrate greater variability in their guild composition, overall diversity and dental adaptations than the contemporaneous canid guilds of the Great Plains (Figure 5). The hesperocyonine pair P. condoni and E. crassidens found in the Great Plains with Caedocyon tedfordi, is altered with the presence of Paraenhydrocyon josephi (Figure 5A). The widespread distribution of this reoccurring hunting set demonstrates a trend that persists throughout canid evolution, as will later dominate canid guilds across the Pleistocene/Holocene boundary with the genus Canis.
The appearance of Cormocyon copei and Mesocyon coryphaeus in the early Arikareean of Oregon forms an interesting complement to the hunting set occupying Wyoming and South Dakota (Figure 5B). The group includes the intermediate outgroup borophagine Rhizocyon oregonensis, which occupied a generalist position within dental ecospace.
Figure 6 Late medial Arikareean of Oregon, A and Wyoming B.
Later, in the medial Arikareean of Oregon (Figure 6A), R. oregonensis is replaced by Enhydrocyon basilatus, which is molarized like C. copei. The canine dominated niche occupied by P. condoni and P. josephi is now occupied by M. coryphaeus. The hunting set formed by P. condoni and E. crassidens has become extinct in Wyoming as well, opening polarity space for later canids.
The medial Arikareean signals a changing trend in canid evolution, as the Nimravidae became extinct (Turner, 1997), initiating the beginning of the ‘cat-gap’ in North America. The late Arikareean begins with no aeluroform carnivores occupying the hypercarnivorous niche, opening previously dominated resources to the hesperocyonine and borophagine canids. This period in the Tertiary can be thought of as a turning point for hesperocyonines, as 5 of the11 genera and 16 of 28 species have evolved and gone extinct (Wang, 1994a). It also could be considered the beginning of the massive radiation of the Borophagini tribe, which dominated canid evolution in diversity of taxa and morphology, until their extinction in the late Blancan.
Figure 7 Medial and Late Arikareean the northern plains: Late medial Arikareean of South Dakota, A; Late Arikareean of South Dakota and Wyoming, B; Nebraska, C.
Overall, abundance and richness of species is high in the Great Plains; the highest it will ever be for the canids, as all three subfamilies coexisted together during this time. A trend towards caninization is expressed as a shift in dental morphology (Figure 7C) some borophagines, such as Desmocyon thomsoni became more caninized. Figure 7A, shows the persistence of the molar-form basal genus Cormocyon in the plains as a likely tolerator, living alongside its primitive ancestral intermediate, Phlaocyon leucosteus. The presence of these two animals forms the basis of a hunting set, which will abide in the changing faunas of North America for the next 6 My. The existence of phyletic intermediate forms (P. leucosteus and C. haydeni) with a later, more derived form (D. thomsoni) probably reflects more comprehensive collections of fossil dogs of the Great Plains, compared to those of the John Day Formation.
South Dakota in the late Arikareean, Cormocyon haydeni and Phlaocyon leucosteus continue as a hunting set, with the more derived, larger borophagine Desmocyon thomsoni evolving as a more caninized form within this group. The relationship demonstrated between P. leucosteus and the more derived borophagines depicted here, is similar to that of Rhizocyon oregonensis and Cormocyon copei in Oregon. Furthermore, the interchange at the close of the medial Arikareean between the more primitive sister species Praenhydrocyon josephi with that of the more molarized P. robustus approximates the position once occupied by Philotrox condoni.
Figure 8 presents the second half of the late Arikareean for the Great Plains, which includes Colorado and Texas. The inclusion of C. haydeni serves as a reference for the Wyoming and South Dakota diagrams, as they are quite dense. The key characteristic of these two figures is the sudden appearance of many canids, many of them with overlapping dental adaptations, possibly competing in similar niches. Of note, is the distribution of pairs of molarized and generalized species throughout the Great Plains. The three member hunting set formed in Figure 7B, consisting of D. thomsoni, P. leucosteus and C. haydeni ; the hunting set P. leucosteus and P. wallovianus in Texas, and the hunting set C. haydeni and P. leucosteus in South Dakota (Figure 7A) and Colorado (Figure 8C).
The late Arikareean of Florida (Figure 9) is presented separately from the other areas of North America, because it represents an interesting point. While the presence of C. copei and P. leucosteus representing a molarized and generalized pair has been demonstrated time and again through the Tertiary, however, now show separation in ecological space, which may indicate habitat partitioning. Here the pair differs in that P. leucosteus is not found in Oregon, but is replaced by P . latidens; the association between the Cormocyon and Phlaocyon is unbroken.
Figure 8 Late Arikareean of Wyoming , A; South Dakota and Nebraska , B; Colorado , C; Texas , D.
The late Arikareean of Oregon shows little variation in specific composition, with E. basilatus, C. copei and M. coryphaeus persisting, while P. wallovianus and D.
Figure 9 Late Arikareean of Florida
thomsoni are immigrants (Figure 10). The trend demonstrated by the canids in the Great Plains towards caninization is at work in Oregon as well. Three of the five animals depicted in figure 9 are hesperocyonines displaying a considerable amount of overlap.
Figure 10 Late Arikareean of Oregon
The caninized, presumably competitor position occupied by D. thomsoni in the plains is selected for as well in Oregon; note the separation between D. thomsoni and C. copei.
Furthermore, the overall separation demonstrated between C. copei and the other canids living with it is further evidence in support of the post-aelurid trend toward caninization of the Canidae.
The early Hemingfordian of Nebraska includes fewer hesperocyonine canids (Figure 11A). The pair Desmocyon thomsoni and Phlaocyon leucosteus persists in Nebraska until the end of the early Hemingfordian, when D. thomsoni becomes extinct.
Figure 11 Hemingfordian of Nebraska : Early, A; Late, B.
A reversal in the trend of canid evolution is demonstrated in figure 11A, as a close overlap of the borophagine hunting set with the hesperocyonine Paraenhydrocyon wallovianus. The end of the cat-gap in the early Hemingfordian is a time of evolutionary retreat of canids from caninization.
The early Hemingfordian marks the beginning of the end for the Hesperocyonines, as the more advanced, larger borophagines evolved. By the late Hemingfordian all but two genera and three species of the Hesperocyoninae have become extinct (Wang, 1994a). Of the two genera (Ectopocynus is not part of the data set for this writing), “Osbornodon has the most extensive distribution of all hesperocyonines, both geographically and geologically” (Wang, 1994a; p. 107). Osbornodon fricki (Figure 11B) is the last living hesperocyonine, becoming extinct in the early Barstovian. It was the largest hesperocyonine canid to have evolved, with an estimated body mass of 95 kg. Its presence in the late Hemingfordian approximates the hypercarnivorous dental niche occupied by Philotrox condoni 10 My before.
Figure 12 Early Barstovian of Nebraska
The data set collected for this research is limited for members of the subfamily Caninae, as they have been a minority in canid guilds for nearly 12 My. The early Barstovian marks the first recorded appearance (Figure 12) of this subfamily in the data obtained from the American Museum of Natural History. The canine Leptocyon vafer has evolved a molar emphasis (possible tolerator polarity), while the borophagines Tomarctus hippophaga and Psalidocyn marianae overlap. T. hippophaga occupies a more generalist position, while P. marianae represents the only borophagine to evolve incisor dominance. Figure 13C demonstrates a southwesterly radiation into California and New Mexico of a diversity of borophagines: Protoepicyon raki, Psalidocyon marianae and Microtomarctus conferta. However, of note in this figure and figure 12 is the reshuffling of the fauna in Nebraska from the late Hemingfordian.
The borophagine hunting set consisting of Aelurodon aesthenostylus, Paratomarctus temerarius and M. conferta (Figure 13A) forms a second group, distributed between California and Nevada; similarly, a pair formed of O. fricki and T. hippophaga is distributed in California and New Mexico (Figure 13B). The evolution of the Aelurodontina subtribe in the early Barstovian is yet another reversal in canid evolution.
Figure 13 Early Barstovian of California and Nevada , A; California and New Mexico , B; California , New Mexico and Nebraska , C.
As the name implies, the cat-like hypercarnivorous morphology of the upper and lower carnassials combined with a lower first premolar having a near-trenchant heel. The reappearance of this hypercarnivorous condition is an evolutionary reversal in the Borophaginae, as the selection of robust upper carnassials and molars, and the reduction of the trenchant heel into subequal cusps on the talonid of the lower first molar are favored for the crushing of bone, an adaptation that is selected for in later borophagines (Werdelin, 1989).
The terminal range for O. fricki (Figure 13B) is depicted along with the borophagine T. hippophaga in California and New Mexico. The caninization of the remaining subfamilies marks the beginning of yet another reversal in canid evolution. While the Borophaginae have been evolving back toward molarization, the Caninae have begun evolving into the lower, more generalist dental ecospace; a position they now partly share with members of the Felidae.
Figure 14 Late Barstovian of Nebraska, A ad B; New Mexico and Texas , C.
The late Barstovian of Nebraska (Figure 14A-B) had the pair of A. aesthenostylus and P. temerarius in what was formally a three-member hunting set in the early Barstovian of California and Nevada. The hunting set overlaps strongly with Paratomarctus euthos, L. vafer and the more derived Aelurodon sister taxa A. Mcgrewi and A. ferox. Similarly, in New Mexico and Texas, the group of A. aesthenostylus, P. temerarius and M. conferta has been reduced to a set lacking P. temerarius. Interestingly, A. aestenostylus has been replaced by its sister taxon A. ferox, an association not present since the late Arikareean between the genera Cormocyon and Phlaocyon.
The late Barstovian of California (Figure 15) demonstrates the end of a long trend in canid evolution. Diagram C is the same three-member hunting set that existed in California and Nevada in the early Barstovian.
Figure 15 Late Barstovian of California , A and C; and New Mexico , B.
Diagram A forms a tightly overlapping hunting set with that of the three-member hunting set of diagram C. The appearance of Leptocyon new species a, is further indicative of the reversed trend back into caninization, and presumably a competitor role.
The relative displacement and overlap between the members of this hunting set form a characteristic pattern that can be traced in canid guilds from the Orellan through every NALMA until this point. In terms of interpreted ecological polarities, there is usually a tolerator with a competitor and a species between these polarities as a generalist. In short, an intermediate position may have been the only place to evolve toward during periods of explosive competitor evolutionary radiation.
In contrast, the canids in New Mexico (Figure 15B) form an overlapping group centered on a generalized ecospace. The complexity of this association is divided between members of the hypercarnivorous Aelurodontina and less derived Borophagina subtribes of the Borophaginae, with L. vafer as a minor constituent. The appearance of these two subtribes in this overlapping association marks the beginning of such ecologically complex interactions, which persist until the early Hemphillian, when the last of the Aelurodontina become extinct.
Figure 16 Early and late Clarendonian of Nebraska , Oklahoma , South Dakota and Texas , A (early); New Mexico , B (late); Nevada , C (late).
The early Clarendonian of the Great Plains (Figure 16A) is characteristic of the trend in the late Barstovian of New Mexico; notably, the three-member group formed by the sister taxa Epicyon haydeni and Epicyon saevus with A. taxooides. The beginning of the late Clarendonian in New Mexico and Nevada (Figure14B and C respectively) further demonstrates the Epicyon-Aelurodon set association, relative to the members of the subfamily Caninae. The early Clarendonian in this data set of the Great Plains lacks members of the Caninae, while the late Clarendonian canid guilds of the adjacent states in the southwest contain L. vafer in New Mexico, and New genus A, new species b in Nevada. The major difference between these faunas is the position of the Caninae. In New Mexico, L. vafer shares an overlapping molarized ecospace with the Epicyon hunting set, while in Nevada, the undescribed canine has evolved caninization, with little overlap.
Figure 17 Early Hemphillian Oregon , A; Oregon and Idaho , B; Nebraska , C and D.
The early Hemphillian faunas of Oregon and Idaho (Figure 17A-B) are dominated by a pair of E. haydeni and Borophagus pugnator. As in the late Clarendonian of New Mexico and Nevada, the only difference is the position of the Caninae relative to the 50% molarization line. In Oregon, Eucyon davisi overlaps with Borophagus as a generalist, and the undescribed species a, overlaps with Epicyon in what can be interpreted as a tolerator polarity.
The diagrams for Nebraska (Figure 17C-D) are split here for easier interpretation, however, due to their overall complexity, they are still quite cluttered. The Epicyon set is joined here by B. pugnator, however now overlapping with B. secundus, and the undescribed species a, in molarization.
By the late Hemphillian of Florida (Figure 18D), the Epicyon hunting set is replaced by the early Hemphillian pair of E. haydeni and B. pugnator.
Figure 18 Late Hemphillian of Nebraska , A; California , B; Oregon , C; Florida , D; Texas , E.
Similarly, in Oregon (Figure 18C), the Epicyon set degrades into the overlapping association formed between the undescribed canine and E. haydeni. In Texas, the genus Canis demonstrates a slight trend toward caninization, which later proves very successful.
The remaining diagrams demonstrate the final stages in the extinction of the Borophaginae; as with the Hesperocyoninae, there is a geologically brief period of overlapping adaptations, followed by a decline in diversity. The guilds that represent this last period are rapidly filling with members of the dominant group, now the Caninae.
There is a gap in the results, which spans the middle Pliocene (Blancan), through the early Rancholabrean. While members of the Caninae for this time are represented in the data, they were not plotted together for the following reasons.
Figure 19 Late Rancholabrean of the North America
The animals that form associations with these taxa in the data set were either poorly preserved (lacking the required dentital complement), so as to disqualify them from use, or they did not exist in the collections I visited. It was my feeling that plotting ecosystems comprised of only a single representative canid would be uninformative. The late Rancholabrean of figure 19 represents the only canids I have for the Quaternary. The genus Canis has three members, C. lupus, C. latrans and C. dirus. Here, as in previous examples, the ancestral C. dirus overlaps considerably with its descendant C. lupus. The late Wisconsinan specimens of C. dirus from the tar pits of Rancho La Brea represent the largest sample of this species, many of which show an excessive amount of tooth breakage (incisor and canine), a sign of increased intraspecific competition within the mega-fauna of that time (VanValkenburgh, 1993).
There are two notable points of figure19: First, the ecological association formed between C. lupus and C. latrans. Similar associations can be traced to the Whitneyan with Mesocyon temnodon and Cynodesmus thooides, and in the medial Arikareean between Philotrox condoni and Parenhydrocyon josephi, for example. The same pattern re-occurs time and again with the appearance new taxa evolving to fill an available ecospace, or to compete for it.
The molarization of borophagines, which may be interpreted as a tolerator polarity, is now vacant within the canids of North America, but remains occupied by hyenas in predator guilds with canids in African grassland ecosystems. The only member of the primitive Hyaenidae (Chasmaporthetes exilus) to migrate into North America in the late Pliocene (Werdelin, 1991; Berta, 1998) were apparently unsuccessful in competition with the advanced Borophagus, with its highly derived durophagus dentition. The dominant trend in evolution is toward hypercarnivory and canine dominance, as what is interpreted as the competitor polarity. The selection for an overall lighter, more gracile form, better adapted for coping with the varied climates and prey species in their ranges has favored multipurpose dentition. The reversed trend into hypercarnivory, is demonstrated in part by the selection of the trenchant heel on the lower first molar on many species, as well as longer, more gracile canines and an overall reduction of the molars to the carnassials. Furthermore, the overall number of felids in North America was reduced relative to their late Pleistocene diversity. This too inevitably aided the later members of the Caninae to evolve toward hypercarnivory and the competitor polarity of predator guilds. The lack of canids in this polarity may also be due to the diversity of the Ursidae in the late Miocene (Hunt, 1998b). These heavily built animals have evolved an impressive battery of large molars, which are the hallmark of a tolerator. The large body masses and corresponding wide geographical ranges of these animals are further lines of consideration that may help to explain why there are no North American canids in the tolerator polarity.
Reconstructing the ecology of extinct organisms is a tenuous task at best. The utilization of skeletal and dental information has proven highly effective, but has failed to offer a portrait beyond two dimensions. However, no exploration of the past could be considered complete unless the animals have been properly framed within the context from whence they came. “A terrestrial ecosystem is a unit landscape composed of both its biotic and abiotic environment. These two domains merge and interact in the soil” (Buol, 1989; page 126). It is to this end that I will present the best framework available; the soils on which they lived and died.
Paleopedology has recognizably been gaining momentum in recent years as a truly universal discipline. The use of fossil soils in stratigraphy, paleoclimatology and vertebrate paleontology is bringing forth new insight and occasionally offering controversial evidence, albeit shedding new light on old questions. These paleosols preserve the remnants of ancient life and climate within their fabric; “Paleosols can be considered trace fossils of terrestrial ecosystems, and have the additional advantage of being in the place where they formed” (Retallack, 1997; page 380).
Local or regional conditions that existed during the formation of a paleosol are recorded within the horizons of that buried soil. Since soil formation has presumably proceeded in the geologic past as it has today, it provides information unavailable in fossil bones and plants alone (Retallack, 1988; Retallack, 1992). Variations in climate, hydrology and biota along a soilscape (Buol, 1989) can be measured from the valley floor to the slopes of adjacent hills and mountains. These variations produce soils of drastically differing pedogenic morphologies. For example, soils forming on the valley floor may be subject to seasonal water logging and gleying, while soils on adjacent slopes are typically well drained (Birkeland, 1999). Such differences are expressed as distinct floral and faunal compositions across a soilscape.
An example of floral variation on such a landscape can be found in the southern Willamette Valley of Oregon. Wooded grasslands of mixed composition of hardwoods and conifers grow on Mollisols (Haploxerolls and Haploquolls) on the valley floor and on the lower slopes of foothills south of Eugene and Springfield. However, higher on these slopes and mantling the tops of some of these hills are older, strongly developed Inceptisols and Ultisols (Xerochrepts and Haplohumults respectively), on which grow late succession forests (Retallack, 2001a). From these examples, it follows that “each kind of soil can be said to occupy an ecological niche in a soilscape” (Buol, 1989; page 149).
The fauna associated within these soilscapes and vegetation zones demonstrate a preference for a particular habitat, forming the basis for niche partitioning on yet a higher level. The animals living in these habitats overlap in range and prey selection, forming the basis of hunting sets with sister taxa, subfamily members, or the predator guild. Within these associations, there will be animals with canine dominance (interpreted as competitors), others that have selected molarization (interpreted as tolerators) and some that have evolved greater incisor occlusal areas (interpreted as breeders), each occupying an available niche corresponding to their given dental specialization. Ecosystems with high predator diversity and considerable overlap will demonstrate niche partitioning across a landscape, such as those sets and guilds found in this writing, throughout the Tertiary.
For example, the coyote (Canis latrans) prefers open and shrubby grasslands formed on the soils below the montane zone, while the grey wolf (Canis lupus) is largely restricted to a range within the timber zone (Gier, 1975; Mech, 1981; Whitaker, 1997) and thus different soil morphologies. The small degree of overlap demonstrated in the Pleistocene by these sister taxa, as well as in the present is due to differences in body mass, and the partial sharing of habitat by C. lupus with C. latrans. Another example can be found today in Africa, where the spotted hyena (Crocuta crocuta), lion (Panthera leo), wild dog (Lycaon pictus), jackals (Canis spp.) and other carnivores are distributed over a range of grassland habitats in close geographic proximity, some alternating on the landscape as nocturnal rather than diurnal predators (Schaller, 1972; Kingdon, 2001).
Deeper in the geologic past, the fossil record of soils in Kenya and Pakistan has revealed similar evidence pertaining to the preference of select hominids and apes to that of specific paleosols and environments (Retallack, 1991; Wynn, 2000). Recent paleosol studies in the Turtle Cove Member of the John Day Formation have produced promising results for further interpreting the ecology of the Canidae.
Exact stratigraphic control that has been made available by these examples is lacking for canids from paleosol sequences outside of the Middle John Day. However, well-studied paleosol bearing formations in the Great Plains may allow similar conclusions to be drawn for the canids found throughout North America.
The well-preserved and complete nature of many of the specimens in the Frick Collection of the American Museum of Natural History is indicative of rapid burial.
The violent transport processes that are associated with fluvial deposition tend to destroy many of the elements of a skeleton, leaving only those that are best shaped and strong enough to survive extended transport (Hanson, 1980; Shipman, 1981). Furthermore, many of the specimens in Frick Collection either contained original matrix in their brain cases, rostrum or were still wholly encased in it. The Munsell color Ò and micritic texture (personal observations) of this matrix resemble the calcareous nodules described in paleosols found in fossiliforous Bk horizons from localities in the Badlands National Park, and Xaxus paleosols of the Middle John Day Formation (Retallack, 1983; Retallack 2002; Retallack 2000). It has also been from my personal observations that complete fossils are most abundant and best preserved in the Bk horizons of paleosols (for further discussion see Retallack et al, 2000; pages 16-17). These observations are based on fieldwork conducted throughout the Turtle Cove Member of the Middle John Day, as well as at localities in the Kimberly Member of the Upper John Day Formation, and numerous other vertebrate bearing paleosol localities in Central and Eastern Oregon.
The recovery of a skull of the Hespercyoninae Mesocyon coryphaeus in wooded grassland paleosols (Xaxus pedotype) and cranial elements from the Borophaginae Rhizocyon oregonensis and Cormocyon copei from dry shrubland paleosols (Xaxuspa pedotype) in the medial Arikareean (Figure 5B) (Retallack, 2002) demonstrate environmentally induced separation in ecological space.
These paleosols occur in repeating triple packages, 105 times in over 365 meters of measured section. The Xaxus forms the basal paleosol, which is overlain by two Xaxuspa profiles. The difference between these variants lies in their depth to the calcic (Bk) horizons, and degree of development, which are functions of climate and biota respectively during the time of soil formation. The depth to calcic horizon (Bk) is used as a proxy for estimating mean annual precipitation (MAP) during soil formation. The depth measurement is taken from the top of each paleosol, down profile to the top of the calcic nodules. This measurement is corrected for burial compaction (Sheldon, 2001) and converted into a MAP estimate using an equation derived by Retallack (2001a). A thorough discussion on the relationship of depth to calcic horizons and climate can be found in (Retallack, 2001a; Retallack, 2000). The Xaxus have deep calcic horizons, which corresponds to a MAP estimate of 500-850mm. This pedotype is thicker, more strongly developed, indicating good bioturbation and longer time for development. The overlying Xaxuspa have shallower Bk depth, corresponding to a MAP estimate of 300-500mm, and are less strongly developed (Retallack, 2002).
The nature of these repeating triple packages is caused by the effects of astronomical forcing, or Milankovitch cycles. These cycles operate on periods of 23 Ky (precession of equinoxes), 41 Ky (oscillation of the earth’s axial orbit) and 100 Ky (eccentricity of orbit) (Imbrie, 1979). An individual frequency will dominate sedimentation patterns in sequences of marine and terrestrial rocks of given latitudes. However, the presence of the other two frequencies is recorded, albeit to a lesser degree as a subsidiary signature. For example, the forcing of climate by the 41 Ky frequencies is recorded in the John Day Paleosols paleosols as alternating calcic horizon depth, profile thickness (which is related to overall development) and alternating floral and faunal composition up section (Rensberger, 1973; Rensberger, 1983; Retallack, 2002).
The small degree of overlap that occurs between taxa is the result of body mass and dental adaptation, as they relate to climatic, pedologic and surrounding biotic change, within and outside of the soils. Mesocyon is the largest of this canid group, weighing an estimated 17.5 kg. Cormocyon and Rhizocyon have an estimated mass of 6 and 2.5 kg respectively. Interpreting the body mass estimates alone allows for overlap, as it has been well established that carnivores tend to hunt within specific size ranges, based on their own mass (Rosenzweig, 1966). This specific size criterion is then resolvable to further niche partitioning of the prey species of given masses, which tend to prefer similar habitats themselves due to floral composition and distribution. Mesocyon seems to have preferred these wetter wooded grassland soils (Xaxus) for this reason, as well as the refuge it provided from larger nimravids and amphicyonids found within paleosols of contemporaneous age in the John Day Formation (Martin, 1998b; Hunt, 1998a). This holds particular interest, as the Hesperocyoninae were broadly a caninized (competitor) group, with a specific evolutionary trend in hypercarnivorous adaptations. The implication then is that there is a possibility that they may have preferred more densely vegetated areas of the landscape, as wolves do now.
Cormocyon and Rhizocyon seem to have preferred drier, more open shrubland grass soils (Xaxuspa) as allowed by their smaller size, in addition to skeletal and dental adaptations. The improved digitigade stance and a shift to molarization, expressed in two differing approaches to carnivory. Rhizocyon is thought to have been mesocarnivorous and Cormocyon possessed altered hypercarnivorous dental morphology. The trenchant heel found in the ancestral stock of these two canids is modified into a pair of sub-eqaul cusps in the talonid basin. Cormocyon differs in its P4/p4 morphology, which it utilizes as a bone crushing unit; evolved in later forms as a true durophagus bone crushing dentition, allowing borophagines to exploit a niche previously unavailable to carnivores (Wang et al, 1999). Here as well, the implication follows that of the Hesperocyoninae, in that the Borophaginae are likely to have preferred the dry, open vegetation of the grassland ecosystem.
Rodent faunas reported from contemporaneously aged deposits within the John Day Formation (Rensberger, 1973; Rensberger, 1983) are likely to have been a reliable fresh supply of meat to these small carnivores. However, the opportunistic nature of carnivorous mammals was evolving the “morphological flexibility unequaled modern families of Carnivora” (Wang et al, 1999; page 10). While Rhizocyon may have been feeding on rodents, carrion and selected floral elements, Cormocyon coexisted on the same soil using carrion. A recent discovery from the early Arikareean of the John Day Formation sheds light on the early development of the borophagine trend into molarization with Cormocyon.
The skull of the artiodactyl Agriochoerus cf antiquus was found to have bite marks belonging to Cormocyoncopei (Hunt, 2002), thereby validating nearly a century of speculation into the hyenid ecology of members of this subfamily (Cope, 1883; Merriam, 1906; Munthe, 1989; Werdelin, 1989; Wang et al, 1999). The abundance of partially utilized carrion available to these small scavengers by larger hesperocyonine canids, nimravids and amphicyonids near the wooded margins of these soilscapes may have aided the strong selection for the trend in molarization that persisted into the late Blancan. Ironically, the same predators that may have provided a steady supply of carrion to the evolving Borophaginae may have expedited their own extinction in part by their own dental shortcomings.
The position Cormocyoncopei occupies alongside Rhizocyon is mirrored east of the continental divide with C. haydeni and Phlaocyon leucosteus. This association is repeated time and again, as the intermediate taxon will fill an ecological niche between two distinct dental morphologies. The migration or extinction of a canid or hunting sets within a given ecospace leaves the niche previously occupied available for other canids to migrate into and evolve. This pattern of evolution, extinction and migration has been referred to as ‘reshuffling’ (Graham, 1986), and occurs throughout the fossil record of the Quaternary, as also demonstrated here with the Canidae. The changing face of these paleofaunas over time is replete with examples of individual taxon as well as hunting set migration from one geographical area into another. The astronomically forced aridifying conditions that were changing the soilscapes from forested areas into the open, tall sod grasslands and evolution of hypsodont and cursorial adaptations in ungulates (Retallack, 1997; Retallack, 2001a; Retallack, 2001b) may have been the final force that led to the extinction of the Hesperocyoninae.
The adaptive radiation of Miocene ungulate prey species (Gentry, 2000; Webb, 1998b) was essentially pushing the canid body plan beyond its design; in effect, the Canidae were becoming increasingly outpaced with the evolution and radiation of grassland ungulates (Bakker, 1983). The late members of the Borophagine subtribe Borophagina (Wang et al, 1999) were inhibited in their cursorial abilities (Bakker, 1983; Munthe, 1989), and may have relied on the carrion supplied by other predators within their predator guild (Dalquest, 1969; Munthe, 1989). The final decline of the Borophaginae correlates with the extinction of Machairodus and Dinofelis, which are well represented in the deposits associated with fossil canids (Dalquest, 1969; Turner, 1997; Martin, 1998a). The migration and evolution of Homotherium, Smilodon, Meganteron, Felis, Miracinonyx and Panthera in North America (Turner, 1997; Martin, 1998a) may represent the killing blow to the last of the Borophaginae in the latest Blancan. Perhaps the tall sod grasses growing throughout North America made hunting difficult for these cursorialy disadvantaged canids, or that the recently immigrated felids overlapped significantly enough to force these animals into less advantageous geographical range, which is apparent in the shifting distribution of the Borophagina subtribe until their extinction.
Before the late Miocene, members of the Caninae were not a significant component of earlier predator guilds, due in part to their small size, however, as their dentition suggests, they began to occupy the niches previously belonging to the hypercarnivorous Hesperocyoninae. This is evident as a selection for caninization, and away from molarization, demonstrated by Leptocyon beginning in the early Barstovian and ending in the late Barstovian. Another example is demonstrated by the ancestor to the coyote, Canis lepophagus, beginning in the late Hemphillian.
At this juncture, an interesting divergence of the utilization of the canid body plan begins to emerge, correlating with the migrations of microtine rodent groups into North America, beginning at approximately 6.7 Ma (early-late Hemphillian) (Repenning, 1987). The gracile body of Canis that had evolved from millions of years of selective pressure became well suited for effectively hunting smaller animals. The gracile skeleton of the Caninae is ideal for sprinting, long distance pursuit, fast turns and leaping, all of which are skills necessary for subduing rodents and their relatives. Such behavior is exhibited in the contemporary forms of this subfamily (Grady, 1994; Mech, 1981), and is likely to have changed little since its Pleistocene radiation in North America. Furthermore, the evolution of the prorean gyrus promoted the advent of pack social structure not exploited by the Hesperocyoninae or Borophaginae (Radinsky, 1969). The selection for this area of the brain undoubtedly aided in closing the gap opened by the evolution of ungulates as large cursorily adapted animals. The1:3 effective predator-prey body mass ratio (Schaller, 1972) was breached by the evolution of coordinated hunting strategy. The struggle for evolving specialized dental morphology was replaced with a more efficient, intelligent machine, capable of surviving in the varied mosaics of climates and environments in the Quaternary.
Niche partitioning, as demonstrated in the Canidae, is complex involving the interactions of many complex factors. The ecological relationships of fossil canids have been demonstrated as interactions within and around given polarities. The changing ecological strategy of members of a genus, tribe or higher taxonomic division within a family can be tracked over geologic time, as its members respond to selective pressures. These movements may be controlled by climatically induced fluctuations affecting soil development and habitat change. The evolution of related taxa in response to hunting set, predator guild and prey evolution follows these stepwise climatic shifts in the fossil record of soils.
Later hesperocyonines evolved caninization (competitors), and may have preferred soils that supported denser vegetation and an assortment of medium to large prey. Borophaginae evolved molarization (tolerators), and may have preferred the open, shrubby grasslands and the assortment of small to medium prey, as well as carrion. Canids occupied intermediate ecospace (including the Caninae), and may have shared range, diurnally or nocturnally with other canids and associated carnivores within their predator guild.
The contribution that soils provide, as demonstrated in the sometimes parallel and alternating environments they support, form the basis from which selective forces originate and operate on a community of animals. Such selective pressure operating over vast stretches of geologic time are expressed in the architecture of the canid. The form and function of all the skeletal components working as a gestalt to drive the incessant need to eat is but one facet of an animals evolution. The changing functional morphology of the dentition itself is the real essence, expressed as trends in caninization (competitor), molarization (tolerator) and incisorization (breeder). The overlap and partitioning of ecological niches as they are expressed in the three-folded nature of dental morphology are the patterns of climatically induced fluctuation, imposed on the evolutionary fabric of the Canidae. Nowhere is the phrase “you are what you eat” more true then here.
I would like to thank my wife and children, whose patience, understanding and inspiration delivered me through the most difficult stretches of this work. Russell and Mary Kahler for their understanding and generous support, Gregory Retallack, who took me in as an undergrad and gave me the opportunity to shine, working for and with him on a number of projects, I have been richly blessed, receiving more knowledge and mentoring then anyone could hope for. Xiaoming Wang for sharing his home, friendship, patience, humility, publications and seemingly infinite supply of vertebrate knowledge, Richard Tedford for the advice and encouragement while I was in New York, Bill and Elizabeth Orr, who lit my fire for geology with fiery paced lectures and outstanding publications and Peter Wetherwax, whose community structure lectures greatly expanded my curiosity. The following staffs deserve special thanks, as they were indispensable in accommodating my every need during my visits: The Longview Ranch, American Museum of Natural History, New York, Museum of Paleontology of the University of California, Berkeley, Museum of Natural History, University of Oregon and the John Day Fossil Beds National Monument.
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