7

PREFACE

The first International Symposium on the Biology papers were read by title and are included either in of the Seal was held at the University of Guelph, On­ full or abstract form in this volume. The 139 particip­ tario, Canada from 13 to 17 August 1972. The sym­ ants represented 16 countries, permitting scientific posium developed from discussions originating in Dub­ interchange of a truly international nature. lin in 1969 at the meeting of the Marine Mammals In his opening address, V. B. Scheffer suggested that Committee of the International Council for the Ex­ a dream was becoming a reality with a meeting of ploration of the Sea (ICES). The culmination of such a large group of pinniped biologists. This he felt three years’ organization resulted in the first interna­ was very relevant at a time when the relationship of tional meeting, and this volume. The president of ICES marine mammals and man was being closely examined Professor W. Cieglewicz, offered admirable support as on biological, political and ethical grounds. well as honouring the participants by attending the The scientific session commenced with a seven paper symposium. section on evolution chaired by E. D. Mitchell which The programme committee was composed of experts showed the origins and subsequent development of representing the major international sponsors. W. N. this amphibious group of higher vertebrates. Many of Bonner, Head, Seals Research Division, Institute for the arguments for particular evolutionary trends are Marine Environmental Research (IMER), represented speculative in nature and different interpretations can ICES; A. W. Mansfield, Director, Arctic Biological be attached to the same fossil material. Readers of this Station, Fisheries Research Board of Canada (FRB) volume should be aware of such differences when read­ represented the International Commission for North­ ing the papers in this section. The twelve papers of west Atlantic Fisheries (ICNAF); and K. S. Norris, S. H. Ridgway’s section on functional anatomy illus­ Director, Marine Mammal Council Executive Com­ trated the fundamental structure of the seal, as well mittee, represented the International Biological Pro­ as its associated control mechanisms. R. J. Schusterman gram (IBP). The Food and Agriculture Organization followed this theme by introducing ten papers on be­ of the United Nations (FAO) also offered its support haviour. He established a major focus on social or­ to the programme and ICNAF has contributed to the ganization and communication and their association financing of this volume. with the functional anatomy of the pinnipeds. D. E. Sponsors of national origin were the Fisheries Re­ Sergeant chaired the population dynamics section of search Board of Canada (FRB), the National Re­ seven papers, covering the modelling of populations search Council of Canada (NRCC), the Canadian and method of analysis of seal populations around the National Sportsmen’s Show (CNSS), the World Wild­ world. In the fifth section, J. R. Geraci, by means of life Fund (Canada) (WWF), and the University of papers and a panel discussion dealt with the care and Guelph. management of captive pinnipeds. W. N. Bonner co­ In his preliminary remarks Professor Ronald intro­ ordinated a presentation in the broad area of ecology, duced the representatives of these groups; namely J. R. and was able to bring together studies on environmen­ Weir, Chairman, Fisheries Research Board of Canada; tal factors and their associated behavioural and gene­ S. Bata, International Director and J. S. McCormack, tic control systems. The physiology section was chaired Director, World Wildlife Fund (Canada); and R. T. by H. T. Andersen, his introductory remarks forming D. Birchall, President, Canadian National Sportsmen’s the initial paper of the section. The other six papers Show and a Director of WWF (Canada). of his section emphasized the underwater responses of W. C. Winegard, President of the University of seals. The final and general section, chaired by J. E. Guelph, welcomed participants to the symposium and King, offered a broad coverage of several of the more commented particularly on how pleased he was to interesting areas in various disciplines. welcome representatives from so many countries. Later, A. W. Mansfield acted as rapporteur for the entire at a banquet sponsored by the Department of the En­ programme, and his report stressed the need for con­ vironment, Canada, he offered an invitation to the tinued cooperation by all biologists so that they might group to return in 1975 for a Second International understand seals and their importance to environmen­ Seal Symposium. tal studies. Altogether 62 papers were presented. A further 14 This volume includes with one exception, those pa- 8 K. Ronald pers either presented, read by title, or abstracted, but mammals of the world’ by D. W. Rice and V. B. the continuing discussion on the biology of the seals Scheffer (U.S. Fish and Wildlife Service, Washing­ led to one further paper that is included here. Some ton, 1968) has been used as the standard reference on of the discussion was formal and, where recordable, is nomenclature. included here, but by far the greater part of discussion The work of the chairmen of each of the seven sec­ was informal and hence must remain as extremely tions of this volume is especially recognized. As well, valuable, but merely mental recollections of the par­ the convenor wishes to thank the programme com­ ticipants in the symposium. mittee for their ability to support a somewhat unortho­ The symposium achieved its purpose of bringing dox procedural system, and particularly the sponsors together scientists interested in the Pinnipedia and it ICES, ICNAF, IBP, CNSS, FRB, NRCC, WWF (Ca­ offered leads into the international examination of nada), FAO, and the University of Guelph for their marine mammals. valuable financial assistance. The editors with little apology recognized that they The convenor is most grateful to Mr. H. Tambs- have not reached a completely uniform format in this Lyche, General Secretary of ICES, for his advice and volume since they have allowed use of both English encouragement from the embryonic stages of the sym­ and metric systems of measurement and both English posium to the publication of the proceedings; he also and North American word usage for the sake of har­ recognizes the considerable amount of expert help pro­ mony. The main editorial structure has been the con­ vided by A. W. Mansfield in co-editing this volume. sistency of usage throughout a particular paper. Finally, the effort put into both the symposium and Attempts have also been made to attain a fairly this volume by Mrs. Ginny Bandesen has been beyond uniform taxonomy for the species, but where there has measure, but I hope that she will accept the results of been any doubt caution has not overridden clarity. As the symposium recorded here as tangible proof of her in other mammalian groups, the systematics of the most valuable contribution. To the members of the Pinnipedia are still open to much interpretation. The Dean of the College of Biological Science’s office, the references are cited according to an Annotated Biblio- university support staff and our host Dr. W. C. Wine- praphy on the Pinnipedia*. The ‘List of the marine gard, I express on behalf of the participants and my­ self, our sincerest thanks. * Ronald, K., L. M. Hanly and P. J. Healey, College of Bio­ K. Ronald, logical Science, University of Guelph, Ontario, Canada. Convenor

The following have kindly acted as Discussion Care and Management Section Leaders of the different Sections and also assisted in J. R. Geraci the editing of the contributions: Department of Zoology, University of Guelph, Guelph, Ontario, Canada. Evolution Section Ecology Section E. D. Mitchell Arctic Biological Station, Fisheries Research Board W. N. Bonner of Canada, Ste. Anne de Bellevue, Quebec, Canada. Seals Research Division IMER, c/o Fisheries Labora­ tories, Lowestoft, Suffolk, England.

Functional Anatomy Section Physiology Section S. H. Ridgway H. T. Andersen School of Anatomy, University of Cambridge, Nutrition Institute, University of Oslo, Blindern, Cambridge, England. Oslo, Norway.

Behaviour Section General Session R. J. Schusterman J. E. King Department of Psychology, California State University Department of Zoology, University of New South Hayward, California 94542, U.S.A. Wales, Kensington, N.S.W., Australia.

Population Dynamics Section Summary D. E. Sergeant A. W. Mansfield (Rapporteur) Arctic Biological Station, Fisheries Research Board of Arctic Biological Station, Fisheries Research Board Canada, Ste. Anne de Bellevue, Quebec, Canada. of Canada, Ste. Anne de Bellevue, Quebec, Canada. 27

Rapp. P.-v. Réun. Cons. int. Explor. Mer, 169: 27-33. 1975.

OTARIOID EVOLUTION

C. A. Repenning U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, U.S.A.

INTRODUCTION otarioid seals owing to the scarcity of specimens. In 1880 J. A. Allen concluded that the only fossil Nevertheless, it is apparent that there was a great otarioid seals from the Atlantic region were walruses, variety of otarioids in the North Pacific Basin in the that the identifications of some European fossils as sea past 15 to 20 million years, most of which have been lions were in error, and that most probably the sea considered to be some sort of sea lion. Although near­ lions and the fur seals have always been limited in ly all fossil walruses have been reported from the their distribution to those areas where they now exist. North Atlantic region it was Kellogg’s belief that the At that time the only described fossil sea lions were walruses and the sea lions had a common origin in the from upper Pleistocene and subrecent deposits in Pacific, a view which has remained in favor, partly Australia and New Zealand. because of the morphological similarities between the In the same year Gervais and Ameghino (1880) living forms. listed Otaria fischeri as a fossil sea lion from Argen­ In the past five years or so interest in the history tina. Ameghino (1889) later described the only spec­ of the otarioids has accelerated, sparked largely by the imen, a toothless mandibular fragment with three discovery of a rather suprising number of new fossils alveoli, as Arctophoca fischeri. The specimen came mainly along the Pacific Coast of North America. Al­ from the bluffs of Parana, Entre Rios, Argentina, and though nearly all of this material is either still under Ameghino believed it to be of late Oligocene age, from study or has not yet been studied, it provides consider­ 25 to 30 million year old. It appears now that it is able insight into the evolution of the Otarioidea. most likely less than 6 million years old and, if the There are four groups of otarioids that can current­ specimen is Arctocephalus, its single-rooted cheek ly be recognized in the North Pacific Basin. Although teeth indicate that it is no older than about 3 million opinions differ regarding taxonomic ranking of these years. groups, I choose to regard them as families; they are The first fossil otarioids from the North Pacific the still extant Otariidae and Odobenidae, the extinct Basin were described by True (1905) and Condon Desmatophocidae, and a fourth family, as yet unnam­ (1906). In a series of papers published between 1921 ed, which is ancestral in position and taxonomically and 1931 Kellogg described several west coast fossil horizontal in structure, presumably including the an­ otarioids. In a more recent series of papers from 1961 cestors of all other otarioids. This fourth family is to the present, Mitchell has described a number of characterized by many terrestrial carnivore features additional records. A few other reports have been that unite them as a familial unit but which make published describing fossil otarioids from both the difficult the assignment of the included genera (or Pacific USA and Japan. species) to any of the three descendant otarioid fa­ More authors are involved in the published history milies. of the walruses. Prior to the works of Mitchell, all fossil The present and largely unpublished fossil record walruses more than 1 million year old were known of the otarioid seals indicates the following historic only from the North Atlantic area. Kellogg had men­ picture : tioned walrus-like features in some of the fossils he 1) separation from the common ancestral group of described from the North Pacific area, but he tentative­ the extinct and living otarioid families began at least ly considered them as primitive sea lions. However, he 20 million years ago and appears to have continued postulated a North Pacific origin for all otarioids, until about 15 million years ago ; based on the conclusions of Allen in 1880. 2) the Desmatophocidae appear to have separated Few people have been active in the study of fossil out first, achieving maximum diversity in the North 28 C. A. Repenning

EARLY MIOCENE MIDDLE MIOCENE LATE MIOCENE PLIOCENE PLEISTOCENE

I OTARIIDAE

^ Q ø \ Arctocephalinae Arctocephalinae

3 V -

Figure 11. Phylogeny of the Otarioidea. Width of branches indicates approximate abundance. 1 = time of establishment of homodont dentition. 2 = time of development of single-rooted dentition. 3 = time of beginning of reduction of lower canines.

Pacific Ocean about 15 million years ago, and appear­ living walrus Odobenus, became extinct in the Pacific ing to become extinct about 10 million years ago; and returned only very recently by dispersal of Odob­ 3) the time of separation of the Odobenidae as a enus from the Atlantic to the Pacific. distinct lineage is not yet known, nor is it yet certain From 12 million to about 4 million years ago the whether they developed out of early desmatophocids second lineage of odobenids, the Dusignathinae, was or directly out of the common ancestral group. How­ very abundant and diverse in the North Pacific. Many ever, they achieved maximum diversity in the North forms had specializations rather closely parelleling Pacific Basin 9 to 12 million years ago. those of the walrus, and these possibly explain the At least two distinct odobenid lineages are recogniz­ presumed extinction of the walrus lineage in the Paci­ able during this time of maximum diversity. The line­ fic. Others had specializations rather clearly indicating age leading to the modem walrus, the Odobeninae, that they took over the adaptive niche now occupied entered the North Atlantic Basin at least 6 million by the sea lions, a niche that had been utilized for the years ago and possibly considerably before that time. preceding 5 or 6 million years by the desmatophocids. This entry into the Atlantic may have been by the Although rare, some desmatophocids are known to be Arctic Ocean, following the dispersal route of their associated with the first abundant records of this line­ favorite food such as the pelecypod M y a (Hopkins age of odobenids, but the desmatophocids appear to 1967; MacNeil 1965), or may have occurred earlier have shortly become extinct; through the Central American Seaway. All known 4) this second odobenid lineage, the Dusignathinae, earlier records of the Odobeninae in the North Pacific flourished along the Pacific shores until about 4 mil­ are from Mexico. lion years ago, when the otariids first began a rather O ther than very recent records of the living species, explosive diversification. However the otariids had there are no records of this odobenid lineage in the had a long history prior to this and appear to have Pacific younger than 7 to 9 million years old; in the developed slowly from the last of the common an­ Atlantic its fossil record is rather extensive during this cestral group some time between 12 and 15 million period when no records are known in the Pacific. It years ago. On the basis of skeletal evidence the earlier thus seems possible that this lineage, leading to the otariids were fur seals, the seal lions not appearing Otarioid evolution 29

until less than 4 million years ago. It appears that the such as a posteriorly very broad and flat palate and period of greatest diversification of the otariids is at a posteriorly projecting paroccipital process. No forms the present time (Fig. 11). are known that suggest ancestry to the odobenids, and it appears equally possible that the odobenids are derived from an early desmatophocid or directly from THE COMMON ANCESTRAL GROUP the ancestral group. Mitchell and Tedford (1973) present the first de­ scription of one of these remarkable, small, and primitive otarioids. Several other specimens are known, THE DESMATOPHOCIDAE none of which are yet studied and some of which have This family of otarioids, whose name derives from yet to be cleaned of rock. They vary in age from the genus Desmatophoca of Condon (1906) has most about 15 to 23 million years and are known from recently been reviewed by Barnes (1972). Also note­ southern California to northern Oregon. worthy is the description by Mitchell (1966) of The principal features of these otarioids are teeth, Allodesmus, the only essentially complete skeleton of ear regions, and brains that resemble those of terrestri­ a fossil otarioid yet described in publication. At least al carnivores in combination with distinctly aquatic two genera, Desmatophoca and Allodesmus, currently adaptations. Possibly, the most striking feature, is the comprise the family and Barnes recognizes three dentition which includes a true carnassial in upper species of Allodesmus as well as three additional un­ and lower jaws, the upper carnassial having a distinct named desmatophocids. The desmatophocids lived protocone, the lower a talonid in addition to a more from about 11 to 20 million years ago and are known or less typically double-cusped shearing blade. Two from California, Oregon, Alaska, and Japan. They approximately quadrate upper molars are present. appear to have achieved maximum diversity in the Relative to the condition in more advanced forms, North Pacific Ocean approximately 15 million years the tympanic bulla of these ancestral otarioids is quite ago at which time most of the known forms existed. inflated and the tympanic membrane quite large, Some had already evolved single-rooted cheek teeth, though not more so than in some odobenids and in an advance of questionable purpose which all otarioid terrestrial arctoid carnivores. Mitchell and Tedford families acquired but which the living sea lions and also report what they infer to be evidence of a rudi­ particularly the fur seals have yet to accomplish uni­ mentary tensor tympani muscle almost like that of versally. terrestrial carnivores. Externally the mastoid and paroc- In the skull the desmatophocids are characterized cipital processes are enlarged in the otarioid manner. by little or no supraorbital processes, nasals penetrat­ The frontal gyri of the cerebrum, including the “ursine ing the frontals, a mortised zygomatic-squamosal ar­ lozenge” and the olfactory bulbs are reduced and the ticulation on the zygoma, greatly enlarged and poster- anterior ectosylvian gyrus is partially opercularized olaterally projecting paroccipital processes of the ex- by the coronal gyrus, as in all otarioids. In general, occipital bones, a trapezoidal to rectangular basi- however, the brain is distinctly ursid and primitive, occipital bone, a small tympanic membrane and very with fewer neocortical convolutions than occur in large auditory ossicles, a very wide and shallow internal living arctoid carnivores of similar size and modern acoustic meatus on the dorsal surface of the petrosal, pinnipeds. It bears a marked resemblance to the brain little enlargement of the petrosal apex, a broad and of contemporary ursid terrestrial carnivores although shallow hypophyseal fossa, and a bony palate that is the neocortex is somewhat more complexly convoluted. markedly broad and flat posteriorly. The cheek teeth The group contains taxa of considerable variety. tend to be bulbous and simple, although more primitive The oldest form, described by Mitchell and Tedford, species with double-rooted cheek teeth have prominent has a broad skull reminiscent of some terrestrial carn­ anterior and posterior accessory cusps. In some species ivores, particularly ursids and mustelids. The skull single-rooted cheek teeth were established at least 15 width nearly equals the skull length, the cheek bones million years ago. The symphysis of the mandible is are roundly flaring and the muzzle is short and broad. deep and narrowly oval. By contrast, the youngest known form, as yet un­ At present no postcranial skeletal elements have studied, has an elongate skull like the sea lions. The been studied except for those of Allodesmus kernensis greatest width of the skull is about half the total (Mitchell 1966). In this species the limbs are elongate length, the cheek bones are flattened and the muzzle is for an otarioid and the bones of the flipper are mark­ long and narrow. A third form is known but, as yet, edly stout. In general features the postcranial skeleton unstudied. This form, which may be as old as the appears to incorporate characters of both the living material described by Mitchell and Tedford, has otariids and odobenids. several features suggesting a desmatophocid ancestry, The persistence of some forms in retaining a strong 30 C. A. Repenning tendency toward double-rooted cheek teeth, as Desmat­ from the common ancestral group or from some ge­ ophoca oregonensis Condon (1906), which lived dur­ neralized desmatophocid. ing the time of maximum divergence of the family, From deposits probably between 7 and 9 million and “desmatophocine A” of Barnes (1972), which years old in Baja California, Mexico, another fossil lived during the time of the latest records of the family, odobenid is known that has one very conspicuous suggests that there may have existed two distinct line­ difference from Imagotaria; the lower canines are ages within the Desmatophocidae. This is reflected in reduced while the uppers are enlarged, the key charac­ Mitchell’s (1968) recognition of two subfamilies: the ter of the subfamily Odobeninae. Although almost all Desmatophocinae and the Allodesminae, as here bones of the skeleton of the dusignathine Imagotaria shown in Figure 11. are distinctly odobenid in nature, all can be distinguish­ The desmatophocids were the dominant seals of ed from comparable elements of living Odobenus. The the North Pacific for about 5 million years. During situation is different in the Mexican odobenine odob­ most of this time they lived with members of the enid; although some skeletal elements clearly are more common ancestral group, which were much smaller in primitive than in Odobenus, others are inseparable size, and presumably with as yet unknown primitive from those of the living walrus. odobenids. Toward the end of their existence the With respect to reduction in the number of cheek desmatophocids became much less common and, about teeth, reduction in size of the lower canine and en­ 12 million years ago, were associated in the North largement of the upper canine, this Mexican fossil Pacific Ocean with abundant and large primitive odobenid is intermediate between the dusignathine odobenids and early otariids little larger than the Imagotaria and the approximately 6-million-year old members of the common ancestral group from which odobenine Prorosmarus alleni Berry and Gregory they are derived. (1906) from the Atlantic coast of North America. The further evolution of the odobenine odobenids into liv­ ing Odobenus is rather clearly displayed in the North THE ODOBENIDAE Atlantic fossil record. Except for the two named genera of desmatopho­ In the North Pacific record, however, there is no cids, almost all records of North Pacific fossil otarioids further evidence of the odobenine lineage in which the are odobenids. Kellogg noted odobenid similarities in upper canines alone become tusklike; rather, all odob­ some of his fossils, as already mentioned, and Mitchell enid fossils found show equal enlargement of both up­ further emphasized the odobenid affinities of many per and lower canines and hence are dusignathine. In of these in his several reports, as well as describing addition, all skeletal elements, while retaining a new odobenid records. distinct odobenid nature, are clearly distinguishable The earliest known odobenid, subfamily Dusignathi­ from comparable bones of Odobenus. nae, is Imagotaria downsi Mitchell (1968), which was The diversification of the North Pacific dusignathine remarkably abundant in the waters then adjacent to odobenids is great throughout the period from approx­ California. It was of large size, and was found asso­ imately 10 million years ago to perhaps 4 million years ciated with the last of the desmatophocids as well as ago; published names include, in addition to Im ag­ with the earliest known true otariid, a small species otaria Mitchell, Pliopedia Kellogg, Dusignathus Kel­ named Pithanotaria starri Kellogg. These fossils are logg, Pontolis True, and Valenictis Mitchell. One or from 11 to 12 million years old. The distinctly odob­ two other forms are known which have not yet been enid nature of both cranial and postcranial bones of studied. Imagotaria, in combination with large size and a In the skull the odobenids are characterized by strong tendency toward single-rooted cheek teeth, little or no supraorbital processes, very weak and suggest that considerable history of the odobenids platelike paroccipital processes on the exoccipital preceded it. Although some fragmentary remains of bones, a broad pentagonal basioccipital bone, a large older otarioids are known that suggest odobenid af­ or small tympanic membrane depending upon the finities and relationship to Imagotaria, none are un­ adaptive habits of the particular genus, very large questionably defendable as odobenids. Therefore, the auditory ossicles, a wide and shallow internal acoustic history of the otarioid seals in its current state, finds meatus on the dorsal surface of the petrosal, great its biggest uncertainty in the origin of the odobenids. enlargement of the petrosal apex, a broad and shallow They do not seem derivable from early otariids, which hypophyseal fossa, and a vaulted bony palate. The in the time of Imagotaria were hardly more than cheek teeth vary from sea lion-like to simple pegs, as members of the common ancestral group that had in the walrus, but have a persistent posterointernal developed homodont, but still double-rooted cheek singular cusp on the uppers, slightly evident even in the teeth. They could have developed either directly modem walrus. In all genera a single-rooted condition Otarioid evolution 31

of the cheek teeth was established about 10 million size and abundance took place in otariid evolution for years ago with the exception of the most posterior the next 8 million years. Between 7 and 9 million upper tooth. The mandibular symphysis is very deep years ago, otariids were relatively common from Mex­ and broadly to narrowly oval, fusing completely in ico and California. the genus Odobenus. The otariids were the only family evolving out of the The dusignathine odobenids were the dominant common ancestral group to develop significant supra­ seals in the North Pacific for about 5 million years. orbital processes. Many of the other morphological Prior to this dominancy, they evidently coexisted with features that characterize the otariids were already the desmatophocids; then, following this dominancy, developed in the latest known member of the ancestral the otariids became larger and increasingly abundant. group of otarioids, however, and the similarity of the It seems that the dusignathines made use of all otarioid postcranial elements is most striking. adaptive specialities; Imagotaria appears to have filled The otariids are characterized by prominent supra­ the niche of the living sea lions, while Dusignathus orbital processes, particularly in the males, frontals and possibly Pontolis appear to have occupied a habitat that penetrate the nasals, a knob-like paroccipital pro­ in the Pacific, similar to that of the early walruses of cess that is pendant and united to the mastoid process the Atlantic and of the living Odobenus. by a crest, a trapezoidal to rectangular basioccipital The earliest record of Odobenus in the North Paci­ bone, a very small tympanic membrane and small fic is considered to be of Sangamon Interglacial age, auditory ossicles, an almost cylindrical internal acoustic possibly only 70 thousand years old. It is possible, meatus on the dorsal surface of the petrosal, mod­ therefore, that during much of the Pleistocene there erate enlargement of the petrosal apex, a deep globular were no odobenids living in the North Pacific. During hypophyseal fossa, and variable development of the this time, however, the Otariidae were increasingly bony palate. The cheek teeth tend to be simple and abundant and dispersed, or had already dispersed, to are sharply pointed. Except for the first premolar, all the Southern Hemisphere. were double-rooted until less than 4 million years ago and many living species still retain double-rooted molars (Repenning, Peterson and Hubbs 1971). The THE OTARIIDAE symphysis of the mandible is usually shallow, not ex­ The earliest known otariid Pithanotaria starri is tending ventrally to the level of the inferior margin about 12 million years old. It is known from three or of the horizontal ramus, and is oval to almost circular four localities in California in most of which it is in form. associated with the sea lion-like odobenid Imagotaria With two possible exceptions, Arctocephalus fischeri and in one locality also with a few remains of the last (Gervais and Ameghino) of at least questionable age of the desmatophocids. Whereas Imagotaria was of and a late Miocene or early Pliocene questionable a size comparable to the living walrus, Pithanotaria otariid from Peru (Hoffstetter 1968), no otariids are was very small, somewhat smaller than the smallest known from the Southern Hemisphere prior to the living fur seal, Arctocephalus galapagoensis Heller. late Pleistocene. In the late Pleistocene, Arctocephalus The youngest known member of the common an­ pusillus is known from South Africa (Hendey personal cestral group, as yet unstudied and not even fully communication) and Neophoca cinerea is known from prepared from the rock in which it was found, is from Australia (McCoy 1877). These are, however, clearly deposits in Oregon about 15 million years old. This assignable to Southern Hemisphere genera and species animal was approximately the same size as Pithanot­ and, as living genera are recognizable in the middle aria, but differed in having reduced but recognizable Pleistocene deposits of the North Pacific fauna, it is carnassial and molariform molars instead of a homo- reasonable to suspect that the otariids arrived in the dont dentition. The skull also lacked significant supra­ Southern Hemisphere earlier than the late Pleistocene. orbital processes. In its postcranial skeleton, this 15 Based upon average size, limb proportions, and lack million year old progenitor of the Otariidae does not of single-rooted cheek teeth, as well as upon the obvi­ appear to have differed greatly from Pithanotaria or ously primitive nature of underfur, the earlier otariids from somewhat more recent primitive otariids. are best considered as fur seals even though underfur Unquestionably more criteria will emerge as further is not likely to be found in the fossil record. specimens are prepared, but at present it seems that The oldest known otariid with single-rooted cheek the development of supraorbital processes and of a teeth is from upper Pliocene or lower Pleistocene de­ homodont dentition are the criteria most useful in posits, roughly 3 million years old, in San Diego, Cali­ separating the Otariidae from the common ancestral fornia (Burleson 1948). Not much of this animal is group. Such criteria developed between 12 and 15 known; a mandibular ramus with three teeth, an million years ago. Little more than increase in average isolated tooth, and two postcranial bones, one of 32 C. A. Repenning which suggests a stronger relationship to otariids of years ago. These temporal limits appear to reflect greater age than to more recent forms. The animal is available sample size and the oldest samples large small and presumably best thought of as a fur seal, enough to evaluate size bimodality. There is a sug­ but the single-rooted cheek teeth preclude even gestion of sexual dimorphism in the samples of some tentative assignment to Pithanotaria, as was done by of the unstudied members of the common ancestral the describer of the mandible (Burleson 1948). Never­ group. As sexual dimorphism appears to correlate with theless, it is the earliest record of single-rooted cheek the rookery behavior and terrestrial territoriality, it is teeth in the otariids and as the sea lions show this of interest to note that many otarioid fossil localities tendency more strongly than the living fur seals (Rep­ contain an abundance of material locally, suggesting a enning et al. 1971), it is the basis for presuming that site of deposition near or at a rookery. the sea lions may have diverged from the fur seals A rather clear picture of changes in faunal compo­ as much as 3 million years ago. Living sea lion genera sition with time is developing out of the fossil records are found in deposits along the Pacific coast of 1 to along the Pacific coast of the United States. Within 2 million years ago. this pattern of faunal change, latitudinal variation Major diversification of the Otariidae, therefore, within one otarioid fauna is also suggested. The odob­ appears to have begun quite recently, perhaps 3 mil­ enine odobenids, prior to their re-entry of the Pacific lion years ago, and there appears to be no reason to by Odobenus, are all southern in occurrence and, so believe that the great variety of otariids living today far as the record now indicates, some genera of the does not represent the same sort of diversity that the dusignathine odobenids appear to be restricted to odobenids experienced 5 to 10 million years ago and central and southern localities and others are known the desmatophocids about 15 million years ago. only from northern localities. Before the Pleistocene, known records of the otariids are also central and southern, although their presumed ancestor is best EVOLUTIONARY TRENDS known from northern localities. In the desmatophocids Many of the anatomical adaptations experienced by the genus Allodesmus appears to have a more souther­ the three families at the three different times just ly distribution than does the genus Desmatophoca. All mentioned, are similar in purpose though often not such latitudinal suggestions are, however, based upon a in manner of execution. Most conspicuous are those small number of samples and new fossil discoveries related to feeding habits. All families have developed will be most significant in better understanding such peg-rooted teeth and haplodonty, from more complic­ faunal distribution. ated tooth patterns to simple blunt pegs or simple sharp cusps; there appears to be a secondary complic­ ACKNOWLEDGEMENTS ation of tooth crown pattern in some living otariids, to judge from the lack of fossil records of such a tooth As the reader will appreciate, this summary of otar­ pattern in otariids between 2 and 12 million years old. ioid evolution is based in large part on studies either All families emerged, at different times, from the in press or incomplete, and upon preliminary examin­ common ancestral group by developing homodont ation of specimens not yet ready for study. It is also dentition with the loss of the typical carnivore multi­ based upon interesting discussion with many colleagues functional dental series. All developed greater sensitivi­ who are well informed on the subject of otarioid seals. ty to waterborne sound and protection of their ears While perhaps a very current summary of progress, the from hydrostatic and hydrosonic pressures; the me­ preceding certainly is not the last word on otarioid thods of these developments vary in pattern between evolution but rather a promise of what is to come. families (Repenning 1972) and with feeding habits In addition, after years of exchanging information and (depth of feeding) within, at least, the Odobenidae. interpretation, I feel I should say that I have no idea Although unknown in the desmatophocids because of whether any interpretation is really my own. I do the scarcity of postcranial skeletal elements, improve­ know, however, that few of my associates would take ment in swimming adaptations are evident in progres­ no exception to what has been said. sive shortening of limbs and strengthening of signific­ Most particularly, for permitting me to read pre­ ant musculature in the otariids and the odobenids. liminary manuscripts and to examine undescribed and Behavioral evolution beyond inferences of food types unstudied specimens in their care, specimens which and environmental preferences is of course difficult to they fully intend to study and which have had a signif­ interpret, but it appears that sexual dimorphism was icant impact on the present interpretation of the clearly present in the otariids at least 9 million years evolution of the Otariidae, I wish to thank Lawrence ago, in the odobenids at least 12 million years ago, and G. Barnes, Edward D. Mitchell, Clayton E. Ray, and in one genus of the desmatophocids at least 15 million Richard H. Tedford. \

Otarioid evolution

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