Polar Dinosaurs

Thomas H Rich, Roland A Gangloff, William R Hammer. Encyclopedia of Dinosaurs. Editor: Philip J Currie & Kevin Padian. Amsterdam: Academic Press, 1997. 

Until 15 years ago, polar habitats were hardly considered fertile ground for dinosaur research. However, studies completed since 1985 strongly suggest that they may have served as both a refuge and birthplace for some dinosaur groups. (The identification of polar dinosaurs refers to dinosaurs that lived within the polar circles of their time, not necessarily within the current polar circles.) In the Northern Hemisphere, eight areas ranging from the Late Jurassic(?) to the Late Cretaceous have produced dinosaurs that lived within the paleo-Arctic Circle. In the Southern Hemisphere, two areas within the Early Cretaceous paleo-Antarctic Circle have yielded dinosaurs, and three others are known from the ?Early Jurassic to Late Cretaceous that fall just outside it. These occurrences are summarized in Tables I and II. Most of the material from both hemispheres was collected during the past 20 years, and much of it remains to be fully described.

North Polar Dinosaurs

Dinosaur remains and associated faunas have been described from the Late Jurassic-Early Cretaceous and the Late Cretaceous of Russia, the Late Jurassic and late Early Cretaceous through latest Cretaceous of Alaska, the latest Cretaceous (Maastrichtian) of Canada, and the Early Cretaceous of Spitzbergen (Table I). Clemens and Nelms (1993) inferred some of the adaptations to a polar environment of the fauna recovered from the Kogosukruk Tongue of the Prince Creek Formation, Colville River, Alaska (Late Cretaceous: Campanian-Maastrichtian). They pointed out that its dinosaurs and mammals closely resemble those of contemporaneous sites in Wyoming, Montana, and Alberta, but there is a striking difference in the paucity of known terrestrial ectothermic tetrapods: frogs, salamanders, turtles, champsosaurs, lizards, and snakes, and crocodilians are completely absent. Hypsilophodontids are unknown from this region so far, though Thescelosaurus, an oddly primitive form, is found in the Late Cretaceous of the northern United States, and hypsilophodontids are common in the Australian polar assemblages (see below). A small Albertosaurus is the only evidence of a top carnivore in the assemblage, though this implies that larger ones were present, and sample size is not sufficient to assess the complete faunal diversity with confidence. The tyrannosaurids are known primarily from teeth that are 40-50% of the adult size of southern conspecifics. Nevertheless, the absence of the typical ectotherms found so abundantly in southern faunas appears to be real and raises questions about the ability of dinosaurs to tolerate climatic conditions that evidently could not be tolerated by other reptiles and amphibians. The possibility of smaller body size in the tyrannosaurids, combined with the abundance of hadrosaur juveniles and even rare hatchlings (Carpenter and Alf, 1994), may reflect year-round adaptations to high paleoaltitudes during the North American Cretaceous.

South Polar Dinosaurs

Antarctic dinosaurs come from Antarctica proper as well as from paleo-Antarctic sites in Australia and New Zealand.

The first Antarctic dinosaur fossils were not discovered until 1986 on James Ross Island off the coast of the Antarctic Peninsula (Olivero et al., 1986). This find, of a Late Cretaceous ankylosaur, was followed 3 years later by the collection of a partial skeleton of a Late Cretaceous hypsilophodont from nearby Vega Island by the British Antarctic Survey (Hooker et al., 1991). These two specimens represent the only Cretaceous dinosaurs known from Antarctica.

During the 1990-1991 austral summer, Early Jurassic dinosaurs were discovered on Mt. Kirkpatrick in the Transantarctic Mountains, approximately 650 km from the geographic South Pole near the Beardmore Glacier (Hammer and Hickerson, 1994). The Jurassic locality produced dinosaur fossils belonging to at least three different theropods and a prosauropod. These animals were found associated with fragmentary remains of a tritylodont (synapsid) and a pterosaur.

There are several reasons why Antarctic dinosaurs were discovered so late and are relatively rare. First, 98% of the surface area of the continent is ice covered, leaving relatively few places to look. The inaccessibility of many areas has additionally delayed exploration of the Mesozoic sediments that are exposed. Second, rocks of Late Triassic, Jurassic, and Cretaceous age, the time when dinosaurs existed, are rare in areas where rocks are exposed. The youngest rocks in the Transantarctic Mountains, the largest region where exploration is possible, are early Middle Jurassic in age, and they only occur on the highest peaks. Although some Cretaceous exposures exist in the Antarctic Peninsula region, they are primarily marine deposits. Because dinosaurs were terrestrial they are rarely preserved in marine rocks. The two individual Cretaceous dinosaurs from the Antarctic both represent unusual occurrences where dinosaur skeletons were washed offshore after death and deposited in marine sediments.

The Jurassic dinosaurs from Antarctica occur at an elevation of more than 4000 m in a tuffaceous siltstone (river bank deposit) high in the Falla Formation. All but 3 of the more than 100 bones collected from this site during a single 8-week field season were concentrated in one small area of exposure approximately 1 m thick and 5 m wide. The Falla Formation overlies the Triassic Fremouw Formation in the southern portions of the Transantarctic Mountains. The Fremouw Formation has yielded faunas of Early to early Middle Triassic age that consist mainly of synapsids and temnospondyls (amphibians) that lived prior to the first appearance of the dinosaurs (Hammer, 1990).

Igneous rocks that have been dated at 177 million years (early Middle Jurassic) intrude the upper portion of the Falla Formation where the dinosaurs occur, indicating they lived before that time. The presence of a large tritylodont and a large plateosaurid prosauropod indicate more precisely an Early Jurassic (Pleinsbachian-Toarcian) age for the fauna from the Falla Formation. Similar tritylodonts from South Africa (Tritylodon maximus) and China (Bienotheroides) are restricted to the Early Jurassic, and plateosaurid prosauropods from other continents do not occur later than the Early Jurassic.

Most of the Jurassic dinosaur specimens from Antarctica belong to a new genus and species of theropod, Cryolophosaurus ellioti (“frozen crested reptile”; Hammer and Hickerson, 1994). Cryolophosaurus was a large carnivore (skull length 65 cm, body length approx 7 or 8 m) with a unique crested skull. The skull is high and narrow and the nasals are high ridges that run along the front of the skull and merge into the highly furrowed crest that sits just above the orbits. The crest consists mainly of the lacrimal bone and extends the entire width of the skull between the orbits. Two horns lie immediately adjacent to the crest on either side. The crest apparently functioned as a display feature during certain types of social behavior such as mating. It is thin, its highly furrowed surface is decorative, and it would have been ineffective as a weapon.

Other theropods, particularly Dilophosaurus and Monolophosaurus, have cranial display crests. However, in the case of both of these animals the display crests are most obvious from a lateral (side) view. The crest of Cryolophosaurus, on the other hand, is most visible from a frontal view. This probably indicates behavioral differences between Cryolophosaurus and the other crested theropods, assuming the crests in all three were visual display features.

A second unusual characteristic of the Cryolophosaurus skull can be seen in the postorbital region, where the lateral temporal opening is actually split into two openings by processes of the postorbital and squamosal bones. Only a few tyrannosaurid specimens show a similar fusion among all the theropods. Aside from the unique features mentioned, the skull of Cryolophosaurus is similar to later Jurassic tetanuran theropods such as Allosaurus and Monolophosaurus.

Elements of the postcranial skeleton of Cryolophosaurus collected include a femur, pubis, ilium, ischium, tibiotarsus, tibia, fibula, two metatarsals, and numerous vertebrae. The femur, tibiotarsus, and metatarsals show primitive theropod characteristics that are also found in early ceratosaurs such as Dilophosaurus. However, the pelvic features are more derived, again similar to the later Jurassic theropods such as Monolophosaurus.

Cryolophosaurus thus shows a mixture of characters, some of which link it to the more derived theropods of the later Jurassic and some of which are more primitive, like the earlier Jurassic ceratosaur Dilophosaurus. It appears most likely that Cryolophosaurus is related to the later Jurassic tetanuran theropods (probably the allosaurids) but because it is much older it is not surprising that it retains some primitive features.

Other Jurassic theropod taxa from Antarctica are represented only by teeth. Some of the bones of Cryolophosaurus show evidence of scavenging and at least two different scavenging theropods are represented by broken teeth found near gnawed bones.

Elements of the prosauropod identified from the Mt. Kirkpatrick site include a partial articulated foot that includes the astragalus and four metatarsals, the distal end of a femur, and, possibly, a few cervical vertebrae. The Antarctic animal is among the largest of the prosauropods, and the size and features of the foot indicate it is related to the plateosaurids such as Plateosaurus from the Late Triassic of Germany and Lufengosaurus from the Early Jurassic of China.

The James Ross Island ankylosaur came from the Gamma Member of the Santa Marta Formation (Olivero et al., 1986). This Campanian (Late Cretaceous) marine unit also contains gastropods, bivalves, ammonites, and plesiosaurs (marine reptiles). The one incomplete specimen of a probable nodosaurid ankylosaur collected includes skull and other skeletal fragments and armor plates.

The Antarctic hypsilophodont was collected from a marine deep-shelf silty mudstone in the Late Cretaceous Lopez de Bertodano Formation on Vega Island. The specimen includes disarticulated elements of the skull, cervical, dorsal, and sacral vertebrae; portions of both pectoral girdles and humeri; and parts of the pelvis (ilium and ischium) (Hooker et al., 1991). The length of the animal, estimated to be 4 or 5 m, makes it one of the largest of the hypsilophodonts (normal size range 2 or 3 m). Preliminary investigations indicate that although this animal shows features consistent with other hypsilophodonts, it also has some unique features, particularly in the dentition. The humeri and pelvis have some characteristics similar to Dryosaurus and Valdosaurus.

Australian dinosaurs are known from the Early Jurassic through the Late Cretaceous. From the Early or Middle Jurassic of southeastern Queensland, Australia, has come a partial skeleton of one of the earliest sauropods, Rhoetosaurus brownii Longman 1927.

A single astragalus suggests that the well-known form Allosaurus may have persisted into the Aptian of southeastern Australia after having become extinct elsewhere at the end of the Jurassic.

On the basis of one or a few bones, the presence of four groups previously known in the Late Cretaceous of the Northern Hemisphere has been suggested in the Early Cretaceous of southeastern Australia.

An ulna from the Aptian bears an uncanny resemblance to that of the Maastrichtian protoceratopsian Leptoceratops gracilis from Alberta, Canada. This suggests that Polar Gondwana may have been the place of origin for the neoceratopsians because they are known nowhere else prior to the Late Cretaceous.

Ornithomimosaurs are represented in the Albian of southeastern Australia by femora and vertebrae distinct enough to base a new genus and species, Timimus hermani (Rich and Vickers-Rich, 1994) on them. Together with the Late Jurassic Elaphrosaurus bambergi from Tendaguru, Tanzania, this material suggests a presence for this group on the Gondwana continents prior to the Late Cretaceous, when ornithomimosaurs are best known in the Northern Hemisphere. The recent publication of Shuvosaurus inexpectatus Chatterjee 1993 from the Late Triassic of Texas implies that ornithomimosaurs might have had a much longer history in the Northern Hemisphere than previously suspected.

Oviraptorosaurs, previously represented exclusively in the Late Cretaceous of the Northern Hemisphere, appear to have been present in the Albian of southeastern Australia based on a partial surangular and a vertebra (Currie et al., 1996).

More than 15 isolated teeth of dromaeosaurids suggest the presence of another typically Late Cretaceous group of the Northern Hemisphere in the Aptian and Albian of southeastern Australia (Currie et al., 1996).

Among polar dinosaurs from the Southern Hemisphere, only one feature of one individual has yet been interpreted as an adaptation to life in a high latitude environment. This may reflect more the fact that very little is known about these animals and less published than that they rarely displayed marked differences between themselves and their lower latitude contemporaries. The feature in question is the enlargement of the optic lobes of the brain of the hypsilophodontid Leaellynasaura amicagraphica (Rich & Rich, 1989) from the Aptian of southeastern Australia in comparison to the same structure on hypsilophodontids from lower latitudes. Hypertrophy of this structure formed the basis for the suggestion that this animal had enhanced ability to see under the low light conditions that would have prevailed during the prolonged periods of continuous darkness each winter.

Hypsilophodontid dinosaurs are generally rare in most dinosaur assemblages. Even where specimens are relatively common as on the Isle of Wight, their taxonomic diversity is not great. Southeastern Australia is a marked exception to that generality. At least six species in five or six genera occur there, just over half the total dinosaurs recognized in that region to date. P. Currie (personal communication) has suggested that hypsilophodontids may have been primarily an upland group at lower latitudes (hence their general rarity there) and are better represented in polar southeastern Australia because of its cooler conditions.

In this regard, it may be noteworthy that the dinosaur known from the Late Cretaceous of Vega Island, Antarctic Peninsula, is a hypsilophodontid or dryosaurid (Milner and Hooker, 1992), and that of the four found at Mangahouanga Stream, New Zealand, one is a probable dryosaurid, a family closely related to hypsilophodontids (Molnar and Wiffen, 1994).

This explanation for the apparent preponderance of these groups at high latitude may well be true. However, in the much more fossiliferous Liscomb bone bed on the Colville River, Alaska, hypsilophodontids are unknown despite the extensive collecting carried out there (Clemens and Nelms, 1993, personal communication). The age of the site is late Campanian to early Maastrichtian (radiometric dates = 68-74 Ma), and by that time there may not have been the variety of hypsilophodontids that there was earlier in the Cretaceous. Several teeth and an ungual of a hypsilophodontid have been collected in older (Campanian) channel deposits upriver from the Liscomb bone bed site.

Today, there are no birds or terrestrial mammalian families restricted to the polar regions. As far as the available record has been analyzed, the same can be said of the polar dinosaurs. Although some new genera and species have been recognized among them, they all belong to families also known at lower latitudes.

However, given the fragmentary nature of much of the evidence for the existence of various groups, this high degree of overlap with low-latitude dinosaur assemblages could in part be an artifact. If one had only a single tooth of the extinct South American litopterns and that group was otherwise unknown, depending on the species, the most parsimonious familial identification might be Equidae. In that case one would be parsimonious but one would be wrong. The recent identification of protoceratopsians in the Aptian of Australia (Rich and Vickers-Rich, 1994) could be a parallel case. On the basis of a single bone, rather than propose an entirely new group of vertebrates that would share an uncanny resemblance in the form of the ulna to protoceratopsians but could in some other as yet unknown way be distinct from them, the specimen was allocated to this known taxon, which extended its record not only to another continent but also backwards in time by at least 15 million years. At our current state of knowledge, that identification is quite plausible but could eventually prove to be fundamentally in error if that Australian “protoceratopsian” proves to be a radically different animal when it is better known.

There is no evidence such as tillites to suggest that continental ice sheets existed during the Mesozoic at high latitudes. However, it has been inferred on the basis of the occurrence of dropstones as large as 3 m across in fine-grained marine sediments deposited during the Late Jurassic and Early Cretaceous that winter seasonal ice did form at high paleolatitudes in both hemispheres (Frakes et al., 1992).

On the North Slope of Alaska, studies assessing the mean annual paleotemperature have been carried out on the sediments producing the dinosaurs. Paleobotanical evidence based on leaf margin and stomata structure together with the overall composition of the flora have been taken to suggest a mean annual temperature of 2-8°C.

In southeastern Australia, similar modes of analysis produced conflicting results. There, the paleobotanical evidence suggested a mean annual temperature of 10°C (Parrish et al., 1991), whereas an oxygen isotope estimate is °2 ± 5°C (Gregory et al., 1989), which is the difference between Chicago and Anchorage today. Although the biological implications of these two estimates are quite different, they are concordant in that the paleoclimate was far from tropical.

No matter what the paleotemperature was, polar dinosaurs would have had to adapt to prolonged periods of annual darkness each winter. Although year-round residency has not been proven in either hemisphere, several lines of evidence strongly suggest this possibility. The suggestion has been made that in the geological past the earth’s rotational axis might have been significantly closer to being oriented perpendicular to the plane of the ecliptic, thus reducing the length of continuous darkness each winter at high latitudes (e.g., Douglas and Williams, 1982). However, the criticism of Laplace (1829) against the earth’s obliquity having shifted more than a few degrees from its present orientation as it does over a period of approximately 41,000 years has never been refuted.

The presence of abundant and diverse assemblages of polar dinosaurs has raised several important questions regarding the behavior, physiology, and extinction of all dinosaurs. Polar dinosaurs are an indisputable fact. They occupied high latitudes in both hemispheres for more than 45 million years. Establishing whether they were year-round residents or long-distance seasonal migrants is directly related to their thermoregulation and energetics. This question also affects extinction theories that depend on short-term weather effects such as a bolide-induced “nuclear winter.” Currently, polar dinosaurs have produced more questions than answers.

The work on the polar dinosaurs is just beginning. Just how distinctive they were from their lower latitude contemporaries is unknown. Beyond a doubt, the known area with the greatest potential for producing further knowledge about polar dinosaurs in either hemisphere is the 200 km of outcrops of terrestrial sediments on the left bank of the Colville River, Alaska. All of the Late Cretaceous is represented in those deposits with fossil vertebrates now known from sites of many different ages. The Falla Formation of Antarctica seems the most promising area in the Southern Hemisphere in terms of future potential. Clearly, to find significantly more information about Gondwana polar dinosaurs outside the area of the Falla Formation, new areas such as the Cretaceous coalfields of New Zealand need to be investigated (Rich, 1975).