Migration

GREGORY S. PAUL

Baltimore, Maryland, USA '0·'

To migrate long distances on land requires threecharacteristics: sufficiently large size, long stridinglegs, and sufficient aerobic power. Mice, for example,cannot traverse long distances, nor can large snakesand turtles. MIgrating on land is harder than migrat-ing in the sea orin the air because the energy cost ofmoving a given distance is highest on land and leastin water (swimming is up to a dozen times moreefficient than walking). Also, sea creatures can takeadvantage of ocean currents. Many marine fish, rep-tiles, and mammals migrate very long distances,sometimes across entire oceans. Insects, bats, andbirds also migrate great distances, in extreme casesfrom pole to pole. In comparison, land migrationsare rather modest. The greatest terrestrial migratorsare polar mammals, who are under strong pressureto seek out the best seasonal conditions. Caribou trekup to 5000km a year, but this movement is confinedto a rather small area of 400 km across. Polar bears-

Migration

move up to 2500 km a year, partly in the water. Inlower latitudes strong wet-dry seasons can inspireseasonal movements. The annual migrations of saigaand Serengeti gnu cover up to 1000 km round-trip.The long migrations often attributed to plains bisonand African elephants before European disruptionare controversial. It is notable that caribou and gnuare specialized for migrating because they have themost energy-efficient legs known. In herding animalsthe size of the juveniles may be a limiting factor;migrating ungulate calves weigh at least 15kg. Carni-vores do not migrate with their herbivore prey; rather,they exploit herbivores as they move through theirterritory. The latter fact exposes the point that migra-tions are dangerous. The tendency of land animalsto migrate in mass aggregations at speeds above 2 or3 km/hr reduces their vulnerability to predation butincreases the possibility of mass disasters at over-crowded water crossings. Migrations are energy-tax-ing and arduous journeys that are undertaken onlyif there are compelling advantages to doing so. Thismay be why few mammals migrate, and no reptilesdo so.

Migrations in the fossil record are difficult to iden-tify but are suggested by the following taphonomiccharacteristics. Migrating animals often take advan-tage of the clear run provided by shorelines (Cohenet al., 1993).Therefore, mass, unidirectional trackwaysparalleling a shoreline are suggestive of migratorymovements, especially if they were made at 2 or 3km/hr or more. Mass aggregations can be killed byvolcanic events, fast-acting epidemics, or suffer highmortality at river crossings. Mass death assemblagesthat are not attributable to gradual accumulation (asin droughts) are therefore strongly suggestive of mi-gratory habits (Coombs, 1990). If a dinosaur specieswas limited in distribution, it probably did not mi-grate very far. A wider distribution is compatiblewith seasonal migrations but may simply indicatedispersed habitation, as per the circumpolar distribu-tion of wolves and moose.

Almost all the larger dinosaurs had the long strid-ing legs needed to migrate moderate distances (Fig.1;the big-clawed, fat-bellied THERIZINOSAURS and short-limbed ANKYLOSAURS may have been exceptions). The2-10 km/hr speeds recorded by the majority of dino-saur trackways are high enough for long-distancemovements (Paul, 1994). No dinosaur had the ex-

Migration

FIGURE 1 The great size of hadrosaurs may have en-hanced their migratory abilities, but they lacked the ex-tremely energy-efficient legs that make gracile caribou thelongest ranging modern land animals. Figures drawn tosame scale.

tremely gracile, highly energy-efficient legs of special-ized long-range migrators.

The traditional view of dinosaurs as REPTILES, manysemiaquatic, tended to suppress speculation that di-nosaurs migrated. An exception was Huene, whosuggested that bone beds of Late Triassic prosauro-pods were death assemblages that recorded massmigrations across a centred European desert. Thisscenario has been challenged by recent work indi-cating that the bone beds accumulated gradually(Coombs, 1990).

The giant SAUROPODS of the Late Jurassic MorrisonFormation have been considered potential migratorsbecause they combined great energy needs with aseasonally dry habitat and because they may haveroamed to the bordering highlands in search of largecobbles suitable for GASTROLITHS. These speculationsare plausible, but there is little direct fossil evidencethat sauropods migrated. Parallel sauropod Jura-Cretaceous trackways following shorelines hint at mi-gratory behavior.

The best evidence for migrating dinosaurs are theextensive bone beds of Late Cretaceous HADROSAURS

and CERATOPSIDS recently discovered in western NorthAmerica from Texas to Alaska, along what used to bethe western, north-south running coast of an ancientinterior seaway (Hotton, 1980;Horner and Gorman,1988;Currie, 1989).It is difficult to explain the accu-mulations as gradual; instead, they appear to be theresult of sudden mass deaths, attributable to vulca-nism or drownings at river crossings. Numbers ofindividuals involved in each event range from thehundreds to the tens of thousands. Age range in thebone beds is from moderately sized (and thereforemobile) juveniles to adults (the implication is that,smaller juveniles either did not migrate or did so

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independently). The provincial organization of west-ern coastal dinosaur faunas into nearshore, shore, andinterior, as well as distinct north-south populations(Lehman, 1996),suggests that the straight-line rangeof any migrations was limited in most or all cases.

The last point contradicts suggestions that Alaskandinosaurs moved far south to avoid the polar winter.In the Late Cretaceous northern Alaska was as littleas 10°from the north paleopole, and the Arctic Circlemay have been near or in northern Alberta (Fig. 2).It has been argued that the high-latitude dinosaurswere migrating north-south in order to escape winterconditions that, although much milder than today,included extended dark, cool and perhaps freezingtemperatures, and floral dormancy near the poles.Similar journeys have been suggested for Australian.dinosaurs living near the south paleopole of the time.In this view dinosaurs moved toward the pole toenjoy the abundant floral growth of long summerdays and then to lower latitudes in order to findwinter food or to seek the warmth needed by dino-saurs with reptile-like energetics. In the latter sce-nario, Hotton (1980) portrayed high-latitude dino-

FIGURE 2 (A) North slope and (B)Montana-Wyomingindicate the possible north-south zone of migration of LateCretaceous dinosaurs .along the northwestern coast of theNiobrara seaway (cartographic distortion causes the coastalplain to appear straight rather than properly curved). Thepaleomagnetic pole is indicated by the open circle; the cal-culated spin pole by the cross. The future locations of NewYork and Los Angeles are marked. Scale bar = 2000 km.

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saurs as "happy wanderers" that moved up to 6000km a year. To move from northern Alaska far enoughsouth to warm reptilian bodies in the winter mayhave required even further movements, up to 9000km round-trip (equal to a New York to Los Angelesround-trip). However, such long migrations mayhave posed insurmountable energy problems. Thecost for a juvenile hadrosaur or ceratopsid of 500 kgto move 6000-9000 km was about 900,000-1,400,000kcal. The annual active energy budget of the samejuvenile reptilian dinosaurs would have been only800,000-1,500,000 kcal, too low to power such a longtrip. Nor could reptilian dinosaurs have producedenough sustainable aerobic exercise power to moveat the 2 or 3 km/hr speeds typical of migrating landanimals. If dinosaurs had mammal-like energetics,then they would have had the power needed for verylong migrations, at least in principle. The great sizeof many dinosaurs also facilitated long movements.In this view the consensus that some large dinosaursmigrated is supported.

However, scenarios that portray dinosaurs asgreater migrators than mammals may be excessive,and it is probable that the great majority of dinosaursdid not migrate at all. The absence of specialized,unusually energy-efficient limbs may have limitedthose large dinosaur~ that did migrate to about 500km straight-line one way each year. In this case, aminority of herbivorous dinosaurs near and far fromthe poles migrated in order to seek out the best intra-regional conditions, and to avoid overexploiting thefood resources in one place, rather than to span partof a continent to find the best seasonal climatic regime(Paul, 1994). In this scenario improvements of high-latitude winter conditions were at most modest andalleviated but did not solve major thermal andfood problems.

See also the following related entry:BEHAVIOR

References-------------------------Cohen,A. S.,Halfpenny,J., Lockley,M., and Michel,E.

(1993). Modernvertebrate tracks from LakeManyara,Tanzaniaand their paleobiologicalimplications.Paleobi-ology 19, 433-458.

Migration

Coombs,W. P. (1990). Behaviorpatterns of dinosaurs. InThe Dinosauria (D.B.Weishampel,P. Dodson,and H.Osm6lska,Eds.),pp. 32-42. Univ. of CaliforniaPress,Berkeley.

Currie, P. J. (1989). Long-distancedinosaurs. Nat. History98(6), 60-65.

Horner, J. R, and Gorman,J. (1988). Digging Dinosaurs.Workman,New York.

Hotton, N. (1980). An alternativeto dinosaur endo-thermy:Thehappy wanderers. In A Cold Look at theWarm-Blooded Dinosaurs (R. D.K.Thomas and E. C.Olson,Eds.),pp. 311-350. MAS, Washington,DC.

Lehman,T. (1996). LateCampanian dinosaur biogeogra-phy in the western interior of North America.In Di-nofest International Symposium (D.1. Wolbergand E.Stump, Eds.),p. 73.Arizona StateUniversity,Tempe.

Paul, G.S. (1994). Physiologyand migration of NorthSlopedinosaurs. In 1992 Proceedings of the InternationalConference on Arctic Margins (D.K. Thurston and K.Fujita,Eds.),pp. 405-408. United States Department ofthe Interior,Anchorage.

Mill Canyon Dinosaur Trail,Utah, USA

see MUSEUMS AND DrsPLA YS

Milwaukee Public Museum,Wisconsin, USA

see MUSEUMS AND DISPLAYS

Mineral Research andExploration Institute, Turkey

see MUSEUMS AND DISPLAYS