On a monospecific assemblage of sauropod dinosaurs from Patagonia : implications for gregarious behavior

ON A MONOSPECIFIC ASSEMBLAGE OF SAUROPOD

DINOSAURS FROM PATAGONIA: IMPLICATIONS FOR

GREGMnOUSBEHAWOR

Rodolro A. CORIA

Museo Carmen Furies, (8316) PlazaHuineul,NEUQUÉN. ARGENTINA

AOSTRACT: A monospecific bonebedof the cetiosaurid sauropodPaJagosaurusfarias;Is reported.Ils implications for hypotheses of dinosaur gregarious behaviorare diseussed,TaphonoDlic datasuggest catastrophic Dla88 accumulation of the assemblage. Associated juveniles and adults sugest the l'emains of a social group or herd, and rnight suggestextendedparental care among cetiosaurids.

RESUMa:Neste artigoérererenciada uma camadaCOolabundantesl'estososteolôgicos de apenas uma espécie de sauropode do grupo doscetiossaurldeos~Patagosaurus furiasi, e discutem-se as hip6teses de compor1amento gregario nos dinossâurios. Os dados iafon6micos pennitem-nos inrerir que 0 conjunto de essos resultou de uma acumulllçio catastr6fica. A presença de juvenis e de adullos sugere que se tRiava de uma manada de dinossaurios e provavelmente que os cetiossaurîdeos dispenssvam cuidados aos juvenis.

INTRODUCTION

The gregarious behavior and habits ofdinosanrs have received much Jess study than their functional morphoIogy. This is because information on the gre-garions behavior of these enormousterrestrial ver-tebrates cannot he obtained

sirnply

from the analyses oftheir anatorny, Consequently, almost all evidence used ta infer dinosaur gregarious behavior cornes from fossil tracks, nestsites and monospecific assem-blages.

BIRD (1944), on the basis of footprints, was the first to consider the possibility that sauropod dino-saurs congregated in herds. Many latter authors agreed with his interpretation (BAKKER, 1968;

Os-TROM,1972, 1986;LOCKLEY, YOUNG&CARPENTER, 1983; LOCKLEY,1991).

During the

summers

of1978-1980, palcontolo

gists

[rom the Fundaci6n Miguel Lillo, the Universidad Nacional deTucumân, the Musee Argentine de Cien-cias Naturales, and the Consejo Nacional de Inves-tigaciones Cientificas y 'Iécnicas, hcaded by Dr. José Bonaparte, found the remains of [ive specimens of cetiosaurid sauropods from a Middle Jurassic locality

at C-erro Condor Norte (Chubut Province, Patagonie, Argentina) (Fig.I),

The preliminary taphonomie and population analysis of this bonebed of cetiosaurid sauropods pre-sented here, was briefly reported previously byBONA·

PARTE

(1982)

asa family-like

groupofsauropods. This report may beconsidered afurthcrattcmpttaprovide information about the social habi ts of these h uge ani-mals.

MATERIALS - SPECIMENS USEDINTHIS STUDyl

The materiels that compose each specimen and the interpretations of their ontogenie stage are pre-sented below. Identification and discrimination of each individual specimen was made on the basis of size and taphonomic distribution of the tossils (BONA-PARTE,pers. commun.).The specimens (individuals) arc described in order of decreasing size,

SPECIMEN1(PVL.4170IIOL01YPE)

Four anterior cervicals, threc postcrior cervicals, three articulated anterior dorsals, rive mid and pos-terior dorsals, two proximal caudal centra, twelve mid

(1) Abbreviations: MANC-CH - Museo Argentine de Cîcncias Naturales, Olubut Collection. PVL - Vertebrate Paleontology Lillo Institute,

RA. CORIA

CERRO CONDOR

NORTE

o

50 100km

« • 1

Fig. 1 - Map of Chubut Province. showing the location ofCerroCondor Norte locality. and distal caudals, rib fragments, chevrons, righlsca

p-ula (prox.imal part) and coracoid articp-ulated,

terr

scapula and coracoid, left humerus (proximal

part),

both Hia, both ischia (partiaHy fused and incomplète proximally), right pubis, both fernora. This specimen is believed to be an adult because the neural arches are coossificd to the centra and sacral vertebrae coos-sified. Il reprcsents a large specimen, with a femur 1364, and

a

pubis835 mm long.

SPECIMEN2(PVL-4116 ANDMACN-Cn·935) Four middle dorsal neural arches, three dorsal centra, four sacral centra with sacral ribs, one sacral centra, three sacral neural arches, onlyone preserving both transverse processes, one proximal caudal cen-trum, one proximal caudal arch, six mid and distal caudal vertebrae, two incomplète neural spines, six proximal fragments of dorsal ribs, three chevrons, fragmentsof both ilia, both pubis and ischia. This îs also considered an adult individual based on its large size. However, the neural arches are not eoossified with thcir centra, nor are the sacral centra coossifled to cach other, This individual, witb a pubis 803 mm long, is smaller than specimen 1.

SPECIMEN3(PVL-4172ANDMACN-CII-932) Axis neural centrum, middle cervical centrum, two cervical centra, dorsal centrum, anterior dorsal, three dorsal centra, six dorsal neural arches, eight dorsal centra, one sacral vertebree with a neural arch. one sacral rib, right scapula and coracoid, right humerus,

radius and ulna, bath pubes, three metatarsals, two mid-falanges, one ungual.

This individual is considered a juvenile because the presacral and sacral neural arches are not fuscd to their centra. This specimen is smaller than Speci-men 2, having a pubis 548 mm long.

SPECIMEN4 (PVL-4171 ANDMACN-ClI-933) Anterior portion of a left dcntary, two cervical centra, anterior dorsal centrum, seven dorsal centra, four dorsal neural arches, one sacral neural arch, il-ium fragment, left pubis, right femur and tibia. The unfuscd neural arches and the si...te of apendicular bones indicatc juvenile condition. The pubis is 414 mm long and the femur 650 mm long.

SPECIMEN.5 (PVL-461.5)

One dorsal centrum,one incomplete ischium. The ischium 18estimated to have been 320 mm long, in-dicating the smallest individual in the assemblage. DESCRIPTION

PALEOENVIRONMENT

The fossil-bearing strara is a thinly hcdded cal-careous tuff containing carbonaceous plant remains, and intercalated with th in sandstones. The paleoen-vironment was a floodplain with abundant water (BONAPARTE, 1986). The rernains show no high en-ergy transport becauso the dorsal arches of young

in-dividuals still preserve their delicate infradiapo-physial and parapophysial-diapophysial lamînae. Moreover various bonesrernain in articulation (Fig. 2). Althougbadetailed sedlmentological study orthe quarrybas not been made. a preliminaryanalysis of the sediments and the preservation ofarticulated, and delicate and small boues indicateft low energy envi-ronment.

TAPHONOMY

The specimens were round in a area 15 m long and 4 mwidc (BONAPARTE, 1986), and were mostly disarticulated and embedded at the same strati-graphieal level (Fig. 2). Practically ail of the material consists of postcranial bones, The only cranial frag-ment is a portion of the left dentary of a juvenile (Specimen 4, MACN.CH-933). The absenceof cra-niai bouesmay be due to incomplète ossification of the bones, post mortem scavenging or erosion. Il should also be noted that the amount ofmaterial(or each individuel is Jess in the younger individuals. Ta-phono-mie blases

(e.g.

immat ure/small bones weather more rapidly th an adult/large bones; see BE-HRENSMEYER, 1978; BEHRENSMEYER, WESlERN &

BOAZ, 1979; CARPENTER, 1982) might explain the usual scarcity of juvenile bones (RICHMOND, 1965;

LEONARD),1981).

The whole assemblage shows no great variation inthe stageofweathering. Bones of the same weath-ering stage maytin sorne cases,he used as an evidence

of catastrophic accumulation (VOORHIES, 1969;BE· HRENSMEYER,1978). AIlfivesauropod specimens

be-10ng ta the same spec ie s, Patagosaurus [anasi BONAPARTE, 1979. This identificationfor four of the specimens is based on the morphologyof the verte-brae, whieh agrees with Bonaparte's specifie diagno-sis (BONAPARTE, 1986). Although the vertebral remains of Specimen 5 are notsufficient for a specifie identification, for this paper, 1 propose to assign this specimen to this same species because the ischium

shows the sarne morphoJogy as the other specimens. No other vertebrate taxa were associated with the ma-terial deseribed here, indicating ft monospccific

85-semblage of cetiosaurid sauropods.

POPULAll0NAL ASPECrs

This assemblage is cornposed of both adult and juvenile individuals in different stages of ontogenie development. Because

sa

few preserved clements are commonto all Civeindividuals, the relative lengthof the pubis was used to show the size/age classes pre-sented in Figure 3. In the diagram, two groups may be seen, one formed by Specimens 1 and 2 repre-senting the adults; the other formed by Specimens 3, 4 and 5, whîch represent juveniles. The adult speci-mens are the largest individuals known for Patagosau-rus [ariasi. Specimens 3, 4 and 5 are considered

juveniles becauseof their smaller size compared to Specimens 1 and 2. Also, the vertebral centra and neural arches are not fused, indicating ajuvenile con-dition. Curiously, Specimen 2 is adult in size (corn-pared with Speci me n L), but it shows juvenile characteristies due to the unfused centra and neural arches. ft is possible that complete coossiflcation of the ve rtebrae in these animals occurred in laie r stages of lire. Alternativelyitcould he an example of sexual dimorphismwith the female showing a juvenile con-dition, as in sorne large living ungulate mammals

Ce.g.

bison).

DISClISSION

Gregarious behavior has been frequently inferred from a monospeciflc dinosaur bonebed. Two such ex-amples are theiguanodon bonebed from Bernissart, Belgium(DOLLO. 1882), and the Plateosaurus bone-bed from Trôssingen, Germany (HUENE,t928). How-ever, these cases are now intcrpreted as aecretional, not catastrophic assemblage of dinosaurs(COOMBS,

1990), due to the stratigraphie relationships of indi-vidual skeletons, In South America, in the Triassic

Fig. 2 - Map

or

the distribution of the principal pieces ofPatag03t2urusfanasiround in Cerro Condor Norte [after BoNAPARTE,1986). QuadricuJe: 1 mbyside.

cm 100 80 60 40 20

.

,

~

2 3 Specimens 4 5 RA.CORIA

respect to adults, AlI these factors suggest, bul do not prove, a catastrophic accumulation ofthe bones. The monospecific nature of the Cerro Côndor Norte

sauropod

assemblage

aIsosuggests

the possible coexistence of adults and juveniles of different ages, perhaps in sorne kind of gregarious group or herd, Probable evidence of posthatching parental care has come

(rom

sites where

juvenile

and

adult

specimens were round associated with nest and eggs (e.g. Pro-toceratops, Maiasaura, sec COOMBS, 1990).

Due tothe sileofjuvenile individuals from Cerro Côndor Norte (startingwith Specimen 5, PVL·46 15),

il îs not possible define them as hatchlings (Fig. 4). No evidence of nest or eggs is known from this locality,

Ifthe hypothesis about thecatastrophic origin of the assemblage of Cerro Côndor Norte is correct,

îl

prob-ably indicates that the sauropod spedesPatagosaums

[ariasi

exhibited gregarious behavior.

Fig. 3 - Size/agedistributionof the assemblage estimatcd from the pubis length (Specimen 5: estimeted], See text foridentification of specimens.

El Tranquilo Formation at Estancia Laguna Col-orada, Santa Cruz Province,Argentine, rnany speci-mens of Plateosaurus "p. were round together

(CASAMJQUELA,1964;BONAPARTE. 1978). Sedimen-tological and paleoenvironrnental studics of this

10-eality will be u sefu l to d e te r minc

if

this mass-accumulation is due to catastrophic or

acere-tional

conditions.

The preservation stage of the assemblage

{rom

Cerro COndor Norte indicate similar condition of ex-posure to weathering, burial at the sarne stratigraphie level and a re la tively high proportion ofjuveniles with

CONCLUSION

The fossil assemblage at Cerro Côndor Norte is believed to represent part of a "herd" of sauropod dinosaurs based on both taphonnmic and systematic data. The monospecific nature of the assemblage con-taining both adult and juvenile specimens suggests mass-accumulation of part of a sauropod herd with

arelatively bread size/age association of individuals of the sarnc species, The populational composition

of

the assemblage

suggestsalso that

sorne

cetiosaurid

sauropod dinosaurs (e.g. PatagosauflLf fariasi) could have developed relativelycomplex social behavior in-velving posthatchinggregarious behaviorand paren-tal care extended to young of several age classes.

1

2

3

4

5

The herd defense hypothesis proposcd for

hadrosaurs (LoCKLEY, YOUNG & CARPENTER.

1983), based on both tracks and nesting site evidence

(HORNER & MAKELA, 1979) may also be applied to the sauropod Patagosaurus [ariasi. At present. no anato-mical features which would provide defense have been recognized in this species, Possibly such herding behavior played an important role in the sur-vival ofthe individual, the population and the species. ACKNOWLEDGEMENTS

1am indcbtedto Dr.JoséF.Bonapartefor bring-ing thePatagosaurusfanas;specimens to

my

attention and for providing helpfulcomments and discussions, and to the authorities of Musco Argentino de Cien-cias Naturales "Bernardino Rivadavia" for Jetting me work with the material deposited in its collections. 1 also thank Dr. Jaime E. Powell for providing data about the material deposited in the collections of the Unîversidad Nacional de Tucumân, Ors. Kenneth Carpenter and John Horner provided useful

corn-menes

and criticisms and improved the English of ear-lier drafls of this manuscript, Also thanks to Ors. Spencer Lucas (New Mexico Museum of Natural His-tory), Adrian Hunt and Martin Lockley (University of Colorado,Denver) for their critical review. Thanks also ta Lie. Leonardo Salgado (Universidad Nacional del Comahue) for his ever present help and criticism. REFERENCES

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