An Antarctic Cretaceous theropod

6 Pages·1996·2.3 MB·

by  Ralph E Molnar

ANANTARCTICCRETACEOUS THEROPOD
R.E.MOLNAR, ALEJANDROLOPEZANGRIMAN&ZULMAGASPARINI
Molnar, R.E., Lopez Angriman, A. & Gasparini, G. 1996 12 20: An Antarctic Cretaceous
theropod.MemoirsoftheQueenslandMuseum39(3): 669-674.Brisbane.ISSN0079-8835.
ThedistalpartofatheropodtibiahasbeenrecoveredfromtheConiacian-SantonianHidden
Lake Fm. near Cape Lachmann, James Ross Island. The piece closely resembles the
correspondingregioninMegalosaurusand,moreclosely,inPiatnitzkysaurus.Thissuggests
that a persistently plesiomorphic tetanuran lineage inhabited the Antarctic. The relatively
small size of the animal argues against a mean annual temperature below 15°C in its
environment. Antarctica, theropod, LateCretaceous, biogeography, tetanuran.
R.E. Molnar, Queensland Museum, P.O. Box 3300, South Brisbane, Queensland, 4I0J
f
Australia;AlejandroLdpezAngriman, BRIDAS,Av. L. N, Alem 1180, (1001)BuenosAires,
Argentina; Zulma Gasparini, Depto. Paleontologia de Vertebrados, Museo de La Plata,
(1900)LaPlata, Argentina;5July 1996.
During the summerof 1988 one ofthe authors the corresponding part of the tibia in
(A.L.A.) discovered a small bone in the Megalosaurus bucklandi, Piatnitzkysaurus
Cretaceous deposits 4km north ofCol Crame, in floresiandPoekilopleuronbucklandii. Seenfrom
the Cape Lachman region, northwestern James the front, the shaft is expanded laterally to form
RossIsland(Fig. 1).Inabasininthenorthwestern a broadly rounded post-fibular flange, and
sectoroftheWeddell Sea, theoriginoftheisland medially into an angularly truncate medial mal-
is related to the opening of that sea during the leolus. A broad, flat-surfaced prominence
breakup ofGondwanaland (Medina et al., 1992). proximomedially bounds the ascending process
The specimenderivesfrom the middlesectionof and dorsal face of the astragalus, extending
the Hidden Lake Fm., Gustav Gr. (Ineson et al., proximolaterally acrosstheanteriorfacefromthe
(1B9u8a6t)o,isre&feLrorepdeztoAntghreimCaonn,ia1c9i9a2an)-.SaInttoisnitahne manegduilaaltimoanlsl.eoTlhuiss.iIststeedrgmeesdatrheemmaerdkieadl bbuyttsrheasrs.p
firstCretaceousAntarctictheropodandtheoldest The posteriorface is almost flat but with a slight
Cretaceoustetrapodfrom Antarcica. longitudinal concavity laterally. Above the post-
Prior to Angnman's discovery, ornithischians fibular flange the surface of this concavity
were the only Cretaceous dinosaurs known from abruptly slopes anteriorly to meet the anterior
Antarctica (Gasparini et al., 1987; Olivero et al., surface ofthe shaft ata sharp edge.
t1h9e9r1o;pHodosokferrometNael.w, 1Z9e9a1l)a.nTdh(eMoLlantearC&retWaicfefoeuns, The broken end reveals a central cavity sur-
roundedby relativelythin-walledbone(Fig.2D).
1994) imply that theropods also lived in An-
The shaft is tear-shaped in section at the break.
tdairscctoivcaeraytvmeraitfiteismtehe(icrf.ocMcoulrnraern,ce1a98n9d)hebluptstfhiilsl Fcarvaigtmyenstusggoefstbotnheatintthheecsahlacfittewfaisll ocfrtuhsehecdentjruaslt
outourknowledgeofAntarctic faunalevolution.
A theropod is known from the Early Jurassic of abovethebrokenendbeforeorshortlyafterburial.
Antarctica (Hammer& Hickerson, 1994). The distal end is slightly concave from the front
Collection designations, ANSP, Academy of and triangular when viewed distally, with the
Natural Sciences, Philadelphia; MLP, Depar- anteriorsurfaceforming thelongestedgeandthe
tamentodePaleontologiaVertebrados,Museode inclined lateral edge the shortest. The distal sur-
LaPlata, La Plata. face is shallowly concave in the middle, becom-
ing mildlyconvexbothmedially andlaterally.
DESCRIPTION The form of the anterior surface of the distal
tibia (Fig. 2C) indicates that the ascending
Th—e specimen (MLP: 89-XII-l-l) is the distal process of the astragalus was moderately low,
end —representingprobably 10-15% ofthetotal relatively narrow and restricted to the lateral half
length of a left tibia. The piece is complete ofthetibialshaft.Thustheastragaluswouldhave
save forvery small portions oftheextremities of been unlike those ofornithomimosaurs or tyran-
themedial malleolusandtheflangethatbacksthe nosaurs in form, but probably much like that of
fibula. In general form it is (Fig. 2) is similar to Poekilopleuron bucklandii.
670 MEMOIRS OFTHEQUEENSLANDMUSEUM
o C. LACHMAN 20 km Coelophysis and Dilophosaarus but present and
angularinSyntarmy(Raath, 1969; Welles, 1984;
Colbert, 1989). Early ceratosaurs, e.g.,
Coelophysis, (and herrerasaurs) lack an ascend-
ing process and the astragalus interlocks with an
offset distal surface ofthe tibia, the offset being
?MARAMBIO visible anteriorly (Padian, 1986, fig. 5.5).
I Theneoceratosaurdistal tibia isknown only in
Ceratosaurus and Xenotarsosaurus, where the
distal expansion is approximately symmetrical
and the fossa for the ascending process low. In
57'W Ceratosaurus the medial buttress is broad with
the 'step' extending nearly horizontally at least
2/3 ofthe way across the shaft (about halfway in
FIG. 1. MapofJamesRossIsland, showingwherethe the HiddenLake specimen) and the medial mal-
theropodtibiawasfound(dot). C=Cape,I=Island. leolusdoesnotextendasfarproximallyalongthe
shaft, so is more pointed in anterior view (Gil-
IDENTIFICATION. The general form and rela- more, 1920); the distaltibiaofXenotarsosaurus is
tively thin-walled hollow shaft indicate that this similar but with a broadly rounded malleolus
tibia derives from a theropod. Dryosaurs have (Martinez et al., 1987). Dromaeosaurs (i.e.,
also been reported with thin-walled limbelements Deinonychus antirrhopus) have a broad, hig—h,
(Chinsamy, 1995)anddryosaursorsimilarforms ascending process, lack the medial buttress
are known from Late Cretaceous New Zealand
& probably correlate—d with the broadening of the
(Wiffen Molnar, 1989), hence probably in- ascendingprocess andhaveabroadlyrounded
habitedAntarcticaatthistime. Howevernoother medial malleolus (Ostrom, 1969). Arctometatar-
dinosaurs have the astragalar ascending process salian theropods also have a broad, high ascend-
set into a depression of the anterior face of the ingprocessandlackthemedialbuttress,andhave
distal tibia, dorsomedially bounded by an abrupt a nearly symmetrical distal end (in anterior or
step or offset in the anterior face. This feature is posterior view) with no distinctive form to the
characteristic of neotheropods (Sereno et al., medial malleolus (Welles & Long, 1974). Other
T1h99e4)t,ruanncdateisnm'etdifaolunmdalilneohelrurseraalssaouroscc(uFrisg.in3)a. caoheilguhr,osbaruorasd(ea.gs.c,eCnadlianmgopsraoucreusss)asnidmilnaorlmyehdaivale
rpersotbraibctleyd rgerporuepseonftsthaersotpagoedsin(Ttahbeleev1o)l,ubtuitonthoisf buttress but the medial malleolus is similar in
distal tibial form (Fig. 3). silhouette, from in front, to that of the Hidden
Lake tibia (Lydekker, 1891). There is little data
The work on theropod astragali of Welles & available on oviraptorosaurs, butjudging from
Long, 1974, (with the correction of Carpenter, Chirostenotes and Microvenator, the form is
1992) accords well with recent phylogenetic similar to that in arctometatarsalians, with the
&
analyses of theropods (Holtz, 1994; Russell exception that the fibular flange seems truncate
Dong, 1993; Sereno et al., 1994). Ageneral ten- in the former genus (Ostrom, 1970; Currie &
dencytowardincreasedheightandbreadthofthe Russell, 1988).Thesegnosaurdistaltibiaremains
ascending process can be seen in the lineage unknown or undescribed. That of Yangchuano-
leading to arctometatarsalians (Fig. 3). Further- saurus shows a low ascending process, a medial
more, a survey of figured tibiae indicates that buttress different in form and an angular medial
distal tibial form is reasonably distinctive forthe malleolus (Dong et al., 1983). In Sinraptor the
recognised theropod groups (Table 1). ascending process is low and narrow, the medial
The distal tibia of ceratosaurs is variable be- buttress essentially similar to that of
cause ofthe evolution ofthe ankle in this group. Ceratosaurus, and with an angular medial mal-
Indistalviewthetibia is notanteroposteriorly com- leolus (Currie & Zhao, 1993). In distal view the
pressed in the Triassic and Early Jurassic forms, tibia is more anteroposteriorly compressed. Al-
as it is in later ceratosaurs and tetanurans. The losaurushasabroader,higherascendingprocess,
ascending process is absent or low and narrow, a narrower medial buttress, and a generally
the medial buttress is absent in Coelophysis and similarmedial malleolusthatis, however,more
Syntarsus but present in Dilophosaarus, and the medially projecting and more roundedinoutline
medial malleolus is quite subdued or absent in (Gilmore, 1920; Madsen, 1976).
ANANTARCTICCRETACEOUSTHEROPOD 671
D
FIG. 2. The theropod distal tibia (MLP: 89-XII-l-l) from the Hidden Lake Fm., James Ross Island, in: A,
posterior;B,medial;C,anterior;D,proximal;E,distal;andF, lateral views. mb=medialbuttress,mm=medial
malleolus, pf=flangebehind Fibula, scale= 1cm.
The distal tibiae of Megalosaurus bucklandi notoccurin PiatnitzkysaurusortheHidden Lake
(Huxley, 1870; Hulke, 1879) andPoekilopleur—on tibia.Thereareslightdifferences: theHiddenLake
bucklandii (Eudes-Desl—ongchamps, 1837) tibiaislesscompresseddistally, its fibularflange
possibly related—forms Erectopus sup—erbus projects less and its medial buttress is slightly
(Sauvage, 1882) ofuncertainaffinities —and broader than in P. floresi. Nonetheless, had the
Piatnitzkysaurusfloresi (Bonap—arte, 1986) a Hidden Lake tibia been found instead in the
plesiomorphic allosauroid are the most Canadon Asfalto Fm. (Callovian-Oxfordian) of
similar. However, the tibiae of torvosaurids Argentina, itwouldlikelyhavebeenattributedto
(specifically Afrovenator abakensis, Eustrepto- an immature Piatnitzkysaurus.
spondylus oxoniensis and Torvosaurus tanneri), ThesimilarityoftheHiddenLaketibiatothose
animals similar in other ways to megalosaurs, ofmegalosaurs and Piatnitzkysaurus, all Middle
differinhavinganangularmedialmalleolus(e.g., Jurassic forms, and Erectopus, an Albian form,
Britt, 1991). Of those with similar distal tibiae, impliesthattheHiddenLaketheropodrepresents
the most similar is that of P. floresi, which a lineage probably derived from relatively
matches that ofthe Hidden Lake theropod in t—he plesiomorphic megalosaur or primitive al-
forms of the medial malleolus and buttress losauroid (butnotsinraptorid) stock, unrelatedto
particularly the curve ofthe step adjacent—to the the lineages culminating in the more orless con-
facetfor the astragalarascending process and temporaneous arctometatarsalians. Interestingly,
the fibular flange. The distal tibiae of the pre- the only other known Antarctic theropod, the
viously noted megalosaurs, and of Erectopus, EarlyJurassic Cryolophosaurusellioti (Hammer
differ in that the fibular flange projects distally, & Hickerson, 1994), is also now thought to be a
formingthedistalmostpartofthetibia.Thisdoes plesiomorphic allosauroid (Hammer, pers.
672 MEMOIRS OFTHEQUEENSLANDMUSEUM
comm., 1996). Generic identifica-
tion ofthe Hidden Lake specimen
is not possible, unless more is Herrerasaurjda
recovered. A _ Ccelophysidae
DISCUSSION Dilophosaurus
The Hidden Lake tibia is the Elaphrosaurus J
same size as the distal end of the Xenotarsosaurus
tibia of the ornithomimosaur
Coelosaurus antiquus (ANSP THEROPODA Ceratosaurus
9222) which is about 40cm long Mbeudtitarless Torvosaurus 4
overall. Iftheproportionswerethe NEOTHEROPGDA
same, then the Hidden Lake Megalosaurus p.b.
theropod would have been about Anteroposteriorly
3-3.5m long. If, on the other hand compressea distal tibia Piatnitzkysaurus
it was more robust, with the tibia Stnraptor HLt
proportioned like those of Plat-
nitzkysaurusfloresi, then it would Truncate medial malleolus-- Allosaurus
havebeen about22-23cmlongand AVETHEttOPOC*
Acrocanthosaurus
the whole animal about 2.5-3m
long. Assuming that it is from a Deinonychus
mature animal, and we have no Ascending process broad & high
Symmetrical Oviraptoridee
reasonto thinkotherwise, in either development of malleolus & fibular fiange
case it represents a moderately (loss of truncate mediCaOlEmLaUlRleOoSluAsU)RIA / Tyrannosauridae,'
small theropod about the size ofa Loss of medial buttress?
large Coelophysis. This theropod Troodontidas l.\
wouldhavebeen too smalltohave
Ornithomimosauria
beenectothermicandtohavelived
underaclimatewithameanannual
temperature of less than 15°C (cf.
Molnar&Wiffen, 1994; Spotilaet FIG.3.Phylogeneticrelationshipsamongtheropods,withastragalarand
al., 1991).ThissuggeststhatJames distal tibia! characters discussed in text assigned to nodes. A and B
Ross Island enjoyed a rather mild indicate presumed reversals of character state: A, to uncompressed
climate, at least in some places, distaltibia;B,toangular(ratherthantruncate)medialmalleolus.Along
therightmarginarediagramsofthedistaltibiaeofthegeneraincluded,
during the Coniacian-Santonian. with that for Poekilopleuron bucklandii (P.b.) added next to that for
The specimen was found in the Megalosaurus, and that for the Hidden Lake theropod (HLt) next to
middle section ofthe Hidden Lake Piatnitzkysaurus. Cladogram afterHoltz (1994), with the positions of
Fm., in the level ofthe calcareous Sinraptor, following Sereno, et al. (1994), and Piatnitzkysaurus, fol-
sand with abundant carbonised lowingMolnar,etal.(1990),added.(Tibialdiagramsredrawnfromdie
material. Associated trace fossils literature,exceptfortheHidden LaketheropodandPiatnitzkysaurus).
include Planolites sp. and
Palaeophycus sp., and remains of logs without suggests that lineages deriving from Middle
the borings ofTeredolites that are often found in Jurassic forms persisted in the Antarctic.
the upper and lower levels of this formation
(Buatois & Lopez Angnman, 1992b). The en- ACKNOWLEDGEMENTS
vironment of deposition was a developing dis-
The specimen was collected with the kind
tributary plain in the central fan of a fan delta
depositionalsystem(Buatois&LopezAngnman, logisticalassistanceoftheInstitutoAntarticoAr-
gentino, and the participation of Drs Christian
1992a) into which, we infer, the specimen had Chemiglasov, Roberto Llorente and especially
been transported afterdeath. Luis Buatois. We alsoappreciate the helpofFer-
This tibia suggests a more primitive theropod, nando Novas (Museo Nacional de Ciencias
or at least one with less advanced distal tibial Naturales,BuenosAires),JaimePowell(Instituto
structure, than was common in the Late MiguelLillo,Tucuman),AlecRitchie(Australian
CretaceousAsiamericaorSouthAmerica.Thusit Museum, Sydney) andH. Phillip Powell (Oxford
.
ANANTARCTICCRETACEOUSTHEROPOD 673
TABLE1.Theropoddistaltibialform.Inthelastcolumn,'lateral'indicatesthatthelateralflangeismorestrongly
developed than themedial malleolus; 'symmetric' that flange andmalleolusareaboutequally developedand
symmetrical inanteriorview; 'both* thatflangeandmalleolus areequallydevelopedbutnotsymmetrical
Medialmalleolar Astragalarascendingprocess Distaltibial
Taxa Medialbuttress
development width height development
Herrerasauridae slight absent narrow low slight
Coelophysidae slight absent narrow lows light
Dilophosaurus slight present narrow low lateral
Elaphrosaurus angular ? ?' 7 symmetric
Xenotarsosaurus rounded ? moderate low symmetric
Ceratosaurus angular present moderate low both
Ton'osaurus angular present moderate low lateral
Megalosaurus truncate present moderate low lateral
Piatnitzkysaurus truncate present ? ? lateral
Sinraptor angular present moderate low lateral
AHosaurus truncate present moderate low lateral
Acrocanthosaurus truncate present ? <) both
Deinonychus rounded absent broad high symmetric
Oviraptoridae slight loworabsent broad •} both
Avimimus slight absent broad high symmetric
Tyrannosauridae rounded absent broad high symmetric
Troodontidae slight absent broad high symmetric
Ornithomimidae slight absent broad high symmetric
Welles&Long(1974)describetheastragalarascendingprocessofElaphrosaurusbambergiashighandbroad,citing
Janensch(1925).HoweveritisnotclearfromJanenschthattheascendingprocessisactuallypreservedsothismaybean
inference.Hencetheproportionsarehereconsideredtobeunknown.
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