Early Cretaceous polar tetrapods from the Great Southern Rift Valley, southeastern Australia

5 Pages·1996·1.7 MB·

by  Patricia Vickers-Rich

EARLYCRETACEOUS POLARTETRAPODS FROM THE GREATSOUTHERN RIFT
VALLEY, SOUHTEASTERN AUSTRALIA
PATRICIAVICKERS-RICH
Vickers-Rich,P. 1996 1220: EarlyCretaceouspolartetrapodsfromtheGreatSouthernRift
Valley, southeastern Australia. Memoirs ofthe QueenslandMuseum 39(3): 719-723. Bris-
bane. ISSN0079-8835.
Early Cretaceous deposits from southeastern Australia record a cold, extensively forested
environment. Tetrapod fossils in channel fills, gravity flows, lag and point bar deposits in
the Aptian Wonthaggi and Albian Middle Eumeralla formations. The fossils occurmainly
in horizontally-stratified, clast-supported conglomerates and massive, matrix-supported
conglomerates.Leafmatsindicatethatseveralspeciesofdeciduousplantsshedtheirleaves
together,presumablyinwinter.—TaphonomyofthelakebedsatKoonwarraindicateseasonal
freezing.Avarietyofdinosaurs• includinghypsilophodonts,anky—losaurs,neoceratopsians,
allosaurs,dromaeosaurs, oviraptorosaurs andornithomimosaurs were presentaswell as
pterosaurs,plesiosaurs,temnospondylsandcrocodilians.Thelattertwogroupsdidnotoccur
togetherand the temnospondyls lived undereithercoolerorhigher-energy conditionsthan
the crocodilians. [ | Cretaceous, Australia, dinosaur, environment of deposition,
palaeoclimate, temnospondyl.
P. Vickers-Rich, DeptofEarth Sciences, Monash University, Clayton, Victoria, Australia,
3168; 5April1996.
BreakupofGondwanabegan in theLateTrias- clothedmanypartsoftheriftvalleyanditsflanks
sic. Australia and Antarctica were the last two at times.
continentsofthesupercontinenttoseparate,com-
mencing in the Late Jurassic. During the Early ENVIRONMENTAL SETTING OFTHE
Cretaceous, between 125 and 105 million years TETRAPOD FAUNA
ago, what is now the southern coast ofVictoria,
Australia, was part of a rift valley formed be- Asmall,butgrowing,collectionoffossiltetrapods
tween the two continents during the initial phase are known from several locales in southeastern
of that separation. As separation continued and Australia (Currie, Vickers-Rich & Rich, 19%;
sped up, the floor of the rift valley sank. As a Gross,&Rich& Vickers-Rich, 1993;&Molnar,Flan-
consequence of that event, volcanoes that nery Rich, 1981; 1985; Rich Rich, 1989;
&
probably lay near the Lord Howe Rise, poured Rich Vickers-Ric&h, 1994; Rich et al., 1988;
1992; Vickers-Rich Rich, 1993; Warren et al,
quantities of ash (estimated at 50,000 cubic
krielwoomrekterdes)byintthoetrhieverirfstavanldlesytrwehaemrs,etithewapsretchuern- t1h9r9e1e).gTeholeosgeicfosusniiltss:ha1v,etbheeenyocuonlgleecrteMdidfdrloem
Eumeralla Formation ofEarly Albian age in the
sors to the green sandstones and mudstones that OtwayGroup; 2, theolderundifferentiated Won-
nowformprominentcliffsforabout200kmofthe thaggi Formation ofAptian age in the Strzelecki
Victorian coastline (Fig. 1). Group;and3,theAptianSanRemoMembernear
Since thatrift valley was formed, Australiahas the base ofthe Wonthaggi Formation.
driftedfartothenorth,whileAntarcticaremained Fossils are concentrated in a few facies within
close to its Early Cretaceous position straddling these units: i.e., 1, mainly those preserved as a
the South Pole. In the Early Cretaceous consequenceofrapiddepositioninanewchannel
southeastern Australia lay well within the An- cut as a consequence ofa single major flood; 2,
tarctic Circle. The dinosaurs, other fauna and as gravity flows ofsediment, either mud or rock
plants that lived in this region thus contended debris; and to a lesserextent, 3, lag andpointbar
with prolonged periods of continuous darkness deposits within majorriversystems (A. Constan-
each year,justas muskox andreindeerdotoday. tine, pers. comm.).
However,thegeochemicalandbotanicalclimatic The bones are most commonly preserved in
indicators suggest that the environment in the horizontally stratified, clast-supported con-
Early Cretaceous ofVictoriawasnotfrigidas are glomerates and massive, matrix-supported con-
similar latitudes today, and extensive forests glomerates. A 'clast-supported conglomerate'
720 MEMOIRS OFTHEQUEENSLANDMUSEUM
refers to one in which the individual pebbles or TheyoungerMiddleEumerallaFormationcon-
boulderscontact oneanother, whereas a 'matrix- sistsprimarilyofsandstone(morethan70%) and
supported conglomerate' is one in which the is characterised by a classic braided river
quantity ofFinegrained rockbetween thepebbles lithofacies andarchitecture. Sinuosityofchannel
orbouldersissogreatthattheydonotcontactone deposits is low, and the channels were wide and
another. If the matrix could be removed from a shallow. There are thick sequences of both
clast-supported conglomerate, the volume oc- floodplain and lacustrine sediments with lenses
cupied by the conglomerate would notdecrease, of sand-sized particles representing individual
whereas in the case of a matrix-supported con- channels stacked one on top of another. Ver-
glomerate, it would. tebrates occur in only a few environmental set-
The horizontally stratified, clast-supported tings, somewhatin cont—rast to the situation in the
conglomerates are typically of the order of 5- Wonthaggi Formation in the Middle Eumeral-
20cm thick and are characterised by pebble to la bones occur predominantly in sediments
cobble-sizedclastsformedofclayandmudstone. formedwhenachannelbrokeitsbankandflowed
Suchdeposits are stratified andcontainlittle car- out over the surrounding floodplain, or in those
bonaceous material. This type of bone-bearing formedbyloading andcollapseofnon-vegetated
conglomerate typically occurs at the base of sandbarsinthemainchannelduringapeakflood
channelcomplexeswheresignificanterosioninto stage that caused liquefaction of sand and then
underlying floodplain claystones and mudstones down-slope mass flow.
isevident.Suchdepositsareinterpretedashaving
formed by streams orrivers breaking their banks THE BIOTA: PALAEOFLORAAND
during major floods, with the resultant flood INVERTEBRATEFAUNAAS
waters flowing out over the surrounding flood- PALAEOCLIMATIC INDICATORS
plain, picking up bones and plant debris and
concentrating them in erosion scours. The palaeoflora of southeastern Australia
Themassive, matrix-supportedconglomerates, during the Early Cretaceous was dominated by
on the other hand, differ in that the pebble to conifers, ferns, cycads, ginkgoes and lower-
boulder-sized clasts are not touching each other. growinghorsetailsandbryophytes. Angiosperms
Instead, they appear to be suspended in a finer- were present, but only as prostrate or small her-
grained matrix composed of fine to medium- baceousforms.Thestructureanddiversityofthis
grained sandstone. Mudstone is again the flora suggest a mean annual temperature of 8-
dominant clast type, and plant remains are not 0°C(Douglas, 1969; 97 Drinnan&Chambers,
1 1 1;
as common. Conglomerates of this type are 1986; Parrish et al., 1991). Some plants in this
interpreted as debris flows, which form as the flora were evergreens, while others were clearly
resultofbank collapses within the confines of deciduous,thebestevidencebeing fossilised leaf
channels. mats suggestive of simultaneous shedding of
Theregionalsettingforthetwogeologicalunits leaves by several species. The evergreen plants
that have produced significant fossil collections possessed leaves with thick cuticle and micro-
shows distinct change through time, from a phyllus (small) leaves. Such leaf morphology is
predominantly 'meandering* to a 'braided' sys- consistent with a climate characterised by sig-
temupsequence.TheolderWonthaggiFormation nificant variation in temperature t—hroughout the
is characterised by thick floodplain deposits and year or a fluctuating water supply conditions
channels with moderate to high sinuosity. There that would beexpected within a continental land
are thick accumulations ofhorizontally stratified massdistantfromtheocean. Leafmats themselves
tolow angle crossbedded sediments, which indi- are indicative o—f leaves having fallen in a short
cate rapid aggr—adation under transitional to high period of time which can be brought on by
flow regimes suggestive of occasional flash pronounced seasonal changes in light, temperature
flooding. The formation consists of about 60% or water availability or a combination of these
sandstone and 40% mudstone and reflects vary- factors. Pronounced seasonal contrasts are ex-
ing flow regimes,perhapsduetodischarge levels pected of such inland environments at such
controlled by snow melt. Vertebrate fossils have latitudes as was the case in southern Victoria in
been recovered from a variety of lithofacies the Early Cretaceous (Parrish et al., 1991).
withintheformation,butmainlyfromhorizontal- One unusual locality to have produced fossil
ly stratified, clast-supported conglomerates and plants, invertebrates, fishes and birds, is Koon-
massive, matrix-supported conglomerates. warra. Koonwarra is an inland site in the Won-
CRETACEOUS POLARTETRAPODS FROMSOUTHEASTERN AUSTRALIA 721
ANTARCTICA
FIG. 1. Sketchofjunctionbetween SEAustraliaandEAntarcticaintheEarly Cretaceous. Separationbegan in
theLateJurassicandresultedinariftvalleybeingformedontheinterplateboundary.Intothisriftvalleypoured
a vastquantity ofvolanogenic sedimentsderived fromvolcanoes perhaps laying to the eastin the vicinity of
theLordHoweRise.Largeriversflowedacrossthefloorofthisriftvalley,fedperhapsinpartbymeltwaterof
snowpresumablylocatedathighaltitudeonmountainsonthemarginsoftheriftvalleyorthevolcanoeswhich
producedthevolcanogenic sediments.TheEarlyCretaceoustetrapodandplant fossilsoccurforthemostpart
in sediments thatwerelaiddown in small streams feeding into the larger rivers on the flooroftheriftvalley.
SubsequenttotheEarlyCretaceous,thesedimentsdepositedontheflooroftheriftvalleywerefirstlithifiedas
they wereburiedunderadditional sediment.Thenlatein theCainozoic, these sedimentswereupliftedtoform
theStrzlecki andOtwayRanges.
thaggi Formation of the Strzelecki Group and Further evidence that temperatures were cool
represents the remains of an ancient lake. Most andthatice may have formed attimesduringthe
othervertebrate sites in southeastern Victoria are year occurs in contemporaneous sequences in
coastalexposures, and fossilsweredepositednot central Australia. Boulders up to 3m in diameter
in thequiet waters oflargelakes, butinthe more havebeenfound in otherwise finegrainedmarine
energetic riverine and floodplain environments. sedimentsoftheBulldogShalenearAndamooka,
&
Insects and other invertebrates, primarily larval SouthAustralia(Frakes Francis, 1988). Frakes
forms,recoveredfromtheKoonwarralocalityare & Francis have suggested that these boulders
mostclosely relatedtoforms typical ofcool Tas- dropped to the bottom of the shallow sea as
manian mountain lakes today (Jell & Duncan, icebergs in which they floated, melted—away. Al-
1986), relationships that are indicative of though—there is no preserved evidence suchas
temperatures similar to those reflected by the tillites for glacial activity in Australia at this
palaeoflora. Waldman (1971), who studied the time, unlike for the earlier Permian time, Frakes
Koonwarrafishconcentration, suggestedthatthe hassuggestedthatmontaneglaciationcouldhave
entire foss—il accumulation may have been due to been active and, in places, these glaciers might
winterkill whenthelakefroze overandoxygen have reached the sea at the base of drainage
supply was greatly reduced. systems.
722 MEMOIRS OFTHEQUEENSLAND MUSEUM
In summary, the suite of animals, plants and ing to think that—Leaellynasaura may have been
sedimentological datasuggestthattheclimateof homeothermic thus allowing the increased
southeastern Australia during the late Early visual acuity to have an adaptive advantage.
Cretaceous when dinosaurs are known to have Other ornithischians in the fauna include an
livedthere, was somewhatcoolerthanatpresent, ankylosaur (based on the cross-sectional outline
but temperatures were by no means frigid as of a rib, a scute and a few teeth) and an Aptian
similar high polar latitudes are today. O /O neoceratopsian (based on an ulna with a remark-
evidence, however, suggests that mean annual able resemblance to that of Leptoceratops
temperatures approached 0°C at times during gracilis from the latest Cretaceous ofAlberta).
deposition ofthe dinosaur-bearing sediments. Allosaurids,oviraptorosaursandornithomimid
theropods are known. An astragalus resembling
VERTEBRATEASSEMBLAGES OFTHE that of Allosaurus has been recognised in the
EARLYCRETACEOUS OFPOLAR Aptian Wonthaggi Formation of the Strzelecki
AUSTRALIA Group (Molnar, Flannery & Rich, 1981; 1985).
&
Oviraptorosaurs (Currie, Vickers-Rich Rich,
&
The Early Cretaceous terrestrial vertebrate as- 1996) and ornithomimids (Rich Vickers-Rich,
semblages of southeastern Australia are 1993)arebothknown fromtheMiddleEumeralla
dominated by dinosaurs, in particular hyp- Formation in the Otway Group, and ornitho-
silophodontids. Thereareatleastfivegeneraand mimids have also been recovered from the older
six species of this family known from Victoria, Strzelecki Group (Wonthaggi Formation).
half as yet unnamed. The diversity of hyp- Footprintsofsmall theropodshavebeenrecorded
silophodontids in these south polar latitudes is in the Otway Group.
unmatchedanywhereelseinthe world,including The neoceratopsian, dromaeosaur and ovirap-
localiteswithhundredsofthousandsofbonesand torosaur fossils from Australia are among the
high diversity in the total dinosaur assemblage. oldest records of these groups anywhere in the
Currie (pers. comm.) has suggested that atlower world.Othercomponentsofthefaunaincludethe
palaeolatitudes, hypsilophodontids may have youngtemnospondyl amphibians, whicharerep-
been uplandforms andthus notfrequentlyrepre- resentedby more than twenty bones, including a
sented at the lower elevations where most pairofmandibleswiththeteethinsitu.Twobones
dinosaur fossils accumulated. Hypsilophodon- of pterosaurs and half a dozen plesiosaur teeth
tids, then,may wellhavebeen preadapted forthe have also been recovered. Evidently the
conditions of southeastern Australia and conse- plesiosaurs were freshwater animals, as all
quently thrived there. Hyspilophodonts are sedimentologicalandpalaeontologicaldatapoint
known from both the Otway and Strzelecki toafluviatilesourceforthecontainingsediments.
Groups and thus have an age range in these se- A few remains of crocodiles have been
quences from Aptian to Albian, with some sites recovered in the Middle Eumeralla Formation in
perhaps being as old as Valanginian in the theOtwayGroup,butneverhavetheybeenfound
Strzelecki Group. together with temnospondyl remains, restricted
Onthebasisoffemoral morphology,fivegenra astheyaretotheolderStrzeleckiGroup.Awarm-
of hypsilophodontids have been recognised. To ing trend up section in the Aptian-Albian se-
date, formal scientific nameshavebeen assigned —quence may explain this apparent faunal change
to only three of these: Fulgurotherium, Leael- the replacement of temnospondyls by
lynasaura and Atlascopcosaurus. Prominent crocodiles.
optic lobes preserved on an endocast of Leael- The Victorian Aptian record oftemnospondyls
lynasaurasuggestthatthisdinosaurhadunusual- is the most recent for the group anywhere in the
ly enhanced ability toprocess visual signals. In a world. These amp—hibians were very crocodile-
polar setting, the most plausible explanation for like functionally in body form and in tooth
thisacuitywouldbethatitimprovedvisualability morphology. Temnospondyls occur in the older
underthelowlightconditions,whichwouldhave Aptiansedimentswheretemperatureswerelower
prevailed during the months ofcontinuous dark- than in theyounger Albian sediments thatbear a
nessofthepolarWinter.Sincebothpalaeobotani- few dermal scutes of crocodiles. Perhaps the
cal and geochemical studies suggest that Winter rising temperatures allowed the invasion of
temperatures wouldhaveprobably dippedbelow crocodilians into an area from which they had
freezing, and since this is a prohibitive tempera- been excluded by the cold waters, thus bringing
ture for activity ofmodern reptiles, it is tantalis- themintodirectcompetitionwiththetemnospon-
CRETACEOUSPOLARTETRAPODS FROM SOUTHEASTERN AUSTRALIA 723
dyls, followed by extinction ofthe latter. Today 1972. The Mesozoic floras of Victoria, Part 3.
amphibians, suchasfrogsandtheGiantJapanese MemoirsoftheGeologicalSurveyofVictoria29.
Salamanders are able to cope with temperatures DRINNAN,A.N.&CHAMBERS,T.C., 1986.Floraof
well below those toleratedby living reptiles. the lower Koonwarra fossil bed (Kommburra
Group),SouthGippsland,Victoria.Associationof
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however, forthere is one otherpossibleexplana- FRAKES, L.A. & FRANCIS, J.E. 1988. A guide to
tion thatcannotbe ruledout. Temnospondyl fos- Phanerozoic cold polar climates from high-
sils areknown from the Strzelecki Grouponlyin latitudeice-raftingintheCretaceous.Nature333:
the high energy sediments that represent 547-549.
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Australia occur in these Early Cretaceous as- MASON,J.,DOUTHITT,R.T.,GREGORY,R.T.
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&
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