(1974)recentlyreporteda number of ammonites and other invertebratesfrom the Lewis
Shalealong the easternedgeof the San Juan
Basin. UNM-V-070 is southeastof their locality D4l5l and northeastof their locality
D5067. Both D4l5l and D5087 are stratigraphically higher in the Lewis Shale than
Uf.ftU-V-OZOand are placed by Cobban and
University'
Yale
History,
others (1974) in the Late Campanian Didyby
' NSpencer
e w H aGvLucas,
e n , CDepartment
T , a n d P eoft Geology
e r K . Rand
e s Geophysics
e r , O i i a rand
t m ePeabody
n t o t AMuseum
n t h r o of
p oNatural
l o g y , U n i v e r s i t y 0 f N e w M e x i c o , A l b u q u e r q u eammonite
,NM
zone. Probmocerascheyennense
ably UNM-V-070 is Late Campanianin age
(no older strata are known in the Lewis Shale)
the
(Cobban and others, 1974)and older than the
in
used
are
abbreviations
following
The
giant
group
of
Mosasaursare an extinct
a
marinelizardsthat flourishedduring the Late text: AMNH-Department of VertebratePa- D. cheyennense zone. Unfortunately,
outShale
Lewis
Natural
limited
the
of
of
Museum
search
American
leontology,
diligent
Cretaceous. Their fossilized remains are
yielded only unknown from all the continentsexceptAntarc- History, New York; UNM-Department of crops around UNM-V-070
shells;
inoceramid
Albuof
New
Mexico,
fragments
of
University
diagnostic
Geology,
tica; the largest and best known collections
preciselydetercome from the Niobrara Formation in Kan- querque;YPM-Peabody Museumof Natural hence,its age cannot be more
mined.
sas. Although marine sediments of Late History, YaleUniversity,New Haven.
cretaceousage are exposedthroughoutlarge
LewisShaleandits fauna
areas of New Mexico, only three mosasaur
havepreviouslybeenreportedfrom
specimens
The Lewis Shalewas named by Cross and
the state. These specimenswere found in others (1899) for exposuresaround Fort
south-centralNew Mexico in the Jornadadel Lewis,an old army post in the La Plata River
Muerto region (Cope, l87la) and in north- valleyin southwestColorado.Primarily comshalewith
easternNewMexico in the Raton Mesaregion posedof dark-grayto greenish-gray
(Lee, l9l7'). Late Cretaceousmarine deposits somesandylayers,limestone,and calcareous
in other partsof New Mexico,includingexten- nodules, the Lewis Shale has been mapped
sive exposuresof the Lewis Shale in and throughouta large areain northwesternNew
Colorado(Reeside'
aroundthe SanJuanBasin,until recentlyhave Mexico and southwestern
failed to produce mosasaur remains. The 1924; Dane, 1936; O'Sullivan and others,
discoveryof a mosasaurin the LewisShaleas 1972).The LewisShalewasdepositedoffshore
reported herein is the fourth report of a in the regressingseawaythat covered this
REMAINS
(Fassettand FIGURE2-VIEw oFExcAvATEDMoSASAUR
mosasaurfrom New Mexico (fig. l) and the regionduringthe Late Cretaceous
postepointing
at
the
is
man
the
UNM-V-070;
m
Lewis
from
the
first report of a mosasaur
Hinds,l97l; Mannhard,1976).
portionof thevertebral
riorendof thearticulated
Shale.This discoveryextendsthe geographic
The faunaof the LewisShaleis mainly a dicolumn.
rangeof the mosasaursinto that part of the verseassemblage
of marine bivalves,gastrosea that coverednorthwesternNew Mexico pods,and cephalopods(Reeside,1924;Dane,
duringthe LateCretaceous.
1936:Cobban and others, 1974;Mannhard'
DlscusstoN-The taxonomyof mosasaurs,
1976).Worm burrows and other trace fossils
like
many groupsof reptiles,is basedheavily
t07'oo'
present.
are
(for example,Ophiomorpha)also
characters(Russell, 1967). The
on
cranial
previouslyreportedfrom
The only vertebrates
cranialremainsof UNM-LK-l
any
of
absence
(for
example,
the LewisShaleareshark'steeth
R i oA r r i b oC o u n t y
jaw and tooth fragmentsthus renthan
other
Lamna sp.) and bony fish scales(Mannhard,
I
ders difficult a preciseidentification of the
t976).
iI
specimen.Fortunately,subfamiliesand tribes
can be diagnosedlargelyon the
o] mosasaurs
i
paleontologY
Systematic
basisof vertebralcharacters(Russell,1967).
ClassREPTILIA
In addition, the tooth and jaw fragmentsof
OrderSAURIA
of
aid in a tentativeassessment
UNM-LK-l
1853
Gervais,
FamilyMOSASAURIDAE
genericidentitY.
its
(Dollo,
I
884)
PLIOPLATECARPINAE
Subfamily
justify assignThe followingconsiderations
Williston.1897
Plioplatecarpinae
the
to
UNM-LK-l
ment of
(Dollo'1884)
TribePLIOPLATECARPINI
as definedby Russell(1967):l) UNM-LK-I
1967
Russell,
CoPe,1869
was a mosasaurof moderatesize, probably
PLATECARPUS
Genus
sP.
cf.PlatecarPus
about 6 m in overall length, basedon comdorsal
RBpeR.R.En
spnclneN-UNM-LK-l; a par- parisonof the lengthof its incomplete
of
lengths
body
and
vertebral
with
series
(14
vertebrae
46
tial skeletonconsistingof
(1967'
given
Russell
by
mosasaurs
various
dorsals, (?)5 pygals, and 21 caudals) of
lVososour
quorry+o
variable preservation,two nearly complete tables 2-3). All plioplatecarpines are
(Russell, 1967);
thoracic ribs and fragmentsof others, and moderate-sizedmosasaurs
within
the
sizerangeof all
well
is
UNM-LK-l
jaw
fragments.
tooth
and
assortedlower(?)
YPM collecin
the
specimens
plioplatecarpine
Loc^c.l-trv-UNM-V-070; in the NE'/+
not be
should
emphasis
undue
Although
iions.
the
Jicarilla
I
E.
on
T.
N.,
R.
sec.
9,
30
SW%
in
repcharacter
a
diagnostic
as
placed
size
on
Apache Indian Reservation southeast of
tiles (animalswith indeterminategrowth), no
Dulce,New Mexico(figs. l-2).
Cot-t-scrons-Originally discovered by evidenceexists to suggestthat UNM-LK-I
Adolph Julian and Darrell Paiz, membersof was a particularlyyoung individual; that it is
of one of the giant
the JicarillaApacheIndian Tribe; collectedby just a small representative
or Tylosaurus
Mosasaurus
like
mosasaurs
pART
the authors.
oF NoR.THwEsrEnNNrw
oF
FICURE l-Mlp
vertebraeof
caudal
All
2)
unlikely.
seems
HoRIZoN nNo ece-In dark-gray fissile
Mrxtco sHowINc rHE LocATIoNor UNM-V-070'
]:F
and
others
THEMOSASAUR
Lewis
Shale.
Cobban
QUARRY.
shaleof the
fromtheLewisShale
A mosasaur
northwestern
(Upper
Gretaceous),
NewMexico
New Mexico GeologY
August l98l
31
UNM-LK-l haveunfusedhaemalarches(fig.
3, C-E). The haemalpedunclesare subcircular, centrally pitted, posteroventrallydirected,
and locatednear the posterior end of the centrum lateralto the ventralmidline.This condition is seenin the plioplatecarpinesbut not in
the mosasaurinesin which the haemal peduncles typically are fused to the caudal centra
(Russell, 1967).3) Other vertebraeof UNMLK-l correspondwell to thoseof plioplatecarpines,althoughby themselves
theyarenot sufficient to diagnosethe group. A typical median dorsal vertebra (fig. 3, J-L) has the
synapophysislocated on the anterodorsal
aspectof the centrum, an anterior articular
cup that is wider than tall, and a posterior articular ball that likewiseis transverselyoval.
This vertebracloselyresembles
mediandorsal
vertebraof Platecarpus(Cope, 1875, pl. 20,
figs. lA-C, pl. 21, figs. lA-B, 2; Williston,
1898,pl. 42, figs.3-4). An anteriordorsal
vertebraof UNM-LK-l (fig. 3, G-I) is similar, although the centrum is slightly longer
and the articular surfacesa little more circular, being less flattenedtransversely.This
vertebrapreservesthe posterior zygopophyses
which are small and locatedon the posterior
edgeof the baseof the neural arch. The vertebra of this specimenalso closelyresembles
anterior dorsal vertebrae of Platecarpus
(Cope, 1875,pl. 20, figs. 5M-N). Most of the
dorsal vertebraeof UNM-LK-I are not as
well preserved
as this one;typically,the neural
arch and spineis no longer present.No evidenceof a zygosphene-zygantrum
can be discernedon any of the vertebrae,but almostall
of the vertebraeare so badly damagedthat
ascertainingwhetheror not at least a small
zygosphene-zygantrum
articulation was present at somepoint on the dorsal vertebralcolumn is impossible.4) As Russell(1967, p. 54)
has pointedout, the marginal teethof mosasaurs are "often generically diagnostic."
Among the tooth andjaw fragmentsof UNMLK-l is a nearlycompletetooth (fig. 3, A-B)
that is long, slender,and hasa pointedtip that
is posteromedially
recurved.This tooth has a
subcircularcrosssectionnear the baseof the
crown and bears distinct anterior and posterior carinaethat run the vertical length of
the crown. The buccal aspectof this tooth
bears six vertical facets whereasthe lingual
-
FIGURE 3-SelncreoELEMENrsoF UNM-LK-I,
AeARTTALsKELEToNoFcFPL.arecenpussp.; buccal (A)
and lingual (B) views of a tower(?) tooth; ventral (C), left lateral (D), and posterior (E) views of an anterior
caudal vertebra; (F) anterior view ofa thoracic rib; anterior (G), left lateral (H), and posterior (I) views of
an anterior dorsal vertebra; ventral (J), left lateral (K), and posterior (L) views of a median dorsal
vertebra. Black bars are 3 cm long (one scale for A and B; one for c, D and E; one for F: and one for G.
H, I, J, K, and L).
A u g u s tl 9 8 l
New Mexico Geology
aspectbearsnumerousthin verticalstriations.
This typeof tooth is typicalof all plioplatecarpines(Russell,1967).
AssigningUNM-LK-l asa plioplatecarpine
to Ectenosaurus,Pl ioplatecarpus, or Platecarpas is difficult without good cranial material
preservingthe charactersupon which these
genera largely are differentiated. Nevertheless,some slight evidencefrom the postcrania argues against assignmentto either
Ectenosaurus or Plioplatecarpus. Thus,
Russell(1967,p. 158), in his diagnosisof
Ectenosaurus,noted that its pygal vertebrae
have relativelylong transverseprocesses.In
contrast,the pygal vertebraeof UNM-LK-I
have relatively short transverseprocesses;
comparisonof two pygalvertebraewith nearly
equal-sizedcentra of UNM-LK-I and YPM
4672, a specimenreferredto Ectenosaurusby
Russell(1967),revealsthat the UNM specimen's transverseprocesses
are approximately
30 percentshorterthan thoseof YPM 4672.
Assignmentof UNM-LK-l to Plioplatecarpus
seemsto be precludedby two features: l)
Dollo (1893) stated that Plioplatecarpus has
only l3 dorsal vertebrae,a claim reluctantly
acceptedby Russell(1967);UNM-LK-I has
l4 dorsalsin its incompletevertebralcolumn,;
and 2) accordingto Russell(1967,p. 159)the
synapophyses
of the anteriordorsalvertebrae
of Plioplatecarpusate locatedin the centerof
the lateral aspectsof the centra. The synapophysesof the anterior dorsals of UNMLK-l, as notedabove(fig. 3, G-I), arelocated
on the anterodorsaledgeof the lateralaspects
of the centra;this is the sameconditionseenin
Platecarpus (Russell, 1967). Assignment to
Platecarpusis further justified by the fact that
UNM-LK-I closely resemblesYPM 1256.
1258,1264,1269,1272,and 1286,specimens
referredto Platecarpusby Russell(1967).
Despite the above characters of UNMLK-l that seemto precludeits assignmentto
Ectenosaurusor Plioplatecarpus, we believe
that there is enough uncertainty about the
diagnosticutility of thesecharactersdue to
potential variability that we only tentatively
assign the specimento Platecarpus. Assignment to a speciesof Platecarpusclearly is impossiblebecausethe speciesof the genusare
distinguishedby cranial characters(Russell,
1967)not preserved
in UNM-LK-1.
Platecarpus(fig. 4) is a Campaniangenusof
mosasaurs
knownelsewhere
in the WesternInterior from the Pierre Shale in Wyoming,
South Dakota, and Kansas(Russell,1967).
Extension of the genus southward into
roughly time-equivalent,though probably
slightlyyounger,depositsis not surprisingin
view of the great mobility of mosasaursthat
gave them an extensivegeographicdistribution during the Late Cretaceous(Russell,
1967).
Taphonomy
Mosasaur remains commonly are found
disarticulatedto a degreesimilar to that of
UNM-LK-I (figs. 2, 5, 6); herewe offer some
observationsand speculationon how this
specimencameto be disarticulatedand then
buried.UNM-LK-I was found in thinly lami-
FICURE 4-RpcoNsrnucrED
1898).
sKELEToNor Ptarrc.nnpus rcrERrcus, AcruAL LENGTHABour 6 M (WtlltsroN,
nated dark-gray shale suggestiveof an offshoreenvironmentof relativelydeepand quiet
water. The anterior and medial dorsal vertebral column was found articulated, though
slightly twisted in the middle. The posterior
dorsal,pygal,and caudalvertebraeare nearly
all disarticulated;they were probably less
axtightly bound togetherby bony processes,
ial muscles,tendons,and ligamentsthan the
more anterior parts of the vertebralcolumn.
The degreeof disarticulationof UNM-LK-I
a fairly long periodof decomposition
suggests
before frnal burial in the sediment. This
decompositionmay have occurredin part as
the dead animal floated in the water column,
but most decompositionprobablyoccurredas
the animal lay on the bottom, sincethe disarticulatedbonesshowa preferentialnorthwestsoutheastalignment shared by the still articulated portion of the vertebral column.
Mixed amongthe disarticulatedvertebraeare
invertebrateshell fragments, shark's teeth,
and bony fish scalesthat couldrepresenteither
gut contentsof the mosasauror organicdebris
presentin the sedimentwherethe deadanimal
cameto rest,or both.
The orientation of most of the bones (a
notableexceptionis the rib in the lower left
hand cornerof fig. 5) suggests
the presence
of
a distinct, though most likely slight, bottom
currentprobablyflowing predominatelyto the
northwestbecausemost of the bonesare scatteredin this direction.The absence
of the skull
and cervicalvertebraeof UNM-LK-l may be
becausethe anterior end of the animal first
was exposedin an arroyo (fig. 5); these
elements,if they were fossilized,have long
since eroded away. The girdles and appendages, on the other hand, may have been
removedin a numberof ways.
A number of grooves and scratcheson
vertebraeand rib fragmentsof UNM-LK-I
may indicatescavenging,a plausiblemechanism by whichthe girdlesand appendages
may
havebeenremoved.However,the specimenis
so damagedby weatheringand the rootsof recent plant growth that we hesitateto support
sucha conclusionfirmly. The girdlesand appendagessimply may havefloated away once
they becamedetachedfrom the vertebralcolumn or may have fallen off elsewherewhen
the decomposinganimal was floating in the
watercolumn.
Most difficult to explain is how the jaw
fragments came to be buried beneath the
mixed-upposteriordorsal, pygal, and caudal
vertebrae.Either the iaw of the animal fell
first andthe restof the carcassfell on top of it,
or the jaw broke up and was carried northwestwardby bottom currentswhereit mixed
with the vertebraeand sank deeperinto a soft
bottom than did the vertebrae.The former explanationis simpler,but the variedaspectsof
the disarticulatedvertebraeexposedon the
quarry surface(fig. 6) suggesttumbling and
sinking at various anglesinto a soft bottom
and might support the later explanation.A
third possibilityis that the jaw fragmentsare
those of a different individual than the
vertebraeand ribs. We seeno clear way to
decide among these three possibilities.Although nothing about the size and morphology of the Lewis Shalemosasaurremainsto
suggestthat they belongedto more than one
individual, this chasteningpossibility must
alwaysbe entertainedwhendealingwith fossil
material in a state of disarticulationor incompleteness.
%%
Summaryof marinereptilesin NewMexico
Among the Reptilia three main groups
adaptedto life in the marine realm: ichthyoand mosasaurs.Somerepsaurs,plesiosaurs,
of other reptile groups, notably
resentatives
turtles and crocodiles, have also invaded
here.
marineniches,but arenot considered
Ichthyosaurswere a widespreadgroup of
fishlike marine reptilesduring the Mesozoic.
Although their remainshave been found in
Triassic,Jurassic,and Cretaceousdepositsof
westernNorth America(McGowan, 1978),to
our knowledgeno ichthyosaurshave beenreportedyet from New Mexico.
Plesiosaurremains,on the otherhand, have
beenreportedfrom two localitiesin New Mexico. Cope(1887)describedplesiosaurremains
from what he termedthe "Fox Hills bed of
the following taxNew Mexico." He assigned
onomic namesto theseremains:Piptomerus
megaloporus new genus and species,P.
microporus new species,P. hexogonus new
species, and Orophosaurus pauciporu,snew
Williston(1903,p. I l) later
genusand species.
listed thesetaxa and summarizedtheir supposeddiagnosticcharacters.Welles(1952),in
a revision of North American plesiosaurs,
consideredall of Cope'staxa to be basedon
materialand thusdeclaredthem
indeterminate
to be nomina vona(vain or void names).The
werecollocalityfrom whichtheseplesiosaurs
lectedwasnot givenby Copeand the Fox Hills
Formation is not presentin New Mexico, being currently restricted to strata in North
IF
verrebroe
o
Y
/e)J
ribrrosmenrs
fl
eog
v
o
/1
-t
l/4m
Mososour
quorry
mop
A
I
e roded
FIGURE 5-Mep oF rHE DrsrRrBUTroN
oF BoNEsoN rHE euARRy FAcEAT UNM-V-070. The jaw and tooth
fragments were found beneath the disarticulated vertebrae in the center of the drawing (fig. 6). Erosion of
an arroyo first exposed the southern end of the articulated vertebral column and may have removed the
skull and cervical vertebrae.
New Mexico Geology
A u g u s tl 9 8 l
pycAI-, AND cAUDAL vERrEFIGURE 6-Donsel,
BRAELYINGMIXEDUPAND DISARTICULATED
ONTHE
euARRy FACEAT UNM-V-070. These vertebrae
are some of those just north of the articulated portion of vertebral column in fie. 5.
Dakota, South Dakota, Montana, Wyoming,
and Colorado as far south as the Colorado
Springsarea(Waage,1968).Despitethis, two
cluesto the provenanceof Cope'splesiosaurs
are available:l) the specimenlabel with the
type of Orophosaurus pauciporus, AMNH
5692, statesthe locality as "Cretaceous.San
Juan Basin,N.M." and 2) the term Fox Hills
bed usedby Cope (1887)would havereferred
to the uppermostmarineCretaceous
belowthe
continentalcoal-bearing
strata of what was
then called the "Laramie Formation," even
outsidethe areaswheretheseformationsare
now consideredto be present(K. Waage,personal communication, 1980). The Laramie
Formation in the San Juan Basin in Cope's
day encompassed
rocks now referred to the
Fruitland and Kirtland Formations(Reeside,
1924).Cope'sFox Hills bed,whichproduced
the plesiosaurs,
must be what is now calledthe
PicturedCliffs Sandstoneor Lewis Shale,or
perhapsincluded parts of both. Cope's indefatigable collector David Baldwin, who
worked in the San Juan Basin during the
1880's,mostcertainlyfoundthesespecimens.
Cunningham(1966) publishedthe second
report of plesiosaurremainsfrom New Mexico by mentioningeight vertebraeof a "Cretaceousvertebrate"from the Colorado Shale
in the Big Burro Mountainsof southwestern
New Mexico. Subsequently,
J. T. Gregoryof
the Universityof California (Berkeley)identified theseas belongingto an elasmosaurid
plesiosaur(J. Cunningham,personalcommunication,1980).
The first previous report of a mosasaur
from New Mexico mentionedat the beginning
of this paperwasCope's(l87la, p. 572)brief
descriptionofa seriesofvertebraefrom "near
Fort McRae. New Mexico" to which he attached the name Liodon dyspelor n. sp.
Cope (l87lb, c) later describedthis specimen
in greaterdetail, Iisting measurements
of the
vertebraeas well. Specimens
from the Kansas
Cretaceous referred to Z. dyspelor were
describedand comparedto the New Mexico
specimenby Cope (1872), but it was Leidy
(1873,pls.35-36)who first illustrated
theNew
Mexicospecimen.
Leidy(1873,p.271-274)referredZ. dyspelorto Tylosaurus;accordingto
Russell(1967,p. 184),T. dyspeloris basedon
indeterminate
materialand thusis bestconsidered,
a nomenvanum.
Augustl98l
New Mexico Geology
Cope's type specimenof Z. dyspelor was
collectedby W. B. Lyon, a surgeonstationed
at Fort McRae,an old army post that wasjust
east of the present site of Elephant Butte
Reservoirin south-central
New Mexico(Cope,
l87lb). In his final description of this
specimen,Cope (1875,p. 167)explainedthat
it came"from the yellowbedsof the Niobrara
epochof the Jornadadel Muerto, near Fort
McRae,New Mexico." Copeprobablywasreferring to bedsof the MancosShalethat are
exposedin the Jornada del Muerto region
(Kelley and Silver, 1952)and are, in part, a
correlativeof the NiobraraFormation.
Lee(1917,p. 46-47)canbe creditedwith the
other two previousreportsof mosasaursfrom
New Mexico. He listed Mososaurussp. (sic)
from two localitiesin the Raton Mesaregion:
l) "North wall of Cimarron Canyon,about 4
mi northwestof Cimarron; 100ft belowtop of
Pierre Shaleand 2) "Van Bremmer Canyon,
New Mexico, a mile from its mouth, in slide
rock: at base of Trinidad Sandstone."Unfortunately,Lee did not describeor illustrate
any specimensso their assignmentro Mosasaurusr emainsunverified.
AcxNowI-pncMENTS-Weare greatly indebtedto numerousmembersof the Jicarilla
Apache Indian Tribe for their aid, interest,
and enthusiasmwhich enabledthe collection
of the specimendescribedhere. We specifically thank LeonardAtole, Tribal President,
under whoseauspices
the work was done;Joe
Muniz, Directorof Tourism,who enthusiastically pursuedthe investigationand authorized
an extendedIoan of the specimento the
Universityof New Mexico;and Adolph Julian
and Darrell Paiz whosesharpeyesfirst found
the specimen.Barry Kues, Niall Mateer, and
Robert Sullivan reviewedan earlier draft of
this paper;their commentshaveimprovedits
contentand clarity. Barry Kueswasespecially
helpful in pointing out obscurereferencesto
New Mexico'smarinereptiles.
References
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Mexico Geological Society, Guidebook 25th field
conference,p.279-282
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other regions in Colorado and New Mexico: U.S.
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and fishes from the Cretaceous formations of the
interior of the United States: U.S. Geological
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266-3tO
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p.
McGowan, C., 1978, Further evidence for the wide
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