IS MONONYKUS A BIRD?

The Auk 112(4):958-963, 1995
IS MONONYKUS
ZHONGHE
A BIRD?
ZHOU 1
Institute
of Vertebrate
Paleontology
andPaleoanthropology,
Chinese
Academy
of Sciences
P.O. Box643, Beijing100044,China
ABSTRACT.--The
avian statusof Mononykus
is refuted in this paper through a detailed
analysisof its supposedavian characters.An extensivecomparisonbetweenit and fossorial
mammals(especiallymoles),as well as variousbipedal archosaurs,indicatesthat someof its
resemblances
to extant birds are digging adaptations.Most of the otherscan be explained
either by its diggingadaptationor by its obligatorybipedalism.In addition,many of its
features,ascomparedwith birds(includingArchaeopteryx),
seemto betooprimitivefor avian
flight, and there is no evidenceindicatingthat the absenceof flight in Mononykus
was
secondarilylost.Received
23 December
1994;accepted
16February
1995.
MONONYKUS
OLECRANUS
is a very controversial
Late Cretaceousanimal from Mongolia. Perle
et al. (1993) published information about it as
a bird in Nature,and a moredetaileddescription
of this fossilwas published recently (Perle et
al. 1994).However, almostfrom the beginning,
its avian statuswas doubted by many paleornithologists (Olson pers. comm., Martin and
Rinaldi 1994, Ostrom 1994, Wellnhofer 1994).
This animal was originally named Mononychus
olecranus,
and shortly thereafter renamedMononykusolecranus
becausethe former genericname
was preoccupiedby the coleopteranMononychus.The materialsof Mononykus
include a partial skull, limbs, thoracicgirdle, portions of ilium and pubis,and mostvertebrae.According
to M. Norell and L. Chiappe (pers.comm.),new
material
of this animal
has been
found
from
diagnosing characters"of a new taxon called
Metornithes, which was defined as a group
within Avialae (seeGauthier, 1986;equivalent
to classAves) that excludesArchaeopteryx.
In
other words, they judged that Mononykuswas
the most primitive bird exceptfor Archaeopter-
yx. The five charactersare: (1) an ossifiedlarge
and longitudinally oriented rectangular sternum; (2) an ossifiedsternal carina; (3) a prominent antitrochanter on ilium; (4) an undivided
fernoral trochanteric crest; and (5) a fibula that
doesnot reachthe tarsus.They did not specify
other charactersusedto placeMononykus
in the
group Avialae; therefore,as I understandtheir
approach,thesefive charactersform their main
evidencefor the avian statusof Mononykus.
In
addition to these, several other characters were
Mongolia, including the previouslyunknown
either consideredto be ambiguouscharacters
of Metornithes or were simply comparedwith
digits II and III.
extant
This fossil was believed
to be a bird because
it was assertedthat several unambiguouscharactersindicated a closerrelationship to modern
birds than to Archaeopteryx.
The purposeof my
paper is to discusswhether or not these char-
acterssupportthe proposedavian statusof Mononykus.
ANALYSIS OF CHARACTERS SUPPORTING
AVIAN STATUS OF MONONYKUS
In the papersby Perle et al. (1993, 1994), five
birds.
Beforeanalysisof thesecharacters,it is necessaryto examine the digging habit of Mononykus.As clearly stated by Perle et al. (1993:
625), "the highly modified forelimb of Mononykusis similar to that of digging animals.The
large processof the humerus, large olecranon
processof the ulna, short massiveforelimb elementsand carpometacarpus,
and sturdy claw
suggestextremely powerful functional capabilities during adduction." These featuresalsoare
present in various digging animals, including
moles, fossorial multituberculates,
rodents, etc.
characterswere proposedto be "unambiguous In fact, digging habits are found in many mammalian orders like Monotremata, Marsupialia,
• Presentaddress:Natural History Museum,Dyche
Hall, University of Kansas,Lawrence,Kansas66045,
Insectivora, Rodentia, Edentata, Pholidota, Tubulidentata,
Carnivora, and Palaeanodonta
(Kielan-Janorwska 1989). It is not difficult to
find in Mononykuseven more evidence for its
USA.
958
October1995]
Is Mononykus
a Bird?
digging habit, including an ossifiedlarge and
longitudinally oriented sternum, and an ossified sternal carina. Recently, Ostrom (1994)
pointed out: "Difficult as it may be to visualize
this creature(Mononykus),there can be no mistake about its fossorialspecializations."
With
this sketch introduction
to the fossorial
habit of Mononykus,
the evidenceby which this
digging animal was deemed a bird can be investigated.Characters1 and 2 are in fact a charactercomplex:(a) sternumossified;(b) sternum
large; (c) sternumlongitudinally oriented; and
(d) presenceof sternal carina.
The sternumis ossifiedin variousgroupsof
tetrapods. In archosaurs,it is known to be ossifted in birds and in pterosaurs(Eaton 1910),
as well as in various dinosaurs (Romer 1956).
959
this analysis,and consideringthe presenceof
digging habitsin Mononykus,
I concludethat its
longitudinally oriented sternum can be most
reasonablyexplainedasfurther evidenceof fossorial capacity.
With regard to the presenceof a sternal carina, note that an ossifiedsternalcarinais present in all known digging animals, including
those in Insectivora (Slonaker 1920, Campbell
1939),Multitubercata (Kielan-Jaworowska1989),
and Rodentia(Barnosky1981).Furthermore,the
changeof the carina in birds and moles is to a
great degree similar to that of the length-towidth ratio. The carina is absent in Archaeopteryxand secondarilylost in ratite birds; it is
initially developed in Early Cretaceousbirds
like Cathayornis
(Zhou 1992)and Concornis
(Sanz
As the seventhskeletalspecimenof Archaeop- and Buscalioni1992). A sternal carina has never
teryxhasan ossifiedsternum(Wellnhofer 1993), been found in any dinosaurs,but it is reported
it is reasonableto believe that a sternummight in pterosaursand bats.Accordingto Campbell
be presentin the ancestryof birds too. That the (1939),in diggingmoles,the ventralridge(keel)
sternumis large is a relative concept.It is dif- becomesdeeper from the simple to complex,
ficult to determine
from this statement when a
and primitive to specializedgenera.Undoubtsternum can be called large. Furthermore,as edly, the sternalcarina,just like flight, had arisMononykusclearly is a digging animal, and a en many timesindependently in tetrapods,and
large manubriumis usually presentin fossorial the two bestknown adaptationsaccountingfor
mammals, it is easy to understand that its rel- its appearanceare flight and digging. With reatively large size is probably related to the fos- spectto the sternal carina in Mononykus,
it is
sorial adaptationof the forelimb.
most reasonablyexplained as evidence for the
The sternum usually is longitudinally ori- digging power of this animal.
entedin birdsexceptin ratites.In Archaeopteryx, Beforeanalyzing character3, it is necessary
the sternumis wider than long, probablyrep- to evaluate the differencesbetween two types
resentingthe mostprimitive condition of this of antitrochantersin archosaurs.
The avian type
structurein birds.The sternumis not markedly is positioned on the postero-dorsalrim of the
longer than wide in Cathayornis
(Zhou in press) acetabulum;the hadrosaurtype is totally difand Concornis(Sanz and Buscalioni 1992). In
ferent. According to Romer (1927), the latter
those dinosaursthat are known to have a pair type of antitrochanteris a strongridge running
of sterna,it is usuallywider than long (Barsbold out at a right angle to the plane of the iliac
1983).Amongpterosaurs,the sternumis longer blade approximately over the ischiadic articuthan wide (Eaton, 1910). Therefore, the ratio of lation. Unfortunately, in Perle et al. (1993), the
the length to width may be an indicatorof the type was not clearly indicated,but accordingto
flight capacity.The increaseof this ratio also the comparisonand figures of this structure, I
can be the result of another totally different assumethat it is of the avian type.
adaptation--digging.The sternumis found to
Character3 is presentin advancedbirds, but
be greatly elongated in moles in comparison absentin Archaeopteryx
and Cathayornis.
A simwith the usual mammalian types (Slonaker ilar structurecalled the supraacetabularflange
1920). Also, accordingto Campbell (1939), not is found on the antero-dorsal rim of the acetabonly is the sternumusually longer than wide, ulum in the ornithischian Fabrosaurusaustralis,
but there is a tendencyfrom the simple to com- which is small in length and probably an obplex (primitive to specialized)for the width of ligate biped (Santa Luca 1984). In the ornithothe manubriumto decreasein proportionto the pod dinosaurTenontosaurus
tilleti(Forster1990),
length. In other words, the increase of the the supraacetabularrim is laterally thickened,
length-to-width ratio is closelyrelated to the dorsally canted, and forms a deep, parabolic
improvementof the digging power. Basedon embaymentin the ventral iliac body. According
960
ZHONGHE
ZHOU
to Kurzanov (1983) and Molnar (1985), an ilium
with
antitrochanter
of the avian rather than the
hadrosaurianform is present in Avirnirnus.Al-
[Auk, Vol. 112
the tarsus.The fibula generally is only about
two-thirds the length of the tibiotarsusin extant
birds (Bellairs and Jenkin 1960). Romer and Parsons (1977), as well as Hinchliffe and Johnson
though a marked projectingrim has not been
reportedamong other theropods,it is found (1980),pointedout that the tetrapodfibulaoften
dorsallyboundingthe acetabulumin Allosaurus is reduced or fused with the shaft of the tibia,
and Dilophosaurus.
and in some taxa it has disappearedentirely.
The supraacetabular
flangemusthave affect- Hinchliffe and Johnson (1980) noted that ared femoral orientation; the femoral head may chosaurshave a seriesof functionaladaptations
have articulatedunderneath the flange, which of the hindlimb and the hindlimb girdle, which
would havetransmittedmassbetweenbody and are seen in many other bipedal types. In dilimb (SantaLuca1984).A lateralexpansionfrom nosaurs,thosewith bipedal habitsusually have
the dorsal rim of the acetabulum
can be exa relatively weaker fibula in comparisonwith
plainedby bipedalism,becausesucha structure those of quadrupedal types (G. Peng pers.
not only extendsthe acetabulararticularsurface comm.).For example,in Deinonychus
antirrhopus,
laterally, allowing a direct transmissionbe- an obligatorybiped beyondany doubt,the fibtween the femur and the ilium (SantaLuca 1984), ula is extremelyslenderand is much lessrobust
but also makes it possibleto not narrow the than the tibia (Ostrom 1969). Therefore, the repelvis at the expenseof the compactcontact duced fibula in Mononykuscan be interpreted
between the femur and the pelvis. (The pelvis as related to bipedalism.Becausethe hindlimb
hasbeen found by Slonaker[1920]to be greatly is very slenderand the forelimb short,it is likenarrowedin moles[G. Peng pers.comm.].)All ly thatMononykus
wasnot only bipedal,but also
of thesefactorsare importantfor a bipedallife. an obligatory biped. A similar conclusionwas
Although the significanceof the differencein reachedby Ostrom(1994).
In summary,among the five purported avian
the exactshapeor position of the supraacetabular rim is not clear, it is almost certain that the
characters,the first two almostcertainlyare digavian type of antitrochanter(or similar struc- ging adaptations.The other three probablyare
tures)appearedmany timesin archosaurs.
This relatedto digging directly or indirectly. Hence,
characterin Mononykus,therefore, is not defin- the five charactersare not phylogeneticallyinformative. In addition, none of them is unique
itive and not particularly informative.
Character
4 is the
undivided
femoral
tro-
chanteric crest. First, how was the conclusion
on its homologyreached?It might be that the
lesser trochanter has degenerated, while the
greattrochanterhasenlarged.Accordingto X.
Zhao (pets.comm.),in somedinosaursthe lesser trochanteris only slightly developed and
maytotallydisappearin someanimals.Besides,
well-developedfernoraltrochantersare present
to birds, and all are restricted to the hindlimb
and pelvic girdle, which are mostvulnerable to
modificationin the adaptationtoward a bipedal
life.
Before elaborating on the parallelism com-
monly observedbetweendinosaursand birds,
I will discuss other resemblances
between
Mon-
onykus
and extantbirds.The humerusis saidto
havea singledistalcondyleand ison the cranial
in the more fossorialmoles(Freeman1979).Reed
(1951) also noted that few major changesoccur
in the femora of soricoidsexcept that the more
fossorialmoleshavebetter developedprocesses
surface,as are both humeral condylesof extant
birds(Perleet al. 1993).A singlecondyleis itself
a very specializedfeaturefor an archosaur.Talpids (moles), which dig with the forelimb,
on the proximalend of the bone.Romer(1956) mainly use the lateral thrust technique in the
noted that with the archosaur trend toward bidigging stroke,and retractionis not important
pedalismand a resultantforwardturning and in the power stroke (Barnosky1982). As there
semierectposeof the hindlimb, the femur has is no evidencethat the head in Mononykuswas
becomegreatlymodified.Sincedigging and bi- used when digging, its digging stroke was
pedalismare both characteristic
of Mononykus, probablymadeby the forelimbsin a way similar
I suggestthat the developedtrochantericcrest to that of moles. The lateral thrust was mainly
also may be related to the animal's fossorial used in the power stroke and when digging
habit either directly or indirectly.
where the earth is pushedaside,the fore paws
Character
5 is that the fibula does not reach
were
thrust
forward
and the elbow
must be
October1995]
Is Mononykusa Bird?
usually acutely angled posteriorly (similar to
folding stateof humerusand forearmin birds).
It is clear why the single condyle is locatedon
961
PRIMITIVE CHARACTERS PRESENT IN
MONONYKUS
AND A COMPARISON
WITH BIRDS
the cranial surface of the humerus, as are both
Primitivefeaturesor synplesiomorphy
should
not be employedas direct evidencefor establishing phylogeneticrelationships.However,
this doesnot imply that primitive characters,
therefore,canbe completelyignored.While not
chian dinosaurs, but also in some saurischians.
acting as direct evidencefor phylogeneticreBarsbold (1983) concluded that the orientation lationships,they can indicatethe evolutional
of the pubic bone in the pelvis originated re- level of the organism and, for practical purpeatedlyin the processof the evolution of the poses,provideindirector auxiliaryevidencefor
differentsaurischian
groups.Unfortunately,the relationships,especiallywhen useful synaporeason for this caudal orientation
is not well
morphiesare lacking or difficult to interpret.
In Mononykus:
the anterior blade of the ilium
known (Romer and Parsons 1977). Mononykus
also resemblesextant birds in lacking an ex- is muchshorterthan the posteriorone;the ulna
panded pubic foot, which is presentin many is not markedlywider than the radius;the scapdinosaurs,but absentin many others.Even as- ula and coracoid are similar to those in nonsumingthat birdsevolved from dinosaurianan- maniraptorantheropods(Perleet al. 1993,1994);
cestorswith an expandedfoot, it is still an un- and the metatarsals are unfused (Perle et al.
solved problem as to whether the immediate 1993, 1994). All of these characters are more
ancestorof Mononykushad such a foot.
primitive than in all known birds, including
In brief, the anterior blade of the
The pubis and ischiumlack a symphysisin Archaeopteryx.
Mononykus,which is another charactersimilar ilium is longer and more robustthan the posterior blade in Archaeopteryx.
This feature also
to birds. Romer (1956) pointed out that
is found in the Early Cretaceousbirds, suchas
Pelvic modification associated, it seems certain, with
Cathayornis
(Zhou et al. 1992), Sinornis(Sereno
humeral condylesof extant birds.
Another striking resemblancebetween Mononykusand extantbirds is a retrovertedpubis.
The pubis not only is retrovertedin ornithis-
a trend towardbipedal habitsare seenin all branches
of the group[archosaurs]
and are mostespeciallydeveloped in pubis and ischium.In ornithischian(paralleling the birds)the ventral symphysisis muchreduced. The two ischia are apposed,frequently for
much of their length, but there is no strongfusion.
The pubesare in contactdistally in a few primitive
ornithopods only.
and Rao 1992), and extant birds. The ulna in
Archaeopteryx
(Wellnhofer 1993) is wider than
the radius. In Sinornis
and Cathayornis,
the ratio
of the ulna to the radius is even greater. In
extant birds, the ulna is almost twice as wide
as the radius. Becausethe ulna supports the
secondaryflight feathers,its robustnesscan be
viewed as a direct indication of flight capacity.
Metatarsalsare at least partially fused in Ar-
Accordingto Slonaker(1920),the greatestvari(Ostrom1976,Wellnhofer1988),Siation found in the pelvis of the different genera chaeopteryx
Perle et al. (1993) comof the Talpidae is in the region of the pubic nornis,and Cathayornis.
of Mononykus
with
symphysis,which varies from a well formed paredtheunfusedmetatarsals
(Sanzand Bonaparte1992),
symphysisto a relativelywide gapbetweenthe thoseof Iberomesornis
pubicbones.In conclusion,the characterof lack but according to E. Kurochkin (pers. comm.)
of symphysisbetween the pubis and ischium and Zhou (1995), the latter probably is a juvemust have arisen at many different times in nile individual. In addition to the above feaarchosaurs for various unknown
reasons. In
tures, Mononykusshowsthe presenceof a long
phylogeneticstudies,this featureis not helpful tail and unfused cervical ribs, which are more
primitive featuresthan thosefound in all birds
in resolving relationships.
Mononykus
alsoresemblesextantbirdsin hav- exceptArchaeopteryx.
In summary,Mononykusshows many strucing an ulna longer than the humerus,but this
almostcertainly is due to the great elongation tural resemblances to dinosaurs, but few to birds.
of the olecranon.A well developed olecranal Although this fact alone will not provide deprocessis characteristicof all fossorialmammals finitive information concerning the relation(Slonaker 1920, Lehmann 1963, Freeman 1979, ship of Mononykus
to birds, it is importantthat
someof thesestructuresare crucialto the flight
Barnosky 1981).
962
z•o•c•E zrtov
[Auk, Vol. 112
of birds. The lack of an avian appearance in
archosaurs,including dinosaurs,in recentyears
these structures
has improved our understandingof parallel-
casts doubt on the "avian"
sta-
tus of this specializedanimal.
DISCUSSION AND CONCLUSION
isms that occur in archosaurs.
For example,Molnar (1985) gave a detailed
analysisof the 18avian characterslistedby Kurzanov (1983) in Avimimus.Fourteen of them were
confirmedby him as valid. Molnar (1985) also
found that Archaeopteryx
haseight of thesecharproblem, I first addresstwo related questions. acters-significantly fewer than Avimimusitself,
First, is there any unique avian character in and alsofewer than are found piecemealamong
Mononykus?
Second,why was Mononykuspre- theropods.Contrary to Molnar's conclusion,
which recommendeda reassessment
of the poviously held to be a bird?
these resemblancesare,
The first questionis easyto answer.No single sition of Archaeopteryx,
characterdescribedfor Mononykusis found only in my view, either synapomorphies
for Avimiin birds. Archaeopteryx
is acceptedby most re- musand birds, or independently developed in
searchers
asa bird becauseit possessed
feathers. them. If these resemblancesare not carefully
Indeed,asis commonlystated,without feathers analyzed,a dinosaurmight easilybe recognized
in specimens
of Archaeopteryx,
it probablywould as a more derived bird than a true early bird.
This unfortunately hasalready happenedwith
not be recognizedas a bird.
The secondquestion,although never clearly Mononykus.
Most birdlike dinosaurs occur late in the Crestated,was implied in the papersof Perle et al.
taceous,
at the end of dinosaurian history,
(1993, 1994). Mononykuswas consideredto be
more closelyrelated to extant birds than Ar- whereas as ones go back through time to Aror beyond, they are less birdlike.
chaeopteryx
becauseit has severalderived fea- chaeopteryx
turesincludinga carinatesternumand reduced Possibly,most of the avian charactersof thefibula.However, they did not explainwhy these ropodsare only convergentlysimilar to birds,
showsushow willing somepeofeatures were chosen. They provide at best a and Mononykus
superficialresemblance
betweenMononykus
and ple are to be deceivedby suchconvergence.
extant birds. Fortunately, the forelimbs were
Finally, Mononykushas some features that
preservedin the specimenand clearly indicate seemtoo primitive for the presenceof flight.
that Mononykus
is an animal specializedfor dig- One could argue that flight has been secondarily lost in Mononykus,
but how would this be
ging (see above).
Among the five ambiguouscharacterslinking known? There is no evidence that can account
Mononykusto extant birds and other resem- for the modificationof theseflight features.Fosblancesbetween them, most can be explained soriallife could force the shortening of the foreby the acquisitionof digging;othersprobably limb elements,but how canwe explainthe presare related to obligatory bipedalism.There is ence of short coracoid, the closeness of the width
no definite evidencesupportingthe avian sta- of the ulna and the radius, and the lack of fusion
tus of Mononykus;
it likely is a specializeddi- of the metatarsals.The most-parsimoniousexnosaur(whether it is a theropodis beyondthe planation is that Mononykusis not a bird and
scopeof this paper),with severalavian simi- that its ancestorsnever possessedthe capacity
larities due to fossorial habits and obligatory for flight.
bipedallocomotion.Thatbirdlikedinosaurs
are
Mononykusis beyond doubt a fossorialani-
mal. However,
is it also a bird? To solve this
not birds does not mean that they have no re-
ACKNOWLEDGMENTS
semblance to birds, but rather that the resem-
blances between them are either synaplesio-
I thank S. Olson, L. Martin,
and A. Feduccia for
morphiesor independentlyacquiredadapta- their critical reviewson this paper.I am alsograteful
to the following people for their commentson the
The analysisof the adaptive significanceof previousmanuscript:S. Olson, L. Martin, C. MourerChauvir•,P. Wellnhofer,W. Bock,J.Ostrom,X. Wang,
avianlike charactersin Mononykusis facilitated
and J. Meng. I have benefitedfrom discussions
with
not only by the completepreservationof the S. Olson, L. Martin, A. Feduccia,L. Hou, G. Peng, X.
specializedforelimbs,but alsoby the existence Zhao, Z. Qiu, and T. Qi. Finally, I am indebted to M.
of various extant fossorial forms (e.g. moles) Norell and L. Chiappe for showing me the castof
that havebeensubjectedto detailedstudy.Also, Mononykusduring my visit to American Museum of
the greataccumulationof early birdsand other Natural History.
tions.
October1995]
Is Mononykus
a Bird?
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