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Pterosaurs as a food source for small dromaeosaurs
David Hone, Takanobu Tsuihiji, Mahito Watabe, Khishigjaw Tsogtbaatr
PII: S0031-0182(12)00094-6DOI: doi:10.1016/j.palaeo.2012.02.021Reference: PALAEO 6050
To appear in: Palaeogeography, Palaeoclimatology, PalaeoecologyReceived date: 11 November 2011Revised date: 6 February 2012Accepted date: 14 February 2012
Please cite this article as: Hone, David, Tsuihiji, Takanobu, Watabe, Mahito, Tsogt-baatr, Khishigjaw, Pterosaurs as a food source for small dromaeosaurs, Palaeogeography,Palaeoclimatology, Palaeoecology (2012), doi: 10.1016/j.palaeo.2012.02.021
This is a PDF le of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its nal form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.
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Pterosaurs as a food source for small dromaeosaurs
David Hone*1, Takanobu Tsuihiji2, Mahito Watabe3, Khishigjaw Tsogtbaatr4.
1. School of Biology and Environmental Sciences, University College Dublin, Dublin
4, Ireland.
2. Department of Geology and paleontology, National Museum of Nature and
Science, 3231 Hyakunin cho, Shinjuku ku, Tokyo 169 0073, Japan.
3. Hayashibara Museum of Natural Sciences, 2-3, Shimoishii-1, Okayama 700-0907,
Japan.
4. Paleontological Center, Mongolian Academy of Sciences, Ulaanbaatr, 210351,
Mongolia.
Abstract
Stomach contents preserved in fossil specimens provide direct evidence for the diet of
extinct animals. Such exceptional fossils remain rare for predatory non-avian
dinosaurs and each can add significantly to our understanding of trophic interactions
between various taxa. Here we present evidence for the dromaeosaurid theropod
Velociraptor scavenging on the carcass of an azhdarchid pterosaur, with a long bone
of the pterosaur being found as gut contents of the dinosaur. Despite previous
inferences of dromaeosaurs as hyper-predators, scavenging appears to have been an
important part of their ecology.
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Keywords:
deinonychosaur, azhdarchid, scavenger, predator-prey, Cretaceous
1. Introduction
The preserved gut contents of fossil carnivorous dinosaurs provide direct
evidence for their diet and help to establish trophic patterns, species interactions and
the ecology of both taxa concerned (e.g. Charig and Milner, 1997; Varricchio, 2001).
Such specimens are known for a variety of predatory non-avian theropod dinosaurs,
though they are rare (see Hone and Rauhut, 2010). However, while these and similar
ichnological records (e.g. bite marks, coprolites) do provide direct evidence of
carnivore-prey interactions it can be difficult to distinguish between active predation
and scavenging without exceptional evidence (e.g. Hone and Watabe, 2010).
In general non-avian carnivorous theropods (and especially deinonychosaurs)
have been considered active predators (Holtz, 2003), however evidence for
scavenging in dromaeosaurs (Currie and Jacobsen, 1995; Hone et al., 2010) is now
known. This should be no surprise few amniote carnivores are exclusively predatory
or carnivorous and even the most dedicated carnivores will not turn down a free meal
in the form of a carcass. Thus the debate of whether or not any given taxon was a
predator or scavenger has moved on (e.g. Holtz, 2008) - the issue is where on the
continuum between these extremes a given taxon may lie, and what evidence is
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available to support this inferred trophic position.
Here a specimen of the dromaeosauridVelociraptor is described, preserved with
part of a bone in the chest cavity. This element can be identified as belonging to an
azhdarchid pterosaur and suggests that small non-avian deinonychosaurs were capable
of consuming relatively large bones. While not the first evidence of theropod feeding
on a pterosaur carcasss (Currie and Jacobsen, 1995; Buffetaut et al., 2004) this is the
first known as gut contents.
2. Institutional Abbreviations
MPC-D: Registered number of dinosaur specimens stored at Paleontological
Laboratory of Paleontological Center, Mongolian Academy of Science, Ulaanbaatar.
3. Locality Information
The skeleton was discovered in 1994 in the middle part of the geological section of
the eolian sandstone complex at Tugrikin Shireh in the Gobi Desert, Mongolia. The
eolian beds are 30 m thick, measured from the base of the cliff to the top and thus the
fossil was recovered approximately 15 m from the base of the cliff. The cliff is south
facing, forming the southern end of the mesa-like structure in Tugrikin Shireh. The
locality is rich in dinosaur and other vertebrate fossils and the sedimentation is solely
eolian in nature. We do not provide GPS co-ordinates owing to the prevelance of
illegal fossil excavation at the site details are available on request. This locality is
correlated with the Djadokhta Formation and thus of Campanian age (e.g.,
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Jerzykiewicz and Russell, 1991).
4. Description
The specimen is of a largely complete and articulated skeleton of the
velociraptorine dromaeosauridVelociraptor mongoliensis (MPC-D100/54), missing
the right forelimb and most of the tail (this material will be described fully in a future
publication). Five dromaeosaurid species are currently recognized from the Djadokhta
Formation or coeval sediments in the Gobi Desert of Mongolia and China:
Velociraptor mongoliensis , V. osmolskae , Tsaagan mangas , Linheraptor exquisitus ,
and Mahakala omnogovae . With a skull length of approximately 230 mm,
MPC-D100/54 is considerably larger than Mahakala (Turner et al., 2007) and this can
be eliminated from further consideration. MPC-D100/54 lacks autapomorphies used
to diagnose V. osmolskae , T. mangas , or L. exquisitus . For example, MPC-D100/54
lacks the elongated maxillary fenestra and elongated promaxillary fenestra that are
characteristic of V. osmolskae (Godefroit et al., 2008), the elongated basipterygoid
process and pendulous paroccipital process without distal twisting observed inT.
mangas (Norell et al., 2006), or an enlarged maxillary fenestra diagnostic of L.
exquisitus (Xu et al., 2010). The skull of MPC-D100/54 does however bear a
longitudinal ridge dorsal to a row of neurovascular foramina in the maxilla (although
only in the posterior part of the bone), which is considered diagnostic forV.
mongoliensis , (Barsbold and Osmlska, 1999). Accordingly, MPC-D100/54 is
referred toV. mongoliensis.
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The specimen has undergone extensive preparation, and numerous elements have
been separated from the matrix, although the chest cavity (dorsal vertebrae, ribs,
sternal plates, gastralia, right scapula and both coracoids) is retained as a single
articulated piece (Fig. 1). The animal was injured or recovering from an injury at the
time of death with one broken dorsal rib showing signs of regrowth.
The specimen represents a young individual, perhaps a sub-adult, based on the
incomplete fusion of the right scapula and coracoid, the separate sternal plates, and
that the sacral ribs are not fully fused to the ilia. The femora are 192 and 194 mm long
(left and right respectively) compared to a length of nearly 240 mm measured from an
adult specimen of V. mongoliensis (Norell and Makovicky, 1999).
4.1 Gut contents
Part of a single bone lies preserved in the chest cavity of theVelociraptor (Figs.
1 4). The anteriorly positioned part is fragmentary, with a more complete posterior
part. The two parts lie in a direct line with one another and as far as can be determined
are identical in bone wall thickness and have similar shapes, strongly suggesting that
this was originally all part of a single piece. The anteriormost part of the bone lies
below the fourth dorsal of theVelociraptor and the posterior piece below the fifth
through seventh dorsals (Fig. 2). While the chest has collapsed somewhat, the relative
positions of the bones have not been affected as this collapse is purely vertical (i.e.
mediolateral directions of the body) and not lateral. The current position of this
element is therefore considered genuine or close to the original position and thus
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would in life have been present in the upper part of the gastrointestinal tract, most
likely the stomach. It is improbable that this entered the area post-mortem given the
lack of disturbance to the material and the narrow spaces between the ribs, in addition
to this being an eolian deposit.
The preserved bone would have been approximately 75 mm in length including
the broken proximal part and 12 mm in diameter (only approximate values were
obtained owing to the position of the overlying ribs and sterna). The bone is slightly
oval in cross section and has a very thin cortex of approximately 0.2 mm in thickness
(as measured from photographs owing to the inaccessibility of the material within the
chest cavity).
The surface of the bone is smooth and in good condition, showing no unusual
traces of marks or deformation that could be attributed to digestive acids. The edges
are broken however, and not smooth but jagged and rough. This suggests that either
breakage occurred before, or as part of, ingestion. This also indicates, that the bone
had not lain long in the gut as it would have been broken up, or at least the rough
edges would be smoothed out. This also therefore suggests that the individual died
shortly after ingesting the bone.
5. Discussion
5.1 Identity of the pterosaurian element
The bone preserved in theVelociraptor chest cavity is here identified as that of a
pterosaur. The extremely thin walled nature of the bone is unique to pterosaurs
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(Fastacht, 2005), even in elements with large diameters such as that preserved here.
Indeed the bone walls seen here are especially thin having a ratio of radius / wall
thickness of 30 (6 mm / 0.2 mm). This ratio is higher (i.e. the bone wall is thinner)
than a number of previously measured pterosaurs (Fastnacht, 2005) though
comparable with others (Witton, 2008). The nature of the bone (see description above)
suggests that it has not undergone damage from erosion or digestion and that the thin
walls are original and unmodified. This discovery also marks the first record of a
pterosaur from Tugrikin Shireh.
The diversity of Late Cretaceous pterosaurs is limited, with only the derived
azhdarchids, nyctosaurs and the unusual istiodactylids being currently known from
this time. The latter is represented by a single isolated jaw from Canada (Arbour and
Currie, 2010, though see Witton, 2012) and as istiodactylids bear numerous diagnostic
teeth which have never been reported from Tugrikin Shireh or related sediments, this
is not considered a likely candidate. A single nyctosaur humerus has been reported
from the Late Cretaceous of Brazil (Price, 1953) but these taxa are both rare and
exclusively confined to marine sediments (Unwin, 2005). Thus an azhdarchid identity
is favoured and at least some azhdarchoids have especially thin walled bones, even
compared to some other pterosaurs (Elgin et al., 2009), which tentatively supports this
identification. Finally, an azhdarchid pterosaur has been recorded at the nearby
Bayshin Tsav locality (Watabe et al., 2009) that is of similar age to (though older than)
this specimen, and the azhdarchids are one of the few pterosaurian groups that likely
favoured terrestrial environments (Witton and Naish, 2008). While clearly the bone is
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not definitively diagnostic, there is strong circumstantial evidence to support the
referral of the material to the Azhdarchidae.
The bone as preserved lacks of any obvious features associated with the proximal
and distal ends of pterosaurian long bones (e.g. a condyle, trochanter, tapering or
expansion of the shaft etc.), and so the bone would originally have been longer than
the measured 75 mm. It would most likely have exceeded 100 mm, and could
potentially have been much longer. In the azhdarchids, long, straight, and thin-walled
elements with a sub-circular cross-section that are significantly longer than their
diameter correspond to a series of major elements including the humerus, the ulna,
radius, wing metacarpal, femur, tibia and the first wing phalanx. Many pterosaur long
bones, especially wing elements, are considerably longer than their diameter and taper
only a little along their length. Based on the near-complete azhdarchid
Zhejiangopterus (Cai and Wei, 1994) wing phalanges and long bones such as the tibia
may more than 20 times longer than the midshaft width (and thus the bone could
potentially have been closer to 250 mm in length originally).
While exact determination of the bone here is uncertain, using Zhejiangopterus
(Cai and Wei, 1994) as a model, a bone of 100 mm in length or 12 mm in diameter
would scale to a variety of wingspans. For example, a humerus of 12 mm diameter
would scale to around 2 m, whereas using the radius as a model would produce a
value closer to 3.5 m. Witton (2008) calculated the mass of a 2.9 m wingspan
Zhejiangopterus as over 9 kg, with a very large azhdarchid (>10 m in wingspan)
potentially weighing around 250 kg. Thus the minimum likely size of the pterosaur
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was 2 m in wingspan, was probably closer to 3 m, and was potentially from an animal
considerably larger still.
5.2 Scavenging or active predation?
Turner et al. (2007) calculated a mass of 24 kg for an adultVelociraptor (of femur
length 238 mm) , and the specimen here is not mature and rather smaller. Based on the
supplementary data of Turner et al. (2007) the animal here was around 13 kg. While
pterosaurs are light for their size, an active predator such as an azhdarchid (Witton
and Naish, 2008) with a wingspan probably greater than the total length of the
dromaeosaur and weighing 9 kg or more would be a difficult, and probably even
dangerous, target from a young dromaeosaur. Thus, unless the pterosaur was already
ill, infirm or injured, it seems unlikely that this would be a case of predation.
Furthermore, such a large animal would provide a substantial bounty for a
dromaeosaur given that it would be a very substantial part of the carnivores mass.
Consuming parts of large bones when there would be substantial amounts of meat
available on a newly dead animal suggests that this was late-stage carcass
consumption (see Hone and Watabe, 2010 and reference therein). The dromaeosaur
was consuming bone (perhaps with some trace flesh attached) presumably because
there was little else to eat on the carcass, although it may simply have been seeking a
source of minerals.
Velociraptorines are known to bite on bones leaving scrape marks (Currie and
Jacobsen, 1995; Hone et al., 2010) during late-stage carcass consumption of sizeable
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carcasses, while smaller food items could be consumed whole (OConnor et al., 2011).
While the articular ends of the bone are missing and may have been bitten through,
the preserved part shows no feeding traces. Nor does the skull show damage to any
teeth indeed all the tooth rows appear complete and undamaged, despite the
propensity of velociraptorines to shed teeth during feeding (Currie and Jacobsen, 1995;
Hone et al., 2010). Thus it appears that the bone was swallowed with little or no oral
(or other form of) processing by the dromaeosaur. Here the element in question likely
had little in the way of muscle tissue attached to it (since the major muscle groups
would attach to the missing articular ends), and the normal delicate feeding of
dromaeosaurs suggest that ingestion of large bone elements was not part of their
normal feeding routine (e.g. see Hone et al., 2010) and nor is this a common pattern of
carcass consumption large bones are consumed whole when there is no alternative.
A bone of such diameter and length would presumably have been a challenge to
consume.
5.3 Paravian carnivory
Evidence for both predation and scavenging in seen for dromaeosaurs (Norell and
Makovicky, 2004; Hone et al., 2010; OConnor et al., 2011) and troodontids
(Makovicky and Norell, 2004), and at least some early birds such as Archaeopteryx
(Elanowski, 2002) are considered carnivorous / insectivorous (although see also
Zanno and Makovicky, 2011). However, it is notable that a similar fossil to that
described here is known from the Late Cretaceous of Canada. Currie and Jacobsen
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(1995) described a large azhdarchid tibia that shows both bite marks and an embedded
tooth from the dromaeosaurSauronitholestes . As with the specimen here, it seems
unlikely that such a small carnivore brought down such a relatively large prey item.
Therefore it is reasonable to infer that at least some carnivorous paravians scavenged
large carcasses regularly and it may have been a common behaviour. At the very least,
based on the increasing amount of evidence for scavenging in members of this group,
they should not be characterised as hyper-specialised predators as they have been on
occasion (e.g. Ostrom, 1990). In addition, the rarity of pterosaur fossils in general, and
specifically those showing evidence of having been killed or scavenged means that
evidence of two separate incidents involving dromaeosaurs and pterosaurs may be
more than a coincidence and may speak of potentially close ecological ties in come
communities.
Notably, while some paravians are known to leave extensive bite marks on large
bones while feeding, clearly at least some also ingested large bones (or significant
parts of them) whole. While this practice is more commonly associated with large
theropods (see Hone and Rauhut, 2010 and references therein), small paravians at
least had the capacity to swallow relatively large food items whole. In this regard, the
gut contents of small theropods such asCompsoganthus (Ostrom, 1978) and
Microraptor (Larsson et al., 2010; OConnor et al., 2011) may represent the normal
condition for ingestion of food by all non-avian theropods: large parts of, or the whole
prey item, would be consumed without extensive oral processing before swallowing.
This pattern is also seen in both modern crocodilians and raptorial birds such as owls
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suggesting a common pattern throughout Archosauria.
Nevertheless, presuming that animals did not normally ingest relatively large
bones (as opposed to a relatively large prey times that contained bone) that they could
not easily digest, this suggests that even small carnivores scavenged regularly. As
such, they may have represented an important part of ecosystem recycling in clearing
up large carcasses. If the recent model of Carbone et al. (2011) is correct, then
numerous small paravians in a given Late Cretaceous terrestrial community might
have reached such carcasses rapidly, and were clearly potentially capable of
consuming not just soft tissues, but significant parts of the skeleton as well.
Acknowledgements
We thank Fabio Dalla-Vecchia for a photograph of the skull of theVelociraptor
specimen, and Corwin Sullivan for useful discussions. We thank Ross Elgin and Mark
Witton for detailed and helpful comments on an earlier version of the manuscript. Mr.
Ken Hayashibara financially supported the fieldwork in Mongolia. The specimen was
prepared by a Mongolian preparator, Lkhagvasuren, with his usual consummate skill.
CT scanning images of the studied skeleton (supplementary data) were taken in
Miyamoto Orthopedic Hospical, Okayama, Japan. The authors thank the division of
radiology of the hospital, especially Mr. Fumihiko Kakuta, for their dedication and
cooperation.
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Figure Captions:
Figure. 1. Chest cavity of Velociraptor , MPC-D100/54 in right ventrolateral view.
The broken rib is marked with a white arrow. The two parts of the preserved
bone are marked with black arrows (the main part to the left and a smaller
broken piece to the right). Scale bar is 50 mm.
Figure. 2. Line drawing of chest cavity of Velociraptor , MPC-D100/54 in right
ventrolateral view. The preserved pterosaur bone is coloured grey. Anatomcail
abbreviations are as follows: co, coracoid; g, gastralia; r, rib; sc, scapula; st,
sternal plate; v, dorsal vertebra. Scale bar is 50 mm.
Figure. 3. Close-up of the posteriorly positioned part of the bone in the chest
cavity in posterioventral view showing the cross section shape and the extreme
thinness of the cortex. Scale bar is 10 mm.
Figure. 4. Close-up of the bone in the chest cavity in ventral view showing the
posteriorly positioned part (to the left) and the associated fragments in line with
this (to the right). Scale bar is 10 mm.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Highlights
- The first theropod dinosaur with a pterosaur bone preserved as gut contents.
- This supports previous interpretations of Velociraptor as scavenging.
- Pterosaurs were perhaps regularly part of the diet of carnivorous dinosaurs.