pterosaurs as a food source for small dromaeosaurs

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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

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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 3


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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.

pterosaurs as a food source for small dromaeosaurs

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