Bulletin 51

New Mexico Museum of Natural History & Science

A Division of the DEPARTMENT OF CULTURAL AFFAIRS

Crocodyle tracks and traces

edited by Jesper Milàn, Spencer G. Lucas, Martin G. Lockley and Justin A. Spielmann

Albuquerque, 2010 Milàn, J., Lucas, S.G., Lockley, M.G. and Spielmann, J.A., eds., 2010, Crocodyle tracks and traces. New Mexico Museum of Natural History and Science, Bulletin 51. 195 CROCODYLIAN-CHELONIAN CARNIVORY: BITE TRACES OF DWARF CAIMAN, PALEOSUCHUS PALPEBROSUS, IN RED-EARED SLIDER, TRACHEMYS SCRIPTA, CARAPACES

JESPER MILÀN1,2, JENS KOFOED3 AND RICHARD G. BROMLEY4

1 Geomuseum Faxe/Østsjællands Museum, Østervej 2, DK-4640 Faxe, Denmark, e-mail: [email protected]; 2 Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark;. 3 NaturBornholm, Grønningen 30, DK-3720 Aakirkeby, Denmark, e-mail: [email protected]; 4 Geological Museum/Natural History Museum of Denmark, Øster Voldgade 5–7, DK-1350 Copenhagen K, Denmark, e-mail: [email protected]

Abstract—Predatory acts of the Recent Dwarf Caiman, Paleosuchus palpebrosus, preying on Red-eared Sliders, Trachemys scripta, are investigated with regard to bite traces left in the shells and the technique applied by the caiman to crack open the carapace of the turtle. The caiman manipulated the turtle into an upright position in the mouth before applying jaw pressure, which caused the bony bridges connecting the carapace and plastron to break, so the shell could be opened and emptied. This act left several types of bite traces in the shells, including round puncture holes, often arranged in rows; elongated scratch traces from the teeth scraping along the carapaces; and large crushed areas on the plastrons and carapaces from repeated bites applied to the same area. These are all traces that would be recognizable in turtle material.

INTRODUCTION OBSERVATIONS OF PREDATION Bite traces in bone are well known in the zoological literature (e.g., The predation took place in the experience center NaturBornholm Binford, 1981; Brain, 1981; Haynes, 1980, 1983). In recent years they on the Danish island of Bornholm, located in the Baltic Sea. The center have been studied increasingly in fossil material (e.g., Deméré and Cerutti, NaturBornholm is designed to interpret the islands complicated geologi- 1982, Cigala-Fulgosi, 1990; Schwimmer et al. 1997, Tanke and Currie, cal past, to the visitors and tourists, by leading them through a series of 1998; Corral et al., 2004; Mikuláš et al., 2006; Schwimmer, 2010; Jacobsen dioramas and exhibits that reconstruct changing environments through and Bromley, 2009). These trace can derive from a wide spectrum geological time. During the Early , the environments on of behavior, ranging from predation (the death blow, slaughter and subse- Bornholm were those of nearshore barrier islands and shallow, freshwa- quent eating), subsequent scavenging, and the later use of the skeletal ter lagoons (Gravesen et al., 1982) with a diverse fauna, in- parts as a source of calcium and/or as a hone for sharpening teeth (by cluding fish, sharks, dinosaurs, mammals, and crocodylians (Bonde and rodents in particular). Christiansen, 2003; Christiansen and Bonde, 2003; Bonde, 2004; Lindgren Thus, several approaches to studying fossilized bite traces are et al., 2004, 2008; Schwarz-Wings et al., 2009). The exhibit depicting the common: the identification of the tracemaker, deducing the mode of Lower Cretaceous environment represents a small lagoon with a life-size behavior used by the tracemaker, the functional reason for the produc- reconstruction of a dromaeosaurid dinosaur and Cretaceous-like vegeta- tion of the trace fossil, and the designation of trace fossil taxa (ichnotaxa) tion. Dwarf Caimans, Paleosuchus palpebrosus, with their prominent for the bite traces. The ichnotaxa must only be based on fossilized type bony ridges above their eyes, were chosen to simulate the small Creta- material, and ichnotaxobases must be of a morphological nature. ceous crocodylians found on Bornholm (Schwarz-Wings et al., 2009) Not all fossilized bite traces are morphologically suitable for the and a number of Red-eared Sliders, Trachemys scripta (sensu Bickart et application of formal names. However, a number of characteristic al., 2007), were donated to NaturBornholm by local residents (Fig. 1). ichnogenera have been established (e.g., Mikuláš et al., 2006; Jacobsen The caimans are adult animals having a total length of about 120 and Bromley, 2009). The morphology of the structure gives clues to the cm and an estimated weight of 7-10 kg. In the first 2 to 3 years, after the tracemaking animal, the behavioral technique of the tracemaker, and, opening of NaturBornholm in 2000, only a few “accidents” with caimans consequently, the reason for the activity. For example, Knethichnus and occurred, mostly during feeding sessions. A caiman would parallelum Jacobsen and Bromley, 2009, has a very characteristic mor- grab for the offered food and instead, by accident, get a turtle leg or neck, phology, consisting of a number of grooves following an exactly parallel because the turtles showed no respect for their sharp-toothed room- course across the surface of bone substrates. The behavior represented is mates. Only on a couple of occasions did this lead to damage to or death the scraping of flesh from the surface of the bone. The tool morphology of a turtle. But when it was decided after four years to cut down on the is the result of sharp-edged teeth having numerous small denticles along number of feedings because the caimans were becoming too fat, and they the sharp edge, which scratch the parallel grooves. But the tracemakers were additionally given a fasting period of several months, a new phe- are of two very different forms: tyrannosaurid theropod dinosaurs on nomenon was discovered: an empty turtle shell, stripped of all flesh and the one hand, and sharks on the other. The denticles on the teeth are divided into carapace and plastron, was found lying on land in the exhibit similarly developed and similarly used, but the tracemakers and their around Christmas 2004. Obviously it was possible for the caimans to respective environments and prey are dramatically different. open the turtles, if they were hungry enough! Close examination of tracemaking carnivores is desirable for ex- In the following years, several empty turtle shells turned up in the tending our understanding of trace fossils in bone substrates. In particu- diorama. Most of them probably died of natural causes, and were prob- lar, this needs close investigation of carnivores and their prey in nature ably mainly emptied by their fellow turtles, but besides this cannibalism, and in captivity. This paper is based on careful observation of predatory crocodylian predation certainly also played a role, as evidenced by nu- attacks by Dwarf Caimans, Paleosuchus palpebrosus, on Red-eared Slid- merous bite traces on the shells. The act of predation and the technique ers, Trachemys scripta, and the resulting bite traces in the turtle cara- for crushing open the turtle shell was observed for the first time by JK in paces. 2009. The male caiman caught the turtle and manipulated it with the jaws 196

FIGURE 1. The Cretaceous exhibit on NaturBornholm with the three Dwarf FIGURE 2. Drawing of the “nutcracker” technique applied by the Caiman Caimans, Paleosuchus palpebrosus, and Red-eared Slider, Trachemys scripta, to crack open a carapace of the turtle by placing it in an upright position in an apparently peaceful co-existence. Photo by JK. betweens its jaws, causing the connection between the plastron and carapace to fail. Illustration by JK. into an upright position between its jaws, before applying pressure to the turtle shell (Fig. 2). This “nutcracker” technique caused the bridges DISCUSSION connecting the plastron and carapace to fail, so the Caiman could divide The caimans responsible for the act of predation are relatively the turtle in two and empty the shell. In this case, however, the predation small animals having a total length of about 120 cm, and the largest of the act was interrupted by JK. While the bony bridges connecting the cara- turtles they succeeded in opening had a carapace length of 17 cm, which pace and plastron of the turtle were fractured by the pressure from the is an admirable act of relative strength. In the wild, chelonians are not Caimans jaws, the turtle survived the attack and is today totally healed. part of the normal diet of dwarf caimans, while several other extant Currently, the amount of food available to the caimans has been balanced crocodylian species are known to prey on chelonians (Cott, 1961; so they neither become too overweight, nor hungry enough to attempt Magnusson et al., 1987, Rene Hedegaard, pers. comm. 2010). predation on the turtles. Only a few confirmed cases of crocodylian chelonivory have been DESCRIPTION OF BITE TRACES reported in the scientific literature. Concentrations of heavily etched and pitted fragments of turtle shell in the Late Cretaceous Laramie Formation To date, six turtles have fallen victim to predation from the dwarf of Colorado have been interpreted as the result of predation by the caimans, and of these, two carapaces notably show very prominent bite alligatoroid Brachychampsa, which commonly appear together with turtle traces from the act of predation. In the first example, the caiman has been remains (Carpenter and Lindsey 1980). Erickson (1984) described successful in cracking open a turtle with a carapace length of 17 cm, and crocodylian bite traces in chelonian shells from the Paleocene of Wannagan dividing it into plastron and carapace, as well as breaking parts off the Creek Quarry in Minnesota. These bite traces appear as rounded holes posterior end of the carapace (Fig. 3). The anterior part of the carapace and pits in the skeletal plates of the carapaces, some totally penetrating has numerous small, rounded to elliptical bite traces in the marginal and the bones, whereas those that had not penetrated the bone appear as pleural scutes. The posterior and left parts of the carapace have been rounded pits in the bones. Deposition of additional bone tissue around crushed, and several small, rounded and elongated bite traces are present some of these trace fossils suggests that in some cases the turtles sur- along fractures. Parts of the horny scutes are missing along the fractures, vived the predation attempt. exposing the skeletal plates below (Fig. 3A-B). The bite traces in the Karl and Tichy (2004) depict a complete turtle skeleton from the plastron are clustered into groups. One group of bite traces forms two Kimmeridgian (Upper ) of Eichstätt, Germany with abundant rows of evenly distributed, rounded bite lesions in both gular scutes and circular crocodylian bitemarks and crushings in the carapace. They note the left humeral, pectoral and abdominal scutes. A second cluster of bite that the dorsal parts of the neck vertebrae were torn off its corpus and traces is four elongated bites in the left femoral and anal scutes. Two that the carapace was destroyed by crushing at both sides of the body, rows of three rounded bite traces and a larger scratched area are distrib- which could be the result of the same predation technique observed in the uted on the right pectoral and abdominal scutes. The last cluster is four Dwarf Caimans of this study. parallel and closely situated scratches on the right humeral scute (Fig. Several large holes and perforations in turtle shells are described as 3C-D). predation from the giant Late Cretaceous crocodylian Deinosuchus In the second specimen, the caiman did not succeed in dividing the (Schwimmer, 2010). These trace fossils appear as rounded to circular, carapace and plastron, but instead it bit off the head and all the extremi- penetrating holes in the shells, or rounded pits in the bones where the ties of the turtle (Fig. 4). The carapace has abundant large and small bite bite did not completely penetrate the bone. traces in the anterior-left part. Small, rounded lesions are distributed None of the bite lesions described in this study has penetrated along the vertebral, pleural and marginal scutes, and a series of elongated through the bones, but several have gone through the horny scutes and scrapes radiates outwards from the vertebral scutes, some even continu- scratched the surface of the bones below, or formed small circular pits in ing down in the pleural scutes (Fig. 4A-B). The plastron has only a few the bone. This is probably due to the relatively small size of the predator but prominent bite traces. These form a row of straight traces oriented in relation to the thickness of the shell of the prey. However, in the cases perpendicular to the length of the plastron. The most prominent runs in which teeth have broken through the horny scutes and into the bones, almost the entire length of the two femoral scutes (Fig. 4C-D). Most of the traces would have a high preservation potential if the turtle was the recorded bites seem to have penetrated the horny scutes and left fossilized. The fossil examples of healed bite-inflicted injuries in the traces in the skeleton below. turtles (Erickson, 1984; Schwimmer, 2010), and the observations from 197

FIGURE 3. Partly crushed shell from a red-eared turtle separated into plastron and carapace by a dwarf caiman. A, The posterior end of the carapace has been fractured into several fragments bearing prominent bite traces. B, Sketch of the carapace with bite lesions indicated in gray. Darker shaded areas indicate exposed bone. C, Plastron of the same animal. D, Sketch of the plastron with bite marks indicated by gray. The names of the different scutes on the carapace and plastron are abbreviated as follows: V, vertebral scute; Pl, pleural scute; M, marginal scute; C, cervical scute; G, gular scute; H, humeral scute; Pe, pectoral scute; A, abdominal scute; An, anal scute. 198

FIGURE 4. Turtle shell with abundant bite traces from the caiman’s unsuccessful attempt to break it open. A, The carapace bears several prominent bite lesions and scrapings in the anterior-left part. B, Sketch of the carapace showing bite traces in gray. C, Plastron of the same animal showing long, prominent, roughly parallel scrapes from the caiman’s teeth. D, Sketch of the plastron with bites indicated by gray. Names of the scutes abbreviated as in Figure 3. 199 the interrupted predation act from this study, show that turtles are turtle into an upright position between the jaws and biting until the bony capable of surviving unsuccessful predation attempts from crocodylians, bridges that connect the carapace and the plastron are broken. The traces even if these have penetrated or fractured the shells. produced in the turtle shell by the act of predation comprise rounded puncture traces, drop-like traces and elongated traces originating from CONCLUSION the teeth scraping along the shell. An 120-cm-long dwarf caiman, Paleosuchus palpebrosus, with an ACKNOWLEDGMENTS estimated body weight of 7-10 kg, has been preying on Red-eared Slid- ers, Trachemys scripta, and is able to crack open carapaces up to 17 cm The research of JM is supported by the Danish Natural Science in length. The observed predation technique involved manipulating the Research Council. David R. Schwimmer and Bent E. K. Lindow pro- vided critical and constructive reviews of the manuscript.

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