Were mammals originally venomous? JØRN H. HURUM, ZHE−XI LUO, and ZOFIA KIELAN−JAWOROWSKA Hurum, J.H., Luo, Z−X., and Kielan−Jaworowska, Z. 2006. Were mammals originally venomous? Acta Palaeontologica Polonica 51 (1): 1–11. The extratarsal spur in extant monotremes consists of an os calcaris and a cornu calcaris. A poisonous extratarsal spur oc− curs only in the platypus (Ornithorhynchus); a possibly secondarily non−poisonous spur is present in echidnas (Tachy− glossus and Zaglossus). Some therian mammals (e.g., bats), reptiles (Chamaeleo), and amphibians have a spur−like struc− ture in the ankle, but this is not homologous to the extratarsal spur of monotremes. Among fossil mammals, the co−ossified os calcaris and ossified cornu calcaris have been found in the eutriconodontan Gobiconodon and in the spalacotheroid “symmetrodontan” Zhangheotherium. Here we describe the os calcaris in several multituberculate mammals from the Late Cretaceous of the Gobi Desert, Mongolia. The multituberculate os calcaris is a large, flat bone, generally similar to that in males of the extant monotreme species, but the cornu calcaris is not ossified. In Gobiconodon and Zhangheotherium the os− sified cornu calcaris is fused to the os calcaris probably to provide the bony support for the keratinous spur. We hypothesize that the os calcaris in these Mesozoic mammal groups is homologous to that of monotremes. However, the extratarsal spur has not been found in non−mammalian cynodonts nor in other synapsids. A platypus−like os calcaris might be an apomorphic characteristic of basal Mesozoic mammals and is secondarily lost in crown therians; the os calcaris is con− firmed to be absent in well−preserved tarsal structures of the earliest known crown therian mammals. We speculate that the os calcaris, the cornu calcaris, and its associated venom gland might have served the function of a defensive structure dur− ing the “dark ages” of mammalian history, when dinosaurs ruled the Earth. This structure is a plesiomorphic character retained in extant monotremes and cannot be used as an autapomorphy of Monotremata. Key words: Multituberculata, Monotremata, Gobiconodon, Zhangheotherium, os calcaris, cornu calcaris, extratarsal spur. Jørn H. Hurum [j.h.hurum@nhm.uio.no], Naturhistorisk Museum, Boks 1172 Blindern, N−0318 Oslo, Norway; Zhe−Xi Luo [luoz@carnegiemnh.org], Section of Vertebrate Paleontology, Carnegie Museum of Natural History, Pitts− burgh, PA 15213, USA; Zofia Kielan−Jaworowska [zkielan@twarda.pan.pl], Instytut Paleobiologii PAN, ul. Twarda 51/55, PL−00−818 War− szawa, Poland. Introduction Various tetrapods have developed a venom delivery system for killing prey and for defense. Most venomous structures are associated with the mouth (Bücherl et al. 1968; Fox and Scott 2005), but a poisonous spur can also be developed on the hind legs. The best known example in extant mammals is the extratarsal spur in monotremes, in which a venom deliv− ery structure is supported by the cornu calcaris and is associ− ated with the os calcaris of the tarsus. Several other tetrapods have bony projections from the tarsus that superficially re− semble the os calcaris of monotremes, but they are not part of a venom apparatus. For example a spur−like prehallux is seen in some placentals and marsupials, in articulation with navi− culare (Emery 1901; Lewis 1964, 1989); many microchiro− pteran bats are known to have a calcar bone, a slender carti− laginous or bony spur that projects from the calcaneus for supporting the interfemoral part of the wing membrane (Schutt and Simmons 1998); in reptiles, males of Chamaeleo calyptratus have a tarsal spur at hatching (Schmidt 1999); and in amphibians the toad Rhinophrynus dorsalis has a large tarsal spur (Mivart 1874). Acta Palaeontol. Pol. 51 (1): 1–11, 2006 Since the discovery of the platypus—Ornithorhynchus anatinus (Shaw, 1799), in the late eighteenth century (Moyal 2001), its venom−conducting spur has caught the attention of many naturalists. Even Charles Darwin in The Descent of Man (1881: 502) commented on it: “[...] the adult male ornitho− rhynchus is provided with a remarkable apparatus, namely a spur on the foreleg, closely resembling the poison−fang of a venomous snake; but according to Harting, the secretion from the gland is not poisonous; and on the leg of the female there is a hollow, apparently for the reception of the spur.” Of course, the spur is in fact on the hind limb in all male monotremes and is part of a venom delivery system in the platypus (Ornitho− rhynchus), but not in the echidna (Tachyglossus), see Grassé (1955) and Griffiths (1968, 1978). Jenkins and Schaff (1988) were the first to describe an extratarsal spur in a Mesozoic mammal. They referred an iso− lated extratarsal spur to the eutriconodontan Gobiconodon ostromi from the Early Cretaceous Cloverly Formation of Montana (MCZ 19860). The next extratarsal spur of a fossil mammal was discovered by Hu et al. (1997) in the spalaco− theroid “symmetrodontan” Zhangheotherium from the Lower Cretaceous Yixian Formation of Liaoning Province, China http://app.pan.pl/acta51/app51−001.pdf 2 ACTA PALAEONTOLOGICA POLONICA 51 (1), 2006 (IVPP V7466). Hu et al. (1997: 140, fig. 1) designated the small L−shaped bone, preserved at the distal end of the fibula on the left side of the specimen, as “an external pedal spur”. In this paper, we will re−examine the anatomical charac− teristics of the extratarsal spur in extant monotremes, de− scribe the extratarsal spur, or its component, the os calcaris in multituberculates, and then examine the comparative mor− phology of this structure in Mesozoic mammals. We follow the transcription of Mongolian names pro− posed by Benton (2000). Institutional abbreviations.—AMNH, American Museum of Natural History, New York, USA; CAGS, Chinese Academy of Geological Sciences (Institute of Geology), Nanking, China; CMNH, Carnegie Museum, Pittsburgh, USA; NGMC−GMV, National Geological Museum of China, Beijing; China; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Academia Sinica, Beijing, China; MCZ, Museum of Compara− tive Zoology, Harvard University, Cambridge, Mass., USA; PM, Paleontological Center of the Mongolian Academy of Sci− ences, Ulaanbaatar, Mongolia; ZMO, Zoological Museum, University of Oslo, Norway; ZPAL, Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland. cornu calcaris cornu - os junction os calcaris 5 mm Fig 1. Basic structure of the extratarsal spur in monotremes (based on cam− era lucida drawing of the Recent platypus Ornithorhynchus anatinus (Shaw, 1799), AMNH 77856, male, left side posteromedial view. For ori− entation of the spur to the pes, see Fig. 3A. Structure of the extratarsal spur in monotremes distal tibial malleolus dorsal In the tarsus of monotremes, there occur two supernumerary bones, the os calcaris and os tibiale; Meckel (1826), in his monograph on Ornithorhynchus, confused them. The confu− sion lasted for almost a century (e.g., Manners−Smith 1894) and Leche (1900) even homologized the two bones. When Emery (1901) studied the hand and foot of Tachyglossus aculeatus (then referred to as Echidna hystrix) and its embry− ological development, he demonstrated that the os calcaris (“Spornknochen” in his terminology) is developed later in embryogenesis than the os tibiale “[…] der Spornknochen ein Hautknochen ist und aus Bindegewebe verknöchert” (Emery 1901: 673), and that the os tibiale is developed to− gether with the other tarsal bones. Lewis (1963) followed Emery and recognized the two bones as separate units. The main argument of Lewis was the attachment of the musculus tibialis posterior to the os tibiale; the muscle continues be− yond the astragalus in Tachyglossus and Ornithorhynchus. Lewis (1964: 198) described the os calcaris in Ornitho− rhynchus as a “[...] flat bony mass formed about the base of the horny perforated spur which conveys to the exterior the secretion of the femoral (poison) gland.” The os calcaris is at− tached to the astragalus by a ligament and articulates through a small synovial joint with the tibia. Between these attach− ments, the os calcaris bridges over the tendons of two mus− cles, the tibialis posterior and the flexor tibialis, as they enter the foot. The tendon of the tibialis posterior has a small inser− tion on the os calcaris in Ornithorhynchus. This was also ob− served and well illustrated in Manners−Smith (1894: 708) and in Lewis’s (1964: text−fig. 2) original drawing of the tar− sus and metatarsus of Ornithorhynchus anatinus in plantar tibia tibia os calcaris lateral astragalus os calcaris astragalus contact for cornu calcaris calcaneus contact for cornu calcaris calcaneus 10 mm Fig. 2. Articulation of the os calcaris to the surrounding bones in the Recent Ornithorhynchus anatinus (Shaw, 1799), AMNH 65833; male, left ankle joint. A. Posterolateral view. B. Posterior view. The cornu calcaris has been removed and the partially disarticulated ankle joint has exposed the contact relationship of the os calcaris to the distal tibial malleolus and to the astragalus. view, with the os calcaris bearing the horny spur lifted away from its site of articulation with the astragalus (talus) and the distal part of tibia. This drawing has been subsequently re−illustrated in several text−books and papers. In this paper, we accept the terminology of Lewis (1963, 1964) with some additions. The os calcaris is connected to the cornu calcaris or the core of the spur itself and has a bone−to− bone contact to the astragalus and the distal end of the tibia near the junction of the two bones (see discussion below on how to distinguish the os tibiale from os calcaris in fossils). HURUM ET AL.—WERE MAMMALS ORIGINALLY VENOMOUS? 3 Fig. 3. Os calcaris in the Recent monotremes. A. Partial left tarsus of Ornithorhynchus anatinus (Shaw, 1799), ZMO 11793, an adult male. Cornu calcaris and its keratinous sheath and os calcaris removed from the tarsus (A1); tarsus partly dissected (A2); isolated os calcaris of the same specimen, stereo−photo of the medial side showing the concave inner side (A3); isolated os calcaris of the same specimen, stereo−photo of the lateral side showing rough surface (A4). B. Left tarsus of Tachyglossus aculeatus (Shaw, 1792), ZPAL Mw−3 in plantar view with os calcaris preserved (B1), and explanatory drawing of the same (B2). Roman numerals I–V denote the fingers. C. Stereo−photo of lateral side of disarticulated os calcaris from the right tarsus of the same individual. The second confusion in terminology is the use of the multiple names: spur, tarsal spur, extratarsal spur, external pedal spur, and os calcaris for the same bony structure. We use the term os calcaris for the plate−like bony base in articu− lation with the astragalus or calcaneus, and sometimes the distal end of the tibia (here some intraspecific variation is ob− served in monotremes); while the term cornu calcaris (sensu Gregory 1947: 36; horny spur of Calaby 1968: 21) is used for the keratinous spur; and the bony core structure inside the keratinous spur is termed ossified cornu calcaris. Finally, we use the term extratarsal spur for the entire structure made up of the os calcaris (the base), the ossified cornu calcaris (the core), plus cornu calcaris, the keratinous spur (Fig. 1). The distinction between these structures of the extratarsal spur of modern monotremes will have important implications for the comparative studies of the similar structures in Mesozoic mammals, as one component, the base of the extratarsal spur tends to be more often preserved than the other two. http://app.pan.pl/acta51/app51−001.pdf 4 In fossil mammals, the most frequently preserved struc− ture is the os calcaris, or the plate−like bony base of the extratarsal spur. The ossified cornu calcaris is co−ossified with the os calcaris in some Mesozoic mammals (see below). The keratinous sheath of the spur is usually not preserved but may be seen as a part of an impression in Aikidolestes (Li and Luo 2006), a rare case of soft−tissue preservation in the Yixian Formation of China. We also point out that the extra− tarsal spur is a sexually dimorphic feature in modern mono− tremes; the keratinous spur is absent in adult female platypus (Calaby 1968; Griffiths 1968, 1978) and vestigial or absent in adult female echidnas. Griffiths (1978: 25) stated: “As in Ornithorhynchus, the inside of the ankle [of Tachyglossus] in all males bears a hollow perforated spur only 0.5–1.0 cm long; juvenile females also can exhibit a small sharp spur which is lost later in life; thus if an echidna lacks a spur on the ankle it is certainly a female.” With respect to Zaglossus Griffiths (1978: 75) wrote: “Of eight Zaglossus specimens I have examined and whose sex was determined by dissec− tion, six were males and had spurs on the ankle. Of the fe− males 7.4 kg and 4.5 kg in weight respectively, the smaller one, which had never bred [...] had spurs; this animal when first examined was deemed to be a male but dissection proved it to be a female. The other female, the larger one, had bred and she had not spurs. This suggests that juvenile fe− male Zaglossus have spurs just as some juvenile Tachy− glossus females do.” It follows that the distribution of this feature may be under represented in fossil taxa due to its sexually dimorphic nature. The os calcaris is known to occur in adult males of all three species of living monotremes. The keratinous spur oc− curs in young females of platypuses and then disappears in adults, and the bony os calcaris possibly disappears as well. During the work on this paper, we studied the tarsi of more than a dozen specimens of Ornithorhynchus anatinus and Tachyglossus aculeatus belonging to both sexes at various ontogenetic stages, and we illustrate some of them. The os calcaris of Ornithorhynchus (Figs. 2 and 3A) is a flat, and plate−like bone. It is situated below and lateral to the horny spur. It is attached to the posterior aspect of the astragalus and the distal tibial malleolus by a ligament. The side facing the astragalus (the inner side) tends to be slightly concave and corrugated. The external surface tends to be un− dulating and has a deep central groove in some specimens (Fig. 3) or a slightly concave surface in others (Fig. 2). Its proximo−lateral margin is slightly convex, and its disto−me− dial margin tends to be slightly concave. The overall outline of this plate−like bone is variable with size. Smaller ossa calcares are bean−shaped. Slightly larger ossa calcares may have a more convex lateral border. The largest os calcaris ex− amined by us (AMNH 77856) is oblong in outline with a tri− angular margin (Figs. 1 and 2), but others are rectangular. The lateral (or proximo−lateral) margin is thickened, bulging and rugose (Fig. 2), for the attachment of the cornu calcaris. The cornu calcaris is attached to the os calcaris by a zone of cartilage, and the periphery of this junction is moderately or ACTA PALAEONTOLOGICA POLONICA 51 (1), 2006 heavily crenulated (Figs. 1 and 3A). The conical core of the spur is slightly curved and bent distal to its junction with the os calcaris, so it points posteriorly. The length of the cornu is intraspecifically variable. The cartilage junction between the os calcaris and the cornu calcaris could absorb some of the impact when the spur is forced into flesh. In Tachyglossus, the os calcaris is situated on the plantar side of the tarsus (Fig. 3B). It is a small bone of roughly rect− angular shape attached to the astragalus. The bone is placed close to the os tibiale and may easily be confused with that bone. The os tibiale is much smaller and more rounded than the os calcaris and is situated partly between the astragalus and naviculare (see e.g., Lewis 1963: fig. 2b; Gambaryan et al. 2002: fig 5c). As in Ornithorhynhus, the os calcaris is at− tached to the astragalus medially by a fibrous union. The me− dial surface is concave and smooth. In Ornithorhynchus, the os calcaris provides a relatively extensive support for the cornu calcaris, but in Tachyglossus the supporting function of os calcaris for the cornu calcaris is nearly lost, the bone be− ing tightly connected to the astragalus. Fig. 3B shows the po− sition of the os tibiale and os calcaris in the left tarsus of Tachyglossus. Fig. 3C shows the disarticulated right os cal− caris from the same individual. The bone is oriented with its lower margin being the surface close to the os tibiale in the articulated specimen. In summary, the size and outline of the os calcaris is vari− able in monotremes. The conical morphology of the cornu calcaris is more conserved but its length and curvature are variable. The attachment of the os calcaris to other tarsal bones is different between the echidnas and the platypus. From the practical purpose of recognizing the extratarsal elements (sep− arate or fused os calcaris and ossified cornu calcaris) in fossils, we also note that in some taxa, the os calcaris and ossified cornu calcaris tend to be fused, such that the entire extratarsal elements would be preserved as one fossilized extratarsal structure (see below). The os tibiale in living monotremes is a sesamoid bone associated with m. tibialis posterior. If present at all, the os tibiale tends to be smaller and more rounded, with a more anterior and more plantar position, than the os calcaris. Hence, if these two supernumerary bones in the tarsus are preserved in fossils, it is possible to distinguish them. Os calcaris in multituberculates In all three multituberculates described below, we found only the os calcaris part of the extratarsal spur. If an ossified cornu calcaris was present it was not co−ossified with the os calcaris. Catopsbaatar catopsaloides (Kielan−Jaworowska, 1974) (Fig. 4A).—The os calcaris is preserved in situ on the left pes of PM 120/107 from the Hermiin Tsav locality, the Upper Cretaceous red beds of Khermiin Tsav (stratigraphic equiva− lent of the Baruungoyot Formation), Gobi Desert, Mongolia. This skeleton of Catopsbaatar received a preliminary de− scription (Kielan−Jaworowska et al. 2002), while its skull has been described in detail by Kielan−Jaworowska et al. (2005). HURUM ET AL.—WERE MAMMALS ORIGINALLY VENOMOUS? 5 calcaneus os calcaris peroneal groove astragalus naviculare ectocuneiforme mesocuneiforme III I cuboideum IV II ectocuneiforme 10 mm Fig. 4. Os calcaris in multituberculates. Left tarsus of Catopsbaatar catopsaloides, PM 120/107. A. Stereo−photo in dorsal view. B. Explanatory drawing of the same. The V finger, preserved separately, has not been reconstructed. Upper Cretaceous, red beds of Hermiin Tsav (equivalent of the ?upper Campanian Baruungoyot Formation), Hermiin Tsav II, Gobi Desert, Mongolia. We provide here information on the os calcaris of this speci− men. Fig. 4A shows the tarsus in dorsal and plantar view. Several tarsals of this multituberculate are slightly displaced. The metatarsals are all present in the specimen, but metatar− sal V is displaced and was found next to the knee joint of the left leg. The displaced metatarsal V is identified by its articu− lation surfaces for the cuboid and for calcaneus. The os calcaris might be correctly situated on the medial side of astragalus but there is a possibility that it has slightly moved from the plantar side of the metatarsus to the medial side. The bone itself is plate−like and its outline is rectangular in medial view. The medial surface is smooth except for a small tuber− cle. The placement of the os calcaris in the ankle of Catops− baatar is more medial than in Ornithorhynchus, but the extratarsal spur (cornu calcaris not preserved in fossil) would point medially in both taxa by comparison of the structural tibia fibula os calcaris medial malleolus of tibia astragalus calcaneus ectocuneiforme cuboid V 5 mm IV III Fig. 5. Os calcaris in multituberculates. A. Stereo−photograph of the right tarsus of Kryptobaatar dashzevegi, ZPAL MgM−I/41, in lateral view, showing partly exposed os calcaris, situated between the distal ends of the tibia and fibula. B. Stereo−photos of the left os calcaris (incomplete) of the same animal, in ?ventral (B1) and ?dorsal (B2) views. Upper Cretaceous Djadokhta Formation (?lower Campanian), Bayan Zak, Gobi Desert, Mongolia. http://app.pan.pl/acta51/app51−001.pdf 6 relationship between the os calcaris and cornu calcaris in Ornithorhynchus. Kryptobaatar dashzevegi Kielan−Jaworowska, 1970 (Fig. 5).—The os calcaris is preserved in the ankle joint of ZPAL MgM−I/41, from the Upper Cretaceous Djadokhta Forma− tion, of Bayan Zag, Gobi Desert, Mongolia. Kielan−Jawo− rowska and Gambaryan (1994: fig. 2) published a photo− graph of the ankle joint of this specimen but did not describe the os calcaris. Wible and Rougier (2000: 7) noted that the “tarsal spur” was present in this multituberculate. Here we provide additional description and photographic documenta− tion of the os calcaris. We interpret that the os calcaris was present in Krypto− baatar, but the ossified cornu calcaris was not co−ossified or not present and therefore missing from ZPAL MgM−I/41. In this skeleton (Fig. 5A), the os calcaris is preserved in its en− tirety on the right side of the specimen between the distal ends of the tibia, fibula, and astragalus. The right os calcaris of the same specimen has been pre− served between the distal part of the fibula and the astragalus (Fig. 5; also see fig. 2a of Kielan−Jaworowska and Gam− baryan 1994). It was impossible to prepare this bone from the surrounding matrix and bones without causing damage to other bones. Nonetheless, half of this bone is exposed and appears to be relatively complete; and it is plate−like with ei− ther an oblong or rectangular outline, with a pointed apex and a ventral process extending horizontally below the fibula, but it resembles very closely the complete os calcaris of Catops− baatar (Fig. 4). The left os calcaris of ZPAL MgM−I/41 has been dis− placed from the vicinity of the tarsus and is preserved close to the base of the epipubic bone (Kielan−Jaworowska and Gam− baryan 1994: fig. 2B). It was possible to remove the bone from the matrix. We interpret that the left os calcaris is not complete (Fig. 5B). As preserved, it is approximately half the size of the complete os calcaris on the right side. The pre− served portion is plate−like, and has a roughly triangular out− line, with rounded lower corners and slightly concave lower margin. In the right lower corner, there is an oblique wide furrow extending from the margin anteromedially (Fig. 5B). Chulsanbaatar vulgaris Kielan−Jaworowska, 1974.—An os calcaris, broken into two pieces, is preserved in ZPAL MgM−I/99b (not illustrated here). The partial left tarsus of Chulsanbaatar vulgaris was described and illustrated by Kielan−Jaworowska and Gambaryan (1994: fig. 25). We now interpret that a partial os calcaris is preserved in situ, and maintains its articulation with the astragalus/calcaneus. This structure was previously labeled as “?tibia” in Kielan−Jawo− rowska and Gambaryan (1994: fig. 25). Another part of this os calcaris is preserved in association with the distal end of the left fibula (Kielan−Jaworowska and Gambaryan 1994: fig. 17A). The bone can be seen in medial and plantar views but it is very fragmented and partially covered with glue and sediments; hence, the rest of its outline is difficult to recon− struct from the two preserved fragments. ACTA PALAEONTOLOGICA POLONICA 51 (1), 2006 Not all multituberculates with well−preserved ankles have shown an extratarsal structure. For example, this structure is absent in the ankle of the only known postcranial specimen of Sinobaatar (Hu and Wang 2002). We were able to confirm the original observation that the extratarsal spur is also ab− sent in the specimens so far known of the Tertiary multi− tuberculate Ptilodus (Krause and Jenkins 1983). Extratarsal spur in eutriconodontans Gobiconodon ostromi Jenkins and Schaff, 1988 (Fig. 6).— Jenkins and Schaff (1988: 18) described the extratarsal spur (referred to as a “spur−like element”) of the eutriconodontan Gobiconodon ostromi as follows: “The base is an oval nod− ule, rounded on the external surface that bears the spur; the internal surface is flat, except along the edge adjacent to the spur, which is raised in the form of a small process. The spur is set eccentrically on the external surface, is constricted about its base, and tapers toward the apex, which is broken off.” (See also Jenkins and Schaff 1988: fig. 18). We supplement their description with more detail photo− graphs of a cast of the extratarsal spur (Fig. 6). We interpret the oval base or plate to be the equivalent of os calcaris. The “spur” itself is the ossified cornu calcaris, and the slight con− 5 mm 5 mm ossified cornu calcaris cornu os junction os calcaris Fig. 6. Extratarsal spur in the eutriconodontan Gobiconodon. A. Ste− reo−photos of a cast of os calcaris of Gobiconodon ostromi, MCZ 19860 (see also Jenkins and Schaff 1988). Lateral extratarsal spur in dorsal view (A1). The same from the apical view of the ossified cornu calcaris (A2). The “under” or “inner” surface of the os calcaris of the same (A3). B. Illustration from camera lucida drawing of the extratarsal spur in ventral (B1) and dorsal view (B2). Lower Cretaceous, Cloverly Formation, Montana. HURUM ET AL.—WERE MAMMALS ORIGINALLY VENOMOUS? striction between the cornu calcaris and the ossified os cal− caris represents the fused junction of these two structures (Fig. 6). Therefore, the “spur−like element” as recognized by Jenkins and Schaff represents the entire extratarsal spur, mi− nus the keratinous sheath on the outer surface of the cornu. The ankle joints of the eutriconodontan Jeholodens jen− kinsi (Ji et al. 1999) are well preserved in specimen NGMC− GMV 2139a; but the extratarsal spur or its components are not present in this fossil. We speculate that the absence of this fea− ture is due to the sexually dimorphic nature of this character. Specimens of the gobiconodontid Repenomamus have been found by the dozens (J.−L. Li et al. 2001; C.−K. Li et al. 2003; Hu et al. 2005), but not described in detail. It might be possible to test the frequency of this dimorphic structure among the in− dividuals of these species, or possibly use this feature to deter− mine the sexes of the relatively well preserved skeletal fossils of this group. Extratarsal spur in “symmetrodontans” Zhangheotherium quinquecuspidens Hu, Wang, Luo, and Liu, 1997 (Fig. 7).—Hu et al. (1997) reported the presence of “an external pedal spur”’ in IVPP V7466 from the Early Cre− taceous of Liaoning Province, China. The bone is shown sche− matically as an L−shaped structure in their fig. 1, which depicts the entire skeleton of the holotype. The “external pedal spur” of Zhangheotherium, as illustrated by Hu et al. (1997), is the equivalent to the bony extratarsal spur structure. The oblong base, os calcaris, and the large ossified cornu calcaris are the two arms of the “L−shaped” extratarsal spur (Fig. 7). The smaller and more pointed arm of the “L−shaped” structure is the ossified cornu calcaris. This structure is similar to that of Gobiconodon in the co−ossification of the cornu and os cal− caris. However, it should be noted that the extratarsal spur is not present in the ankle of a second Zhangheotherium speci− men (CAGS−IG99−07352; Luo and Ji 2005). The tarsals of this specimen are scattered as preserved. It is not clear if the absence of extratarsal spur (or os calcaris) is due to poor pres− ervation, or that this specimen is a female. 5 mm 7 Rougier et al. (2003) reported on a second zhangheo− theriid species, Maotherium sinensis (NGMC−97−4−15). Al− though the extratarsal spur was not described, this structure is preserved in both ankle joints in the holotype of Maotherium, as shown in the photographs (Rougier et al. 2003: plate II−D). The in situ preservation and the orientation of the ossified cornu calcaris are almost identical to the preserved extra− tarsal bony spur of IVPP V7466 (Zhangheotherium). Z. quin− quecuspidens and Maotherium sinensis demonstrate that the bony part of the extratarsal spur (os calcaris and ossified cornu calcaris) is a basic feature in acute−angled “symmetro− dotontans” and is a plesiomorphic feature in trechnotherians (sensu McKenna and Bell 1997; Luo et al. 2002; Kielan− Jaworowska et al. 2004). Discussion and conclusion Are the extratarsal spur structures seen in the different Meso− zoic mammals homologous with those of extant monotremes? The answer is clearly yes. However, the most frequently pre− served component of the extratarsal spur structure in fossil state, is its base of support, the os calcaris; the ossified cornu calcaris and its keratinous sheath are often not preserved as in the case of multituberculate specimens we have examined. But from the anatomical association of the os calcaris and cornu calcaris in living monotremes, we infer that the entire extratarsal spur structure possibly was present in multituber− culates. In Gobiconodon, the extratarsal spur can be recognized and it consists of the base (os calcaris) and a bony inner sup− port for a spur (ossified cornu calcaris), but the horny sheath of the extratarsal spur is not present. The extratarsal spur in Zhangheotherium closely resembles that in Gobiconodon. The flat rectangular bones found in multituberculates have only tiny ridges on the surface, suggesting that they are re− duced compared to those of Zhangheotherium and Gobi− conodon. The bones lack the bony inner support for a spur fibula tibia os calcaris ossified cornu calcaris ossified cornu calcaris os calcaris tuber calcanei 1 mm Fig. 7. Extratarsal spur of Zhangheotherium quinquecuspidens. A. Stereo−photo of a silicone cast of the tarsus and extratarsal spur of Zhangheotherium, IVPP V7466 (see also Hu et al. 1997). B. Interpretive drawing of the structure of extratarsal spur (reconstruction from IVPP V7466 and a CAGS specimen from Luo and Ji, 2005). Lower Cretaceous, Yixian Formation, Liaoning Province, China. http://app.pan.pl/acta51/app51−001.pdf 8 ACTA PALAEONTOLOGICA POLONICA 51 (1), 2006 lower jaws of Zhangheotherium 10 mm lower jaw of Sinobaatar 0.1 m Fig. 8. Two mammals (represented by three specimens) in a stomach of a small carnivorous dinosaur. A. Sinosauropteryx prima (GMV 2124), entire speci− men. B. Enlarged abdominal contents in the pelvic area of the same. Lower Cretaceous, Yixian Formation, Liaoning Province, China. and appear to have served only as a base for a keratinous sheat. The placement of the os calcaris in Catopsbataar is more medial than in Ornithorhynchus, but the spur would have pointed medially as in the monotremes. In Gobiconodon and Zhangheotherium the spur is not only supported at its base by the os calcaris, as is seen in mono− tremes and inferred in multituberculates, but a bony spur is present. In monotremes the spurs are hollow and therefore may be used to inject poison. The bony spur of Gobiconodon has probably been sheathed by a keratinous spur, but a groove for a poison canal is not seen in the bony spur. This may have limited the function of the spur, suggesting that it may have been non−venomous. The same may be inferred from the small L−shaped bone in Zhangheotherium. It is difficult at present to venture an opinion whether this is a secondary loss of a venom delivery system, as seen in Tachyglossus, or whether the an− cestral condition would be non−venomous. Beginning with their appearance in the Carnian (about 225 MA ago) until the end of the Cretaceous (65.5 MA ago), mam− mals were almost exclusively small creatures of probable noc− turnal habits (Jerison 1973; Kielan−Jaworowska et al. 2004). With exception of Early Cretaceous Repenomamus, few Me− sozoic mammals attained the size of a fox. Most Mesozoic mammals were probably too small to serve as prey for large theropod dinosaurs; but some small−size dinosaurs (troodon− tids, dromaeosaurids, and oviraptorosaurians), large lizards, crocodiles, birds, and even large sphenodontians could well have preyed on them. Some of the large mammals, such as the large individuals of the stem mammal Sinoconodon in the Early Jurassic (Crompton and Sun 1985; Crompton and Luo 1993), the gobiconodontid Repenomamus of the Early Creta− ceous (J.−L. Li et al. 2001; Hu et al. 2005), and the marsupial Didelphodon of the Late Cretaceous (Clemens 1966) could also have preyed on smaller mammals. One of us (JHH) found that in the abdomen of a specimen (CAGS GMV 2124) of the feathered dinosaur Sinosauropteryx prima, three lower jaws of mammals have been preserved (Fig. 8), rather than two as previously mentioned by Ackerman (1998). Two of them belong to Zhangheotherium, the third to the multituberculate Sinobaatar (Hu and Wang 2002). This finding unequivocally demonstrates that small Mesozoic mam− mals were prey for larger carnivorous vertebrates. For these early mammals that were small, the presence of an extratarsal spur could serve as a defensive weapon. If it was also part of a venom delivery system it would have been doubly effective. In the venom delivery system of the platy− eutherians metatherians ?"eupantotherians" "symmetrodontans" Zhangheotherium multituberculates Catopsbaatar Kryptobaatar ?morganucodontans docodontans australosphenidans Ornithorhynchus Tachyglossus eutriconodontans Gobiconodon pus, the spur is connected to the venom−secreting femoral gland. The spurs are situated on the inner side of the tarsus, and the left and right spurs are directed towards one another. Calaby (1968: 26) described the use by platypuses of their poisonous spur on man, as follows: “When attacking, the platypus drives the hind legs toward one another with consid− erable force so that the spurs are embedded in the flesh caught between [in case of man, usually the hand or wrist]. In at least some cases difficulty was experienced in forcing the legs of the animal apart so that the victim could be released.” Undoubtedly, the extratarsal structure is useful for defending against natural enemies. It has also been documented that the apparatus was employed to attack other male platypuses and possibly for hunting. To date, the bony extratarsal spur or its os calcaris have been described only in three groups of Mesozoic mammals: eutriconodontans, “symmetrodontans” and multitubercula− tes. This structure is most likely a characteristic of the entire monotreme group (Fig. 9). The presence of an extratarsal spur is not a synapomorphy of crown mammals as indicated by distribution of this feature in the phylogeny of major Me− sozoic mammal groups (Luo et al. 2002; Kielan−Jaworowska et al. 2004). Although tarsals are not preserved in the Late Ju− rassic docodontan Haldanodon (Krsuat 1991; Martin 2005), the os calcaris is now known from a new docodontan from China (Luo, personal observation 2005). Presence of the extratarsal spur in morganucodontans (Jenkins and Parring− ton 1976) has not been demonstrated as yet since the tarsals are incompletely known in this group. Modern marsupials and placentals clearly have lost the extratarsal spur. So far, this feature has not been reported in any Cretaceous eutherians (Kielan−Jaworowska 1977, 1978; Novacek et al. 1997; Horovitz 2000; Ji et al. 2002). It is also true that this structure is absent from the relatively complete ankles as known for early marsupials and stem metatherians (Muizon 1995; 1998; Szalay and Trofimov 1996; Luo et al 2003). It follows that the extratarsal spur (and possibly its as− sociated venom gland) is absent from therian mammals. The condition of the extratarsal spur is unknown in major Mesozoic mammalian groups more derived than spalaco− therioids but more primitive than crown therians. The ankle joint as known for the dryolestoid Henkelotherium is not com− plete (Krebs 1991; Vázquez−Molinero et al. 2001; Vázquez− Molinero 2003). The astragalus and calcaneus of Vincelestes have been very well described, but it is not known if any su− pernumerary tarsal elements are present in this taxon (Rougier 1993). In light of the above−discussed data, we put forward the hypothesis that the extratarsal spur is a basic feature of Mammalia and was retained in all lineages of basal Mesozoic mammals. Its main adaptive function is for defense, possibly made more effective by being associated with a venom gland. This structure is also useful to a lesser extent for intra−specific competition or predation. It is conceivable that this feature originated earlier than crown Mammalia. Within Mammalia, the extratarsal spur is secondarily lost in crown 9 tritylodontids HURUM ET AL.—WERE MAMMALS ORIGINALLY VENOMOUS? loss of extratarsal spur (os and cornu calcares) acqusition of extratarsal spur (os and cornu calcares) Fig 9. Tentative evolutionary pattern of extratarsal spur of the main groups of early mammals (cladogram simplified from Luo et al. 2002 and Kielan− Jaworowska et al. 2004). Theria, if not earlier in the precursors to crown therians (Fig. 9). The extratarsal spur in extant monotremes is a sym− plesiomorphic character and cannot be used to characterize the Monotremata as a monophyletic group as has been done repeatedly by some neontologists (e.g., Ax 1984; Sudhaus and Rehfeld 1992). This character shows that the reconstruc− tion of phylogeny based only on evidence from living mam− mals can be misleading. Acknowledgements We thank Chuan−Kui Li and Yao−Ming Hu (Institute of Vertebrate Pa− leontology and Paleoanthropology, Beijing) for providing a cast of Zhangheotherium, figured by us. Our colleague Richard L. Cifelli (University of Oklahoma, Norman) read the first draft of this paper and discussed it with us, while the journal’s reviewers Richard C. Fox (Uni− versity of Alberta, Canada), Inés Horovitz (University of California, Los Angeles), and Thomas Martin (Forschungsinstitut Senckenberg, Frankfurt am Main) provided many useful comments; we especially benefited from meticulous corrections provided by Richard C. Fox. The photographs in Figs. 3, 4, and 5B have been taken by Per Aas (Natural History Museum, University of Oslo), that in Fig. 5A by the late Maria Czarnocka (Institute of Paleobiology, Warsaw), while Arild Hagen (NRK, Norwegian National Broadcasting) kindly allowed us to use his excellent photos of Sinosauropteryx in Fig. 8. The work of JHH has been funded by the University of Oslo, that of Z−XL by the National Science Foundation (USA), National Natural Science Foundation (China), National Geographic Society, and the Carnegie Museum of Natural History, Pittsburgh, that of ZK−J by the Institute of Paleo− biology, Polish Academy of Sciences, Warsaw. Z−XL is grateful to Qiang Ji (Institute of Geology, Chinese Academy of Geological Sci− ences, and formerly at National Geological Museum of China, Beijing) for access to comparative materials of Zhangheotherium, Maotherium, and gobiconodontids, Nancy B. Simmons for a visiting study of the col− lection of monotremes at the American Museum of Natural History, Farish A. Jenkins and Charles R. Schaff (Museum of Comparative Zo− http://app.pan.pl/acta51/app51−001.pdf 10 ology, Harvard University, Cambridge, Mass.) for access to study the Gobiconodon fossil, David Krause (State University of New York, Stony Brook) for an opportunity to study the ankle structure of Ptilo− dus, and John R. Wible and Susanne McLaren (Carnegie Museum of Natural History, Pittsburgh) for the use of the Carnegie Museum collection of extant mammals. To all these persons and institutions we express our sincere thanks and gratitude. References Ackerman, J. 1998. “Dinosaurs Take Wing”. National Geographic 194 (1): 189–192. Ax, P. 1984. Das Phylogenetische System. Systematisierung der lebenden Natur aufgrund ihrer Phylogenese. 349 pp. Gustav Fischer Verlag, Stuttgart. Benton, J. M. 2000. Mongolian place names and stratigraphic terms. In: M.J. Benton, M.A., Shishkin, D.M. Unwin, and E.N. Kurochkin (eds.), The Age of Dinosaurs in Russia and Mongolia, xxii–xxviii. Cambridge Uni− versity Press, Cambridge. 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