The Enigmatic Birdlike Dinosaur Avimimus portentosus Comments and a Pictorial Atlas

PATRICIA VICKERS-RICH, LUIS M. CHIAPPE, AND SERGEI KURZANOV

|3pa The phylogenetic position of Avimimus is a

^Siijfc^- puzzle that is yet to be solved. Since its an-^ is* nouncement by Kurzanov in 1981, Avimimus * '*f has been alternatively regarded as a nonavian theropod close to the ancestry of birds (e.g., Thulborn, 1984; Norman, 1990; Holtz, 1994; Currie, 2000) or as a flightless, basal avian (e.g., Chatterjee, 1991, 1995, 1999). It is hoped that the comments and the high-quality stereophotographs provided in this short chapter (see Figs. 3.1-3.19) will yield some clues that will eventually help the relationships of Avimimus to be decoded.

Avimimus portentosus was first described by Kurzanov (1981) on the basis of three partial specimens from the Upper Cretaceous (Campanian; see Jerzykiewicz and Russell, 1991) Barun Goyot ("Burungoyotskaya" Svita) and Djadohkta ("Djadochtinskaya" Svita) Formations of Mongolia Om-nogovi and Ovorkhangai Aimaks, respectively) (Weis-hampel, 1990). Kurzanov (1982, 1983, 1985, 1987) published a number of subsequent studies on Avimimus, several of them translated into English (Kurzanov, 1982, 1983, 1985). The specimen illustrated in this chapter (PIN 3907/1, Pale-ontological Institute, Russian Academy of Sciences) is from Udan Sayr in the eastern part of the Mongolian Gobi Desert and was made available for our study as a result of the organization of The Great Russian Dinosaurs Exhibition (1993-1997), a joint venture between the Paleontological Institute (Moscow), the Monash Science Centre, and the Queen Victoria Museum and Art Gallery (the last two institutions in Australia) (Vickers-Rich and Rich, 1993).

Kurzanov's opinion about Avimimus in 1993 was that it was "a [nonavian] dinosaur that had features in parallel with birds, perhaps even feathers" (Kurzanov, 1993). Norman (1990:286) concluded that Avimimus was an "unusual [nonavian] theropod from the evidence of the published description possessing, as it does, a very distinctive mixture of features, some of which are seen either in other groups of [nonavian] dinosaurs or in birds." Norman (1990) listed a few salient points: (1) The premaxilla (Fig. 3.10b) is reminiscent of lambeosaurine hadrosaurids in that it has a crenulate margin. (2) The braincase (Fig. 3.2) resembles that of birds and sauropods. (3) The vertebral column is typical for small, nonavian theropods, with the exception of several cervicals in the holotype specimen being radically different (notably, the lack of pneumatic foramina on the centra and the presence of large ventral processes—which are more commonly seen in birds more advanced than Archaeop-teryx; see Chiappe et al., 1996). (4) The pelvis (Figs. 3.11, 3.12) is typical for nonavian theropods except for the unusual form of the iliac blades. (5) The hindlimb resembles that of nonavian theropods; the fusion of the distal end of the fibula with the tibia and proximal tarsals (Figs. 3.15, 3.16) occurs commonly in birds but occurs in some nonavian theropods as well. (6) The retention of the vestige of metatarsal V (Fig. 3.17) is unusual in nonavian theropods from the Late Cretaceous (although present in Vorona and Rahonavis, both birds from the Late Cretaceous of Madagascar [Forster et al., 1996, 1998, Chapter 12 in this volume], as well as the Late Cretaceous alvarezsaurids [Chiappe, Norell, and Clark, Chapter 4 in this volume]). (7) The fore-limb is most puzzling; the humerus (Fig. 3.9) is typical for a bipedal, nonavian dinosaur; the metacarpal fragment (Fig. 3.10d-f) is similar to that of a bird.

Norman (1990:287) further suggested that "evidence can be brought forward to suggest [nonavian] theropod, sauro-pod, ornithopod, and avian affinities with some of the individual remains in this apparently associated material." Norman (1990:287) also pointed out that "using Gauthier (1986) as a guide it is clear that Avimimus displays some basal theropodan characters: spur-like metatarsal V, enlarged preacetabular portion of the ilium and large brevis fossa, and bowed femoral shaft and a fibular attachment crest on the lateral surface of the tibia. It also exhibits a small number of tetanuran and ornithomimid characters and one maniraptoran feature (development of hypapophyses on cervical vertebrae). However, it seems clear that our knowledge of this form is insufficient to establish precise relations with currently recognized higher taxa." Avimimus was also considered as a nonavian theropod by several other authors. Thulborn (1984) placed it as the sister group of his "Aves," which he used to name a group formed by Enantiornithes and other more advanced birds (a clade equivalent to Chi-appe's Ornithothoraces; see Chiappe, Chapter 20 in this volume). Thulborn (1984) cited the presence of an intercotylar prominence and a straplike coracoid as the evidence supporting his proposed sister-group relationship. This support, however, is clearly weak at best, since Avimimus lacks an intercotylar prominence and its straplike coracoid appears to be of another taxon, something even Thulborn (1984) himself admitted.

In a discussion about the origin of birds, Molnar (1985) implicitly regarded Avimimus as a nonavian theropod, although he did not discuss its precise placement within the phylogeny of theropods. Paul (1988) followed Thulborn (1984) in placing Avimimus as the sister group of a clade equivalent to Chiappe's Ornithothoraces (see Chiappe, Chapter 20 in this volume). Paul also followed Thulborn (1984) in placing Archaeopteryx in a more basal position than dromaeosaurids, troodontids, and other nonavian theropod taxa. In Paul's (1988) opinion, Avimimus was secondarily flightless, a form derived from volant ancestors. Paul (1988) regarded Avimimus as the most birdlike of all nonavian theropods and allocated it within his "proto-birds," as the sister group of his "birds." The intimate relation between Avimimus and the origin of birds proposed by Thulborn (1984) and Paul (1988) was dismissed by Holtz (1994, 1996), who placed it within his Arctometatarsalia, not directly related to the origin of birds (see also Padian, 1997).

Even though, from a strict phylogenetic approach, Thul-born (1984) and Paul (1988) regarded Avimimus as a member of Aves (the common ancestor of Archaeopteryx and ne-ornithine birds plus all its descendants), it is clear that this was not their intention. Chatterjee (1991, 1995), however, considered it to be a true, but flightless, bird. In 1991, Chat-terjee placed it as the sister taxon of a group composed of Protoavis and Ornithurae (hesperornithiforms, ichthyor-nithiforms, and neornithine birds). Among the apomor-phies cited by Chatterjee (1991) for Avimimus is the presence of teeth in the premaxilla and their absence in the maxilla. Yet, as pointed out by Kurzanov (1987) and Norman (1990) and confirmed by observations of one of us (Chiappe), the premaxilla of Avimimus lacks teeth, and the maxilla is not known for any specimen. (This fact appears to have been noticed by Chatterjee [1997], who mentioned the presence of "toothlike denticles" in the premaxilla of Avimimus.) Subsequently, Chatterjee (1995) supported the sister-group relationship of Avimimus and Mononykus, allocating them within Aves as the sister group of Ornithothoraces (see Chiappe, 1996, for the phylogenetic definition of this clade). Chatterjee (1995) cited the presence of a free orbital process of the quadrate, its ventral condylar articulation with the pterygoid, a lateral cotyla for the quadratojugal, and the presence of three condyles in the distal end of the quadrate as synapomorphies uniting Avimimus and Mononykus. Yet Norman (1990) remarked on the fusion of the suspensorium, and Kurzanov's (1987) illustrations show neither the free orbital process nor the pterygoid condyle of the quadrate (Figs. 3.2, 3.3a). Furthermore, the quadrate does not have a lateral cotyla or three condyles on its mandibular articulation. In his 1997 book, Chatterjee was more cautious about the avian identity of Avimimus and discussed it within a section entitled "Dubious Flightless Birds," although he concluded that it "may represent a flightless bird" (1997:121). This conclusion was more strongly reaffirmed in his recent monograph on Protoavis (Chatterjee, 1999). In this paper, Chatterjee (1999) regarded Avimimus, and its alleged sister-taxon Mononykus, as the sister-group of Or-nithothoraces, although he did not include confucius-ornithids (see Chiappe et al. [1999] and Chiappe [Chapter 20 in this volume] for the phylogenetic position of these birds) in the cladistic analysis.

Chiappe made the following observations, complementary to Norman's and Kurzanov's descriptions, when viewing the specimen at the New Jersey State Museum (Trenton) when The Great Russian Dinosaur Exhibition (Vickers-Rich and Rich, 1993) was in residence there during 1996 and 1997.

An axial centrum is preserved. It is very small compared with the remaining vertebral elements. This condition resembles that of alvarezsaurids (see Chiappe, Norell, and Clark, Chapter 4 in this volume). The axis has a small pneumatic foramen on the centrum and a short odontoid process.

Several thoracic vertebrae have open neurocentral sutures, which suggests that this specimen was not fully grown (see Sereno and Novas, 1993; Brochu, 1996). Some, but not all, have one small pneumatic foramen (sometimes two together) on the lateral side of the centrum. This is more evident in the caudal thoracic centra. The cranial thoracic vertebrae have prominent ventral processes (Fig. 3.4)—much larger than those in Deinonychus (Ostrom, 1969). The cranial and midthoracic vertebrae have a compressed and ventrally keeled centra. The caudal thoracic centra are more rounded. All the articular facets are platy-coelous. The ratio between vertebral canal and centrum (in cranial view) is roughly 0.5, a condition typical for birds among theropods (see Chiappe, 1996). Hyposphene-hypantrum accessory articulations are well developed in the thoracic vertebrae.

The humerus (Fig. 3.9) appears to have a single condyle (as in Alvarezsauridae; see Chiappe, Norell, and Clark, Chapter 4 in this volume). There is neither a pneumatic fossa nor foramen. The head is flat when viewed proximally, and there is a very weak deltopectoral crest with a knoblike prominence toward the midshaft. The main planes of the proximal and distal ends are displaced by a fairly large angle.

The ulna is quite compressed (Fig. 3.10). The ulna was suggested to have quill knobs by Kurzanov (1987). Chiappe confirms that there are bumps on the caudal margin, but their function remains unclear. The proximal end is not very well preserved, but it does not seem to have two cotylae.

The carpometacarpus (as interpreted by Kurzanov [1987]) is a bone representing some sort of fusion of elements (Fig. 3.10). The semilunate carpal appears to be fused to metacarpals II and III in a cast displayed at the exhibition, but this is not so clear in the actual specimen.

The pubis has a large pubic foot (Fig. 3.12), primitively designed and not reduced cranially. It has a long pubic apron, compressed craniocaudally. The ventral surface of the pubic foot is flat with a central depression.

On the proximal end of the femur (Figs. 3.13, 3.14), the lesser trochanter has a globular-condylar appearance. The head does not have a fossa for the capital ligament. On the shaft, caudally and cranially, there is a prominent intermuscular line. The distal end, cranially, has no trace of a patellar groove. Interestingly, the caudal end has condyles that are connected below the popliteal fossa by a transverse ridge, a condition otherwise known only for birds among dinosaurs (see Chiappe, 1996). The lateral condyle is not distally projected, unlike that of alvarezsaurids (see Chi-appe, Norell, and Clark, Chapter 4 in this volume). The fibular condyle is rather large.

The tibiotarsus is a true tibiotarsus, including astragalus, calcaneum, and tibia (Figs. 3.15, 3.16). The fibular crest is quite short. The cnemial crest projects only slightly cranially, and in medial view it has a crescentic shape. Distally, the cal-caneum did not have a fossa for the articulation of the fibula, and it is uncertain whether the fibula reached the tarsals. Even though only a faint ridge and suture imply some portions of the ascending process, it is clear that it extended for roughly one-quarter of the length of the tibiotarsus.

The metatarsus (Figs. 3.17, 3.18) is fused proximally and includes the distal tarsals, as well. It has a metatarsal V, which is slender and small and runs along the caudal surface of metatarsal IV. There is no hypotarsus. Metatarsal IV is somewhat longer than metatarsal II. Metatarsal III is arc-tometatarsalian, yet it does not have the proximal expansion seen in tyrannosaurids or ornithomimids.

The pedal digits (Fig. 3.19) are short when compared with the metatarsus. The phalanges of digits IV and II are unique in that they are not flat ventrally but have a sharp ridge. Consequently, the cross section is oval, instead of triangular.

Final Comment

With these observations and the high-quality photographs made by Steve Morton (Monash University), it is hoped that this chapter will stimulate further research and discussion on this intriguing form. It should be kept in mind for further work on PIN 3907/1 that the association of all elements is not absolute, as at least three other taxa were found in the same concentration that yielded Avimimus (E. Kurochkin, pers. comm.). Yet a new articulated specimen from Udan Sayr, recently collected by the Mongolian-Japanese expeditions (M. P. Watabe, pers. comm.), supports the association of the holotypic hindlimb elements with the holotypic skull.

Avimimus Fossils
Figure 3.2. A. portentosus (PIN 3907/1). Stereo pairs ofthe skull in dorsal (a), ventral (b), occipital (c), rostral (d), and right lateral (e) view. Scale bar = 2 cm.
Figure 3.3. A. portentosus (PIN 3907/1). Stereo pairs of the skull in left lateral view (a). Stereo pairs of the caudal portion of the right mandible in dorsal (b), lateral (c), medial (d), and ventral (e) view. Scale bar = 2 cm.
Figure 3.4. A portentosus (PIN 3907/1). Stereo pairs of cervical (a-e) and thoracic vertebrae (f-o) in right lateral view (sequence follows Kurzanov [1982]). Scale bar = 3 cm.

Figure 3.5. A. portentosus (PIN 3907/1). Stereo pairs of cervical (a-e) and thoracic vertebrae (f-o) in left lateral view (sequence follows Kurzanov [1982]). Scale bar = 3 cm.

Figure 3.5. A. portentosus (PIN 3907/1). Stereo pairs of cervical (a-e) and thoracic vertebrae (f-o) in left lateral view (sequence follows Kurzanov [1982]). Scale bar = 3 cm.

Figure 3.6. A. portentosus (PIN 3907/1). Stereo pairs of thoracic (a-j) and cervical vertebrae (k-o) in cranial view. Scale bar = 3 cm.
Figure 3.7. A. portentosus (PIN 3907/1). Stereo pairs of thoracic (a-j) and cervical vertebrae (k-o) in caudal view. Scale bar = 3 cm.
Dinosaur Lesser Trochanter
Figure 3.8. A. portentosus (PIN 3907/1). Stereo pairs of vertebral column in dorsal (a-o) and ventral (p-dd) view (sequence follows Kurzanov [1982]).
Figure 3.9. A. portentosus (PIN 3907/1). Stereo pairs of left (a, c) and right (b, d) humeri in dorsal (a, b) and ventral (c, d) view. Scale bar = 3 cm.
Figure 3.10. A. portentosus (PIN 3907/1). Stereo pairs of left ulna in dorsal view (a); stereo pairs of premaxilla in rostral (b) and caudal (c) view; stereo pairs of "carpometacarpus" (?) in several views (d-f). Scale bar = 2 cm.
Figure 3.11. A.portentosus (PIN 3907/1). Stereo pairs of right (a, c) and left (b, d) ilia in ventral (a, b) and dorsal (c, d) view. Scale bar = 3 cm.
Figure 3.12. A. portentosus (PIN 3907/1). Stereo pairs of pubes in caudal (a) and cranial (b) view. Scale bar = 5 cm.
Figure 3.13. A. portentosus (PIN 3907/1). Left femur in caudo-lateral (a) view and right femur in caudal view (b). Scale bar = 5 cm.
Theropod Femur
Figure 3.14. A. portentosus (PIN 3907/1). Stereo pairs of left femur in medial view (a) and the right femur in cranial view (b). Scale bar = 5 cm.
Figure 3.15. A. portentosus (PIN 3907/1). Stereo pairs of right (a) and left (b) tibiotarsi in cranial view and of right fibula in medial view (c). Scale bar = 5 cm.
Figure 3.16. A. portentosus (PIN 3907/1). Stereo pairs of right (a) and left (b) tibiotarsi in caudal view and of right fibula in lateral view (c). Scale bar = 5 cm.
Figure 3.17. A. portentosus (PIN 3907/1). Stereo pairs of right (a, c) and left (b, d) metatarsal II and IV in dorsal (cranial) (a, b) and plantar (caudal) (c, d) view. Scale bar = 5 cm.
Avimimus Anatomy
Figure 3.18. A. portentosus (PIN 3907/1). Stereo pairs of right (a, c) and left (b, d) metatarsal III in dorsal (cranial) (a, b) and plantar (caudal) (c, d) view. Scale bar = 2 cm.
Figure 3.19. A. portentosus (PIN 3907/1). Stereo pairs of pedal digits (II—IV) in dorsal (a-f) and ventral (g-l) view; digit II (a, f, g, l), digit III (b, e, h, k), digit IV (c, d, i, j). Scale bar = 3 cm.

Acknowledgments

We are grateful to S. Morton (Monash University) for photographing PIN 3907/1 and to L. Meeker (American Museum of

Natural History) and L. Rhoads (Natural History Museum of Los

Angeles County) for mounting and labeling the photographs.

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