Report

A Basal Dromaeosaurid and Size Evolution Preceding Avian Flight

Science  07 Sep 2007:
Vol. 317, Issue 5843, pp. 1378-1381
DOI: 10.1126/science.1144066

Abstract

Fossil evidence for changes in dinosaurs near the lineage leading to birds and the origin of flight has been sparse. A dinosaur from Mongolia represents the basal divergence within Dromaeosauridae. The taxon's small body size and phylogenetic position imply that extreme miniaturization was ancestral for Paraves (the clade including Avialae, Troodontidae, and Dromaeosauridae), phylogenetically earlier than where flight evolution is strongly inferred. In contrast to the sustained small body sizes among avialans throughout the Cretaceous Period, the two dinosaurian lineages most closely related to birds, dromaeosaurids and troodontids, underwent four independent events of gigantism, and in some lineages size increased by nearly three orders of magnitude. Thus, change in theropod body size leading to flight's origin was not unidirectional.

Which nonflying maniraptoran dinosaurs are the closest relatives to birds (Avialae) has been debated (15). Dromaeosaurids and troodontids are the two clades consistently found to be most closely related to avialans (18). Discoveries of these dinosaurs, which illuminate the features ancestrally present in the first flighted theropods, have remained rare. Here we report a basal dromaeosaurid theropod: Theropoda Marsh, 1884; Maniraptora Gauthier, 1986; Paraves Sereno, 1997; Dromaeosauridae Matthew and Brown, 1922; Mahakala omnogovae, new taxon. The new taxon is small (∼70 cm long) and possesses features absent in other dromaeosaurids but shared with early troodontids and avialans.

Holotype. Specimen number IGM (Mongolian Institute of Geology, Ulanbaatar) 100/1033, a partial skull and postcranial skeleton (Figs. 1 and 2).

Fig. 1.

Mahakala omnogovae IGM 100/1033, holotype. (A) Skull in occipital view. (B) Braincase in left lateral view. (C) Sacrum and partial right leg in ventral view. (D) Frontal in dorsal (left) and ventral (right) views. (E) Axisvertebrain left lateral view. Scale bars, 5 mm in (A), (B), (D), and E) and 1 cm in (C). Abbreviations are as follows: cav, caudal vertebra; ctr, caudal tympanic recess; dtr, dorsal tympanic recess; ep, epipophysis; f.l, lacrimal facet; f.po, postorbital facet; fm, foramen magnum; mt, metatarsus; oc, occipitalcondyle; od, odontoid; pap, paroccipital process; pf, pneumatic foramen; ph, phalanx; prz, prezygapophysis; q.pr, contactsurface on prootic for quadrate; q, quadrate; ti, tibia; tl, tectal lobe; sac, sacrum; v.o, occipital vein track.

Fig. 2.

Mahakala omnogovae IGM 100/1033, holotype. (A) Rightulna in lateral (right) and medial (left) views. (B) Ilium in medial (top) and lateral (bottom) views. (C) Femurinposterior (left) and lateral (right) views. (D) Tibia in anterior view. (E) Left metatarsus in anterior view. (F) Right raptorial claw. (G) Midcaudal vertebrae. Scale bars, 1 cm in (B) to (E) and 5 mm in (A), (F), and (G). Abbreviations are as follows: aa, ascending process of astragalus; as, astragalus; bf, brevis fossa; brs, brevisshelf; bs, biccipital scar; ca, calcaneum; cc, cnemial crest; ch, chevron; fc, fibular crest; fi, fibula; gtr, greater trochanter; lr, lateralridge; lc, lateral crest; mco, medial condyle; mt, metatarsal; obr, oblique ridge; pat, posterior antitrochanter; prz, prezygapophysis; pt, posterior trochanter; ts, trochanteric shelf.

Etymology. “Mahakala,” Sanskrit for one of the eight protector deities (dharmapalas) in Tibetan Buddhism. The specific epithet refers to the southern Gobi provenance of this taxon.

Locality and horizon. The Tugrugyin Member of the Djadokhta Formation (Campanian) (9, 10), Tugrugyin Shireh, Ömnögov, Mongolia (10, 11).

Diagnosis. A small paravian diagnosed by the following combination of characters (autapomorphies are noted by *): a strongly compressed and anteroposteriorly broad ulna tapering posteriorly to a narrow edge*; an elongate lateral crest on the posterodistal part of the femur*; anterior caudal vertebrae with subhorizontal, laterally directed prezygapophyses*; a prominent supratrochanteric process; and the absence of a cuppedicus fossa.

Estimated at 70 cm long, Mahakala is similar in size to the basal avialan Archaeopteryx and basal members of other maniraptoran clades such as the oviraptorosaur Caudipteryx and the troodontid Mei long. The specimen is a young adult or near adult, based on the degree of neurocentral and astragalocalcaneal fusion, braincase coossification, and histological analysis (fig. S4). Thus, it can be distinguished from the contemporaneous Archaeornithoides, which is of similar size but is a juvenile (12).

The braincase, quadrate, and frontals are well preserved. Unlike dromaeosaurids but similar to troodontids such as Sinovenator (7) and Mei (8), the frontals are dorsoventrally vaulted and the interorbital region is narrow, indicating proportionally large orbits. The anterolateral corner of the frontal lacks the articulation notch present in other dromaeosaurids. The frontals transition smoothly from the orbital margin to the postorbital processes as in troodontids (13), but unlike the abrupt transition and sharply demarcated postorbital processes of dromaeosaurids. The supratemporal fossa margin is weakly curved, not sinuous as in all other dromaeosaurids except Tsaagan (5) and Dromaeosaurus [AMNH (American Museum of Natural History) FR 5356]. The quadrate is incipiently bistylic, unlike the single-headed ball-shaped process in other dromaeosaurids. A depression on the prootic may correspond to a secondary articulation surface for the quadrate. This depression corresponds topographically to the braincase articulation facet in birds and alvarezsaurids but is also present in the troodontid Byronosaurus (14). The lateral braincase wall lacks any indication of a well-developed otosphenoidal crest like that in troodontids (14). The paroccipital processes are relatively short and distally twisted rostrolaterally as in other dromaeosaurids.

The axis bears a single pneumatic opening and has small epipophyses that do not overhang the postzygapophyses. The sacrum comprises six apneumatic, coossified centra as in Rahonavis and mature specimens of Velociraptor (IGM 100/986 and IGM 100/985). The fused neural arches of the posterior sacral vertebrae form a bony lamina as in other dromaeosaurids. The tail is long as in basal avialans (such as Archaeopteryx and Jeholornis), basal troodontids (such as Jinfengopteryx and Mei), and other dromaeosaurids. The transition point occurs between caudals (Cd's) 11 and 12 and is more posteriorly located than in Rahonavis (proximal to Cd 9) or Velociraptor (proximal to Cd 10). The distal caudal postzygapophyses are smaller than the prezygapophyses. As in to other basal paravians, the postzygapophyses do not exceed the posterior margin of the vertebral centra. The lateral surface of the proximal caudals bears a low ridge similar to that in Buitreraptor (4) and Rahonavis [UA (University of Antananarivo) 8656]. The chevrons in Mahakala are platelike as in many derived coelurosaurs.

The scapula is narrow and straplike and has a strongly compressed ovoid cross section. The preserved portion of the incomplete humerus suggests that the entire humerus was reduced in contrast to the condition of most coelurosaurs. The ulna is distinctly bowed as in most maniraptorans (1) but is strongly compressed and possesses a small biceps tubercle. The distal region of the radius is expanded and flattened as in paravians (11). A semilunate carpal covers the proximal surfaces of metacarpals I and II; a plesiomorphic conformation lost in most avialans.

The ilium is dolichoiliac. A prominent supratrochanteric process is present as in Unenlagia, Rahonavis, and many avialans. The brevis shelf is triangular and does not extend laterally as in some other basal dromaeosaurids. No antiliac shelf is present, therefore Mahakala lacks a defined cuppedicus fossa—an absence unique within dromaeosaurids but characteristic of avialans such as Apsaravis (15) and Yixianornis (16).

The femur is anteriorly bowed. The lesser trochanter is well developed, and its anterior edge is continuous with the greater trochanter. The fourth trochanter is present as a smooth and weakly developed ridge. Unlike Velociraptor (IGM 100/986), the lateral ridge is poorly developed, and the moundlike trochanteric shelf is proximodistally elongated and closely connected to the posterior trochanter. A prominent crest extends from the distal third of the shaft to the ectocondylar tubercle. The tibia is longer than the femur and possesses a single cnemial crest. The lateral surface of the calcaneum is distinctly concave and lacks the notch for the articulation with the distal fibula that is present in dromaeosaurids and other nonavian theropods. This condition is shared with Rahonavis, basal avialans (2), and derived alvarezsaurids. Unlike most troodontids and microraptorines, but similar to Archaeopteryx and derived dromaeosaurids, the foot of Mahakala exhibits the plesiomorphic unconstricted condition for metatarsal III, further indicating that this avian trait may be the primitive condition for paravians. The distal end of metatarsal II is composed of an asymmetrical ginglymoid articular surface and phalanx II-2 has a well-developed proximal heel and hypertrophied ginglymoid trochlea. This suite of characters is present only in dromaeosaurids.

Phylogenetic analysis identifies Mahakala as a basal dromaeosaurid and supports paravian monophyly with birds (Avialae) as the sister group to a monophyletic Deinonychosauria (Dromaeosauridae + Troodontidae) (Fig. 3 and fig. S1). Although discovered in relatively young Cretaceous deposits, the basal position of Mahakala has several implications regarding our understanding of the early history of deinonychosaurians (17). First, Shanag from the Early Cretaceous of Mongolia (18) nests within the purported Gondwanan lineage of dromaeosaurids, Unenlagiinae. This topology complicates recently proposed vicariance-driven origin hypotheses for these groups (4, 19). Second, these dinosaurs are united with Jehol microraptorines (Microraptor, Graciliraptor, and Sinornithosaurus) to form the sister group to derived dromaeosaurids from Laurasia (velociraptorines and allied forms). Third, the purported avialan Jinfengopteryx (20) is a troodontid. Jinfengopteryx has feathers; it thus demonstrates the presence of feathers of modern aspect in a troodontid.

Fig. 3.

Phylogeny and body size change within paravian theropods. A temporally calibrated cladogram depicting the phylogenetic position of Mahakala and paravian body size through time and across phylogeny is shown. Characters uniting Mahakala with other dromaeosaurids include the absence of an accessory tympanic recess dorsal to the crista interfenestralis, and elongate paroccipital process with parallel dorsal and ventral edges that twist rostrolaterally distally, and the presence of a distinct ginglymus on the distal end of metatarsal II (17). Silhouettes are to scale, illustrating the relative magnitude of body size differences. Left-facing silhouettes near open circles show reconstructed ancestral body sizes. Ancestral paravian body size is estimated to be 600 to 700 g and 64 to 70 cm long (17). The ancestral deinonychosaur, troodontid, and dromaeosaurid body size is estimated at ∼700 g. Large numbers (1, 2, 3, and 4) indicate the four major body increase trends in Deinonychosauria. See the supporting online material for further ancestral body size reconstruction data. Ma, Maastrichtian; Ca, Campanian; Sa, Santonian; Co, Coniacian; Tu, Turonian; Ce, Cenomanian; Ab, Albian; Ap, Aptian; Bar, Barremian; Hau, Hauterivian; Va, Valanginian; Ber, Berriasian; Ti, Tithonian; Ki, Kimmeridgian. Ma, million years ago.

Decrease in body size is a trend in coelurosaurs (3, 8, 21) and is thought to have played an important role in the origin of birds and flight (6, 11, 2224). Dromaeosaurids and other coelurosaurs, however, may have undergone clade-specific increases in body size (8, 25). Testing these trends requires empirical size reconstructions for each node of the coelurosaur tree. We estimated ancestral body sizes for each internal node (ancestral node) using body mass estimates from femoral length measurements. These data were treated as a continuous additive trait and optimized across the phylogeny (16).

Our analysis (fig. S2) indicates that small body size was not a derived condition at Archaeopteryx or Avialae, where flight evolution in theropods is currently inferred. The ancestral dromaeosaurid, troodontid, and deinonychosaurian are reconstructed as small, each with a body mass around 700 g (Fig. 3). The basal members of these lineages are the same size as the early avialan Jeholornis. Additionally, our results indicate that deinonychosaurs underwent four parallel trends of body size increase. Three of these events occurred within Dromaeosauridae: Deinonychus increased in size by more than two orders of magnitude, as did Unenlagia, and the Achillobator + Utahraptor clade increased by three orders of magnitude. A single trend of body size increase was observed in troodontid body size. These events were contemporaneous with a decrease in avialan body sizes. Our analysis implies that the ancestral paravian had a body size of 600 to 700 g and was ∼65 cm long, roughly the size of the largest specimens of Archaeopteryx or Sapeornis and entailing the size range reconstructed for basal deinonychosaurs. Thus, miniaturization preceded the avialan node and the origin of flight, and as a result, hypotheses relating ontogenetic or metabolic controls on miniaturization to flight origin in theropods must be equally capable of explaining the size reduction within ancestral paravians and the iterative trends of size increase in deinonychosaurs.

Supporting Online Material

www.sciencemag.org/cgi/content/full/317/5843/1378/DC1

SOM Text

Figs. S1 to S5

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