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A description of Rahona, an unusual "primitive" bird which suggests a theropod ancestry for birds.

Below article from Science reprinted for fair use purposes only -- copyright acknowledged.

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Science

Copyright 1998 Information Access Company, a Thomson Corporation Company;

ASAP

Copyright 1998 American Association for the Advancement of Science

March 20, 1998

SECTION: No. 5358, Vol. 279; Pg. 1915; ISSN: 0036-8075

The Theropod Ancestry of Birds: New Evidence from the Late Cretaceous of Madagascar.

by Forster, Catherine A.; Sampson, Scott D.

AUTHOR-ABSTRACT: A partial skeleton of a primitive bird, Rahona ostromi, gent et sp. nov., has been discovered from the Late Cretaceous of Madagascar. This specimen, although exhibiting avian features such as a reversed hallux and ulnar papillae, retains characteristics that indicate a theropod ancestry, including a pubic foot and hyposphene-hypantra vertebral articulations. Rahona has a robust, hyperextendible second digit on the hind foot that terminates in a sicklelike claw, a unique characteristic of the theropod groups Troodontidae and Dromaesauridae. A phylogenetic analysis places Rahona with Archaeopteryx, making Rahona one of the most primitive birds yet discovered.

BODY: The origin of birds has been debated for more than 100 years, with theropod dinosaurs (1-6) and basal archosauriforms (7, 8) most frequently hypothesized as their ancestors. Several workers have argued explicitly against the "birds as dinosaurs" theory (812). We report here a new raven-sized primitive bird that adds new morphological data to the question of bird ancestry The holotype specimen of this new bird, Rahona ostromi, gen. et sp. nov. (13), was recovered from a small quarry (site MAD93-18) in Upper Cretaceous rocks in northwestern Madagascar. This quarry has produced a diverse, well-preserved vertebrate fauna, including the primitive bird Vorona berivotrensis (14).

The skeleton of Rahona exhibits a striking mosaic of theropod and derived avian features (Fig. 1). The specimen appears to be adult, based on the complete fusion of neural arches to vertebral centra (Fig. 2). The single camellate cervicodorsal vertebra bears a large hypopophysis and bilateral pneumatic foramina, as in maniraptorans and birds, as well as a large vertebral canal (88% of the centrum height; Fig. 2B). Pneumatic foramina also occur on the dorsal vertebrae, lying within well-developed pneumatic fossae, as in some enantiornithes (Fig. 2A). The vertebral canals are large (42 to 62% of the centrum height), as in birds. The dorsal vertebrae have accessory hyposphene-hypantra articulations, a unique character of theropod and sauropod dinosaurs, retained only in Patagonykus (15) among birds. There are six sacral vertebrae, one more than in most theropods and Archaeopreryx. They are completely co-ossified into an avianlike synsacrum (Fig. 2C). Like Archaeopteryx, Rahona retains a long bony tail. Thirteen caudal vertebrae (Cd) are preserved, but the complete number is unknown (Fig. 2D). The transition point is proximally placed at Cd9.

[Figures 1-2 ILLUSTRATION OMITTED]

The antebrachium of Rahona is avian, it is elongate (the ulna is 150% of the length of the femur) and the radius is reduced to 50% of the diameter of the ulna (Fig. 3, B and C and measurements in Table 1). The caudal (anconal) margin of the bowed ulna bears six low, slightly elongate papillae that become less distinct distally (Fig. 3D). We interpret these to be quill knobs for the attachment of secondary flight feathers. These six quill knobs, which are regularly spaced (1.6 cm apart), cover only a portion of the ulnar shaft. We estimate that there is space for four additional feathers, for a total of approximately 10 secondary remiges, which is fewer than the 12 to 14 secondaries suggested for Archaeopteryx (16) but within the range known for extant birds.

[Figure 3 ILLUSTRATION OMITTED]

Table 1. Lengths (in millimeters) of pelvic and limb elements of Rahona ostromi. Dash indicates that measurement is not possible because of an absent or incomplete element.

Element / Left / Right

Synsacrum 41.9
Scapula 82.2 / -
Radius - / 126.9
Ulna - / 132.3
Ilium 66.7 / 67.7
Pubis - / 60.8
Ischium 27.3 / -
Femur 88.0 / 87.1
Tibia 119.8 / 120.2
Fibula - / -
Metatarsal I 8.9 / -
Metatarsal II 44.7 / 44.1
Metatarsal III 48.0 / 48.1
Metatarsal IV 45.3 / 47.7

The main axis of the glenoid fossa is centered on the ventral edge of the scapular blade, as in Archaeopteryx and theropods, rather than lateral to the ventral edge as in Neornithes (Fig. 3A). Otherwise, the scapula is quite derived. It has a facet for the coracoid, indicating a mobile joint as in derived birds, rather than the plesiomorphic sutural contact of theropods and Archaeopteryx. A well-developed acromion process projects well cranial to the coracoid facet, as in Unenlagia, Archaeopteryx, and birds. On the basis of these forelimb characters (enlarged acromion process, coracoid facet, elongate ulna, and ulnar papillae), the scapula of Rahona was probably positioned dorsally on the ribcage rather than more ventrally as in theropods, resulting in a more laterally directed glenoid fossa. This orientation allows for the more extensive vertical excursion of the humerus needed to produce a flight stroke (17) and contributes to the wing-folding mechanism (18).

The pelvic elements of Rahona closely resemble those of Archaeopteryx and Unenlagia (18). The ilium has a long preacetabular process (55% of the ilium length) and a short postacetabular process that is drawn back into a narrow, pointed posterior end. The pubis (90% of the ilium length) is oriented vertically (as in some maniraptorans, Archaeopteryx, and Unenlagia). Distally, the pubis sweeps caudally and expands into a foot; a well-developed hypopubic cup is present (Fig. 4A). A pubic foot is absent in nearly all avians, but is present in theropods, Archaeopteryx, Patagonykus, and enantiornithines (for example, Sinornis and Cathayornis). Like that of Archaeopteryx, the ischium of Rahona is short (45% of the length of the pubis), platelike, and has a pointed process at the anterodistal end (Fig. 4A). We interpret the latter as the obturator process, based on its shape and position. Behind the iliac articulation is a small dorsally projecting process [the "proximodorsal process" of Novas and Puerta (18)], a character shared exclusively with Unenlagia and the primitive birds Archaeopteryx, enantiornithines, Iberomesornis, and Confuciusornis. A second, smaller process is midway down the caudal ischial margin, as in Archaeopteryx and Confuciusornis. There is no evidence of an ischial symphysis. All pelvic elements are unfused, a plesiomorphic character state shared with nonavian theropods, Archaeopteryx, Unenlagia, and Iberomesornis.

[Figure 4 ILLUSTRATION OMITTED]

The femoral head is identical to that of Archaeopteryx, lacking both a neck and a fossa for the capital ligament. It also bears an avianlike undivided trochanteric crest (Fig. 4B). The tibia is long and straight (137% of the femoral length) and lacks a medial cnemial crest as occurs in more derived ornithurine birds. The greatly reduced, splintlike fibula is birdlike in proportion (15% of the tibial diameter), and the tubercle for the m. iliofibularis faces posteriorly, as in Ornithurae (Fig. 4C). The right fibula is preserved in articulation with the tibia, and its distal portion shifts onto the cranial surface of the tibia. If this is its natural position (as in Patagopteryx), it could not have articulated with the calcaneum. Loss of contact between the fibula and calcaneum characterizes birds.

The much reduced calcaneum is tucked into the lateral margin of the broad short astragalus (14% of tibial length), as in maniraptorans and Archaeopteryx. The astragalus and calcaneum are partially fused to one another but are not fused to the tibia (Fig. 4C). A free distal tarsal caps the right metatarsal IV. Plesiomorphic free tarsals are also retained in the primitive bird Iberomesornis and in some specimens of Archaeopteryx.

The foot of Rahona is primitive in many respects; notably the metatarsals are not fused to one another (Fig. 4D). In some specimens of Archaeopteryx, the metatarsals also lack any fusion (for example, the Eichstatt specimen), although other specimens exhibit partial fusion of the proximal metatarsals (for example, the London specimen). The digits of the left foot of Rahona were found in articulation and show that digit I is reversed relative to the other digits (Fig. 1B), a configuration known only in birds (10).

The most striking feature in the nearly complete left foot, however, is the structure of digit II. It is extremely robust relative to the other digits (the first phalanx of digit II is 140% of the width of that of digit III at midshaft) and distinctive in morphology. The phalanges have large, ventrolaterally placed flexor keels, expansive distal extensor surfaces, and deep, dorsally placed, collateral ligament pits. The digit ends in an enlarged sickle-shaped claw. Although unguals are missing from digits III and IV, their preserved distal phalanges indicate that they bore substantially smaller claws. On the left foot, digit II was found in hyperextension, whereas digits III and IV were flexed (Fig. 1B). This distinctive morphology of an enlarged hyperextendible digit II is found only in dromaeosaurid and troodontid maniraptorans (for example, Deinonychus, Velociraptor, and Troodon), resulting in the predatory "slashing" foot (19).

The general skeletal morphology of Rahona is birdlike. Rahona is only slightly larger than the London Archaeopteryx specimen (though smaller than its avian contemporary Vorona) and extremely lightly built (the long bones are hollowed to the same degree seen in other birds). These factors, combined with the elongate feathered ulna and raptorial slashing foot, suggest that this bird was lightweight, active, predatory, and capable of powered flight. The combination of derived wing morphologies with a vertically oriented pubis in Rahona counters the recent suggestion that the development of an avian-style lung ventilation system suitable for the high metabolic demands of flight was coupled with a fully retroverted pubis (11). The vertical pubis of Rahona also bears a well-developed hypopubic cup, a morphology associated with suprapubic musculature and avian-style lung ventilation (11). Rahona thus shows that a hypopubic cup and opisthopuby did not develop in concert.

It has been hypothesized that birds belong to a derived clade of theropods called Maniraptora (2-5). However, the arrangement of taxa within Maniraptora, including exactly where birds fit, is debated. Both Dromaeosauridae (4, 5) and Troodontidae (20) have been hypothesized to be the closest relatives of birds.

We ran a phylogenetic analysis (21) with two separate data sets, one including and one excluding forelimb elements for Rahona (22). The most parsimonious tree for both data sets shows the same arrangement of taxa within Aves (which includes Rahona). That is, the exclusion from the phylogenetic analysis of the strongly avian forelimb assigned to Rahona does not alter its phylogenetic position within Aves. Rahona is supported as a member of Aves [Avialae of others; for example, (3, 5)] by seven unambiguous derived characters; bootstrapping of the data set (500 replications) shows a 90% confidence level for our Aves node (Fig. 5A; the analyses depicted include forelimb characters for Rahona).

[Figure 5 ILLUSTRATION OMITTED]

Our most parsimonious analysis places the purported maniraptoran theropod Unenlagia within Aves as the sister taxon to a Rahona-Archaeopteryx clade (Fig. 5A). This three-taxon clade is united by four unambiguous characters of the pelvis and femur (node 3 in Fig. 5A). Uniting these three taxa in a single subclade places them on a side branch of early bird evolution and supports the suggestion that Archaeopteryx was not a direct precursor of modern birds (12, 23). However. this clade collapses to a paraphyletic configuration of (in order) Unenlagia-Archaeopteryx-Rahona--other birds, or Archaeopteryx-Unenlagia-Rahona--other birds, with only one additional step (see Fig. 5B for one of these trees). This suggests that the characters uniting these taxa may represent primitive features for birds rather than synapomorphies of a separate primitive bird lineage. These alternative hypotheses may prove more tenable, as Rahona shares a number of characters with more derived birds exclusive of Archaeopteryx (for example, six fused sacral vertebrae, a mobile scapulocoracoid joint, and an undivided trochanteric crest). Rahona remains firmly nested within Aves in all trees.

In addition to its numerous bird features (for example, a reversed hallux, a splintlike fibula, and ulnar papillae), Rahona retains specific theropod synapomorphies. The accessory hyposphene-hypantra articulations on its dorsal vertebrae are a synapomorphy of Saurischia (Sauropodomorpha + Theropoda) and are unknown in any other amniote clade (24). The singular pedal morphology is known only in derived maniraptoran theropods, which are the purported precursors of birds (25). Thus, the combination of morphological characters found in Rahona strongly supports its membership in Aves, as well as its theropod ancestry, and thus the dinosaurian origin of birds.

REFERENCES AND NOTES

(1.) E. D. Cope, Proc. Acad. Nat. Sci. Phila. 1867, 234 (1867); T. H. Huxley, Geol. Mag. 5, 357 (1868): S. W. Williston, Kans. City Rev. Sci. 3, 457 (1879); L. Witmer, in Origin of the Higher Groups of Tetrapods, H.-P. Schultze and L. Trueb, Eds. (Comstock, Ithaca, NY, 1991), pp. 427-466; P. C. Sereno and C. Rao, Science 255, 845 (1992); L. M. Chiappe, Nature 378, 349 (1995). To increase readability and conserve space, we use the shorthand terms "theropod" and "maniraptoran" in place of "nonavian theropod" and "nonavian maniraptoran," respectively.

(2.) J. H. Ostrom, Biol. J. Linn. Soc. 8, 91 (1976).

(3.) J. A. Gauthier, Mem. Calif. Acad. Sci. 8, 1 (1986).

(4.) T. R. Holtz Jr., J. Paleontol. 68, 1100 (1994).

(5.) F. E. Novas, Mem. Queensl. Mus. 39, 675 (1996).

(6.) L. M. Chiappe, Ibid., p. 533.

(7.) G. Heilman, The Origin of Birds (Appleton, New York, 1927); A. S. Romer, Vertebrate Paleontology (Univ. Of Chicago Press, Chicago, IL, 1966); P. Brodkorb, in Avian Biology, D. S. Farner, J. R. King, K. C. Parkes, Eds. (Academic Press, New York, 1971), pp. 19-65.

(8.) S. Tarsitano, in Origin of the Higher Groups of Tetrapods, H.-P. Schultze and L. Trueb, Eds. (Comstock, Ithaca, NY, 1991), pp. 641-576.

(9.) A. Feduccia and R. Wild, Naturwissenshaften 80, 664 (1993); A. C. Burke and A. Feduccia, Science 278, 666 (1997).

(10.) A. Feduccia and L. Martin, Mus. North Ariz. Bull. 60, 186 (1996).

(11.) J. A. Ruben et al. Science 278, 1267 (1997).

(12.) L. D. Martin, in Origin of the Higher Groups of Tetrapods, H.-P. Schultze and L. Trueb, Eds. (Comstock, Ithaca, NY, 1991), pp. 485-540.

(13.) The holotype specimen of Rahona ostromi is cataloged as Universite d'Antananarivo (UA) 8656. Locality: MAD93-18, Upper Cretaceous (?Campanian) Maevarano Formation, Mahajanga Basin, northwestern Madagascar; collected by a joint expedition of the State University of New York at Stony Brook and the Universite d'Antananarivo in 1995. Etymology: Rahona (RAH-hoo-nah; Malagasy): meaning menace/threat or cloud; intended interpretation: "menace from the clouds"; ostromi: in honor of Dr. John H. Ostrom. Diagnosis: Rahona ostromi is distinguished from all other avians by retention of a robust, hyperextendible, pedal digit II; from all other avians except Patagonykus by hyposphenehypantra articulations on dorsal vertebrae; from Archaeopteryx by six fused sacral vertebrae and a greatly reduced fibula lacking contact with the calcaneum; from nonavian theropods, Archaeopteryx, and alvarezsaurids by its relatively elongate ulna with ulnar papillae and mobile scapulocoracoid articulation; from all other avians except Archaeopteryx and alvarezsaurids by retention of a long tail lacking a pygostyle; and from nonavian theropods by neural canals at least 40% of the height of the dorsal vertebral centra, proximal tibia of equal width and length, lack of a medial fossa on the fibula, and a reversed pedal digit I.

(14.) C. A. Forster et al, Nature 382, 532 (1996).

(15.) The placement of Patagonykus and other alvarezsaurids

(Mononykus and Alvarezsaurus) within Aves, although supported by cladistic analyses (for example, see (5, 6) and this analysis], is questioned by other researchers (10). Elimination of Alvarezsauridae from the phylogenetic analyses presented in this report does not alter the placement of Rahona within Aves. (16.) B. Stephan, Urvogel Archaeopterygiformes (Ziemsen, Wittenberg, Germany, 1974); S. Rietschel, in The Beginnings of Birds, M. K. Hecht, J. H. Ostrom, G. Viohl, P. Wellnhofer, Eds. (Bronner and Daentler, Eichstatt, Germany, 1984), pp. 251-260.

(17.) F. A. Jenkins, Am. J. Sci. 293-A, 253 (1993); S. A. Poore, A. Sanchez-Haiman, G. E. Goslow Jr., Nature 387, 799 (1997).

(18.) F. E. Novas and P. F. Puerta, Nature 387, 390 (1997).

(19.) J. H. Ostrom, Peabody Mus. Bull. 30, 1 (1969).

(20.) P. J. Currie, J. Vertebr. Paleontol. 7,72 (1987); P. J. Currie and X. Zhao, Can. J. Earth Sci. 30, 2231 (1993).

(21.) Morphological information from Rahona was combined with that of six bird and eight maniraptoran taxa into a 113-character matrix and analyzed with the PAUP and MacClade programs. Characters were unordered and unweighted, and trees were optimized with the use of delayed transformations. Tree statistics are as follows: The most parsimonious tree shown in Fig. 5A is 228 steps; consistency index (CI) = 0.579, homoplasy index (HI) = 0.421, retention index (RI) = 0.712. The tree shown in Fig. 5B is 229 steps; CI = 0.576, HI = 0.424, RI = 0.709. The character matrix and character list for this phylogenetic analysis are available at www.sciencemag.org/feature/data/972697.shl.

(22.) The three forelimb elements of Rahona were found either next to or touching the hind portion of the skeleton (Fig. 1B). Because they were not in direct articulation with the rear of the animal, we recognize the possibility, albeit remote in our opinion, that they do not belong to the same individual or taxon. Although material of more derived avians was found elsewhere in the quarry, with the exception of one articulated partial tibiotarsus-tarsometatarsus (14) all avian material occurred as widely scattered, isolated elements. The only articulated skeleton found anywhere in the quarry is that of Rahona. Because of the taphonomic distribution of bone in the quarry and the juxtaposition of these forelimb elements with the rear portion of the skeleton, we believe they belong to the same specimen and are confident in assigning them to Rahona. Nevertheless, to test the effect of an erroneous association, the phylogenetic analysis was run with two data sets, one including and one excluding forelimb elements for Rahona. Each data set resulted in two most parsimonious trees; the ambiguity in these trees was due to the switching of the positions of the theropod taxa Oviraptoridae and Ornithomimidae. The topology of taxa within Aves was consistent across all four most parsimonious trees with Rahona firmly nested within this clade.

(23.) J. H. Ostrom, The Beginnings of Birds, M. K. Hecht, J. H. Ostrom, G. Viohl, P. Wellnhofer, Eds. (Bronner and Daentler, Eichstatt, Germany, 1984), pp. 161176; L. Hou, L. D. Martin, Z. Zhou, A. Feduccia, Science 274, 1164 (1996); N. Bonde, in The Continental Jurassic, M. Morales, Ed. (Museum of Northern Arizona, Flagstaff, AZ, 1996), pp. 193-199.

(24.) It cannot be ascertained whether Archaeopteryx possesses hyposphene-hypantra articulations. Among more derived birds, only the alvarezsaurid Patagonykus retains this character.

(25.) The foot of Unenlagia is not known. However, it has been suggested that Archaeopteryx retains vestiges of an enlarged, hyperextendible, second pedal digit and claw. This observation was first advanced by J. Gauthier (3) and more recently revived by G. Paul [Programs and Abstracts, Society of Avian Paleontology and Evolution (Washington, DC, 1996), p. 15].

(26.) We thank B. Rakotosamimanana, P. Wright, B. Andriamihaja, the staff of the Institute for the Conservation of Tropical Environments, the people of Berivotra, and al expedition members for their help; and L. Witmer, J. Clark, and an anonymous reviewer for discussions and critiques. D. Varricchio, J. Clark, M. Norell, H. Osmolska, and P. Wellnhofer provided valuable information on theropods and Archaeopteryx. Rahona was prepared by V. Heisey and photographed by M. Stewart and F. E. Grine (with a scanning electron microscope), and figures were drawn by L. Betti-Nash and C.A.F. This work was supported by grants from NSF and The Dinosaur Society (to C.A.F., S.D.S., and D.W.K.) and the J. S. Guggenheim Foundation and F. Chapman Memorial Fund (to L.M.C.).

9 January 1998; accepted 5 February 1998

Catherine A. Forster, To whom correspondence should be addressed. E-mail: cforster@mail.som.sunysb.edu

C. A. Forster and D. W. Krause, Department of Anatomical Sciences, State University of New York at Stony Brook, Stony Brook, NY 11794, USA. S. D. Sampson, Department of Anatomy, New York College of Osteopathic Medicine, Old Westbury, NY 11568, USA. L. M. Chiappe, Department of Ornithology, American Museum of Natural History, 79th Street at Central Park West, New York, NY 10024, USA.

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