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A New Family of Bird-Like Dinosaurs Linking Laurasia and Gondwanaland

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A New Family of Bird-Like Dinosaurs Linking Laurasia and Gondwanaland Journal of Vertebrate Paleontology 5(2):133-138, June 1985 A NEW FAMILY OF BIRD-LIKE DINOSAURS LINKING LAURASIA AND GONDWANALAND M. K. BRETT-SURMAN' and GREGORY S. PAUU 'Department of Geology, Division of Geobiology, George Washington University, Washington, D.C. 20052 23109 N. Calvert St., Baltimore, Maryland 21218 ABSTRACT - A new family oftheropod dinosaurs is described based on metatarsi f:T0m~ o~h ~merica and Argentina. Because birds are theropod descendants, there are.often proble~s ill assigning isolated metatarsi to the proper group. Differences between the metatarsi of ground birds ~d theropods are detailed. In particular, it is shown that the length/width ratio of articulated metatarsi cannot be used to distinguish higher taxa. This new family represents the first occurrence ofthe same.genus of theropod dinosaur from both Laurasia and Gondwanaland at the end of the Cretaceous, a time when the two supercontinents were supposedly still separate. INTRODUCTION nia (Berkeley); UNT = Universidad Nacional de Tu- cuman. In 1975, an expedition from the University ofCal- ifornia (Berkeley) collecting in the Hell Creek For- mation (Cretaceous, Maastrichtian) of Mo?tana, ~e- THEROPOD AND BIRD METATARSI covered fragments of fossil bird bones associated WIth A series of studies conclusively demonstrates that dinosaurian and other reptilian remains. This collec- theropod dinosaurs and birds have an ancestor-de- tion included a complete metatarsus (UCMP 117600, scendantrelationship (Ostrom, 1974, 1976;Bakker and Fig. 3A-I) that was called dinosaurian by neo0!lli- Galton, 1974; Bakker, 1975; Osmolska, 1981; Padian, thologists but avian by most dinosaur paleontologists! 1982; Paul, 1984; Benton, 1984; Paul and Carpenter, The question then arose as to how to separate the unpublished). As a consequence, there is considerable metatarsi of dinosaurs morphologically from those of morphological similarity and parallel evolution be- birds. While pursuing this problem, the authors ex- tween the two clades. In particular, there often are amined the material from the Lecho Formation (Cre- problems in assigning isolated theropod or bird-like taceous Maastrichtian?) of Argentina, at that time on metatarsi, such as those discussed herein, to the proper loan to'the University of Florida and to the British group. Museum (Natural History) in London. This material Traditionally, two characteristics used to separate (with the exception of a metatarsus on. loan to the birds from theropods are the length/width (L/W) ratio University of Florida), was recently descnbed as a new of the metatarsus and the degree offusion of the meta- subclass of birds (Walker, 1981), the Enantiornithes. tarsus. As an example, Sagittarius (the Secretary bird) In this unassociated and mixed assemblage are two has a LlW ratio of 31/1 while Allosaurus (a Jurassic metatarsi (Figs.2K-L, 3J-P) that closelyresemble, and theropod) has a LlW ratio of only 2.5/1. Figure 1shows share derived characters with, the metatarsus from that the overlap of the two clades (birds and theropods) Montana (UCMP 117600)described here. These meta- is almost total for animals with a metatarsus longer tarsi mark the first occurrence of the same Cretaceous than 80 mm, based ori a sample of 31 families of di- dinosaur genus from both Laurasia and Gondwana- nosaurs and birds. Consequently, the length/width ra- land at the end of the Mesozoic, a time when the two tio of the metatarsus can no longer be considered a supercontinents were supposedly still separate ~Kauff- valid criterion for separating birds from theropods and man, 1977; Rage, 1981). This supports an earher ~y- other dinosaurs. It must be emphasized that in the pothesis (Brett-Surman, 1979) of a land connection fossil record, size alone cannot be used to distinguish between North and South America based on the pa- taxa without reference to actual structural and mor- leobiogeography of hadrosaurs and ceratopsians phological features. (Reptilia; Omithischia). Birds have a fused metatarsus while that of thero- Institutional Abbreviations- BMNH = British Mu- pods is typically unfused. Fusion of this element, or a seum (Natural History), London; HMN = Humboldt less extensive coossification, is known to occur in the Museum Berlin' JM = Jura Museum, Eichstatt; PU = theropods Ceratosaurus (Gilmore, 1920), Coelophysis Princeto~ Univ~rsity; UCMP = University of Califor- (Raath, 1969, =Syntarsus, Paul, 1984), Elmisaurus © 1985 by the Society of Vertebrate Paleontology l33 3 TABLE L Characteristics of metatarsi that separate birds and dinosaurs. Birds Dinosaurs 1) Metatarsal fusion distal 1) Fusion rare, restricted to 2-----/ at first, then: proceeds proximal end; bundle I I proximally; metatarsal coossification rare; com- I I I I bundle fusion complete. plete bundle fusion ab- I I sent. I I Metatarsal III proximally Often proximally I 3 I 2) 2) 2 pinched, not visible in pinched and visible over / I anterior view at proxi- entire length. mal end. II 4A " I 5/ 1~ I 3) Foramen present be- 3) No such foramen. 18BI 6 ~~ tween metatarsals III & IV. 9 19 /10 8/ /1 4) Distal tarsals always 4) Complete fusion absent, 11 / completely fused to except in Elmisaurus. HA'®P 13 Q//16 E metatarsals in adults. I 12 / F 150 / I 5) Hypotarsus very promi- 5) Hypotarsus moderately 1 I // nent, developed behind developed behind meta- II,J / metatarsal III (except tarsals II & IlL 14 .•.• L K/G H esperornis ), N ~M in Archaeopteryx is more closely related to age and R. health. In Elmisaurus, it may be the normal condition w (Osmolska, 1981). Table 1 lists five characteristics of metatarsi. Each characteristic may occur in both groups with varying 2 3 degrees of frequency, but taken as two "character l suites," they reliably separate the metatarsi of thero- FIGURE 1. A log-log scale of the length (X-axis) versus pods from those of post-Archaeopteryx birds. the midshaft diameter or width (Y-axis) of metatarsi in nat- Theropod and avian metatarsi represent an excellent ural articulation. Integers on the axes are powers of 10 mea- example of both the similarities and the differences sured in millimeters. Numbers refer to bipedal dinosaurs and that can be found within an ancestor-descendant re- letters refer to birds. @ = A visaurus, • = Dipus (Rodentia, lationship. The similarities include a laterally com- Mammalia). Dinosaurs: 1 = Tyrannosaurus, 2 = Allosaurus, 3 = Ceratosaurus, 4 + 7 = Ornithamimus, 5 + 11 = Dryo- pressed, tridactyl pes in which metatarsal I no longer saurus, 6 = Ornitholestes, 8 = Gallimimus, 9 = Hypsilopho- articulates with the ankle, digit I is a semi-reversed don, 10 = Nanosaurus, 12 = Laosaurus, 13 = Coelophysis, hallux, and metatarsal V is either extremely reduced 14 = Archaeopteryx (postcranially a dinosaur; based on or lost. This adaptive suite is not found in any other BMNH 37001, HMN MB.1880/8L4598), 15 = Compsog- tetrapod clade and may have evolved only once. As nathus, 16 = Struthiomimus, 17 = Macrophalangia, 18 = such, it is an excellent minimum definition of the ther- Velociraptor, 19 = Elmisaurus. Birds: A = Pachyornis, B = opod-bird clade. Diornis (juvenile), C = Diatryma, D = Diornis (adult), E = The differences, however, are also important. In Rhea, F = Dromaeus, G = Sagittarius, H = Pygoscelis, I = birds, fusion of the metatarsal bundle starts distally Pandion, J = Sula, K = Apteryx (juvenile), L = Falco, M = and migrates proximally with ontogeny (Fig. 2). In Aramides, N = Opisthocomus, 0 = Apteryx (juvenile), P = Apteryx (adult), Q = Hesperornis, R = Ichthyornis. The theropods, the coossification is always more pro- regression equation for both birds and dinosaurs, excluding nounced proximally (Fig. 2). This is also the case in the penguin, is Y = L06X - 0.92 and where r = 0.8. the Montana and Argentina metatarsi (Fig. 3). Note that metatarsal fusion/coossification in theropods oc- curs erratically, being found in some, but not all, basal and derived taxa. Almost all birds (except for some (Osmolska, 1981), and in Archaeopteryx (Ostrom, psittacids and spheniscids) including Hesperornis and 1976). Possible occurrences have also been reported Ichthyornis, have a metatarsal III that is proximally in A vipes ('f on Huene, 1932) and in H eterodontosaurus pinched and hidden behind metatarsals II + IV. Vary- (Santa Luca, 1980). Indeed, metatarsal fusion is typical ing degrees of proximal and lateral compression of gracile ungulates, and is also known in a 'rodent (pinching) of metatarsal III is a derived feature found (Rich, 1973). Fusion or coossification in dinosaurs and in the theropod dinosaurs most closely linked to bird 134 JYP 5(2), June 1985 l •••••ABC DE -~"F G ,'; " I' ,!, , I i ; ! i if! i I fl 1 .~ t . ~, !li\ I' \!l y1; ~f •. ~ r ( : I I 'It ~.; I ~ III ._---/ J K L M k-',: :1 .~ . .!( k-' ,~' " i. FIGURE 2. Left metatarsi of dinosaurs and birds in anterior view (shaded) and proximal view (black). A, Ceratosaurus; B, Velociraptor, C, Elmisaurus; D, Archaeopteryx (based on BMNH 37001, HMN MB.1880/81.4598); E, Ichthyornis; F, Hes- perornis; G, Rhea (juvenile); H, Pygoscelis; I, "Enantiornithes," UNT 4021; J, "Enantiornithes" UNT 4053; K, Avisaurid UNT 4048; L, Avisaurus sp., UNT uncataloged specimen; M, Avisaurus archibaldi (holotype); N, mid shaft cross-section of Avisaurus archibaldi; 0, midshaft cross-section of Elmisaurus. Knob for proposed origin ofm. tibialis anticus = k. Figures 2 and 3 by the junior author. ancestry (paul, 1984), and in true birds. Examination the above criteria. However, it is beyond the scope of ofArchaeopteryx(BMNH 37001, JM SoS 2257) shows this paper to settle the higher systematics of Archaeop- a moderate degree of proximal compression of meta- teryx only on the basis of its metatarsus. tarsal III (contra Ostrom, 1976). The Montana and It is readily apparent that the Argentina and Mon- Argentina metatarsi contrast in having a proximally tana metatarsi are more like those of theropods than robust metatarsal III, a condition found elsewhere in of birds. It is possible that these metatarsi are those of such basal theropods as Coelophysis ( =Syntarsus) and enantiornithids, and that the enantiomithids represent Ceratosaurus.
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