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A New Partial Mandible of Ichthyornis

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A New Partial Mandible of Ichthyornis SHORT COMMUNICATIONS A NEW PARTIAL MANDIBLE snakes. The recently recognized mandibular fragment OF ZCHTHYORNZS is an almost complete posterior segment of a left mandible consisting of angular, surangular, articular, and prearticular bones (fig. 1) . The intramandibular PHILIP D. GINGERICH articular surface of the angular is well preserved. The Peabody Museum of Natural History Yale University quadrate articular surface of the articular is also well New Haven, Connecticut 06520 preserved, although its postero-medial portion, in- cluding the posterior articular cotylus, is displaced In 1872, 0. C. Marsh described a new Cretaceous slightly forward and upward. bird Ichthyornis &pm based on a postcranial skele- Most of the sutures separating individual bones are ton (Marsh 1872a). Later in the same year, he de- closed in this posterior mandibular segment, indicat- scribed a new reptilian genus Colonosaurus for two ing that the jaw belonged to a mature individual. As toothed jaws found associated with the type speci- nearly as can be determined, the individual bones men of Ichthyomis dispar (Marsh 187213). In the making up the segment are very similar to those of following year, Marsh referred the toothed “Colono- Hesperornis in arrangement. The angular forms the saurus” jaws to Ichthyornis d&par stating that: “On ventral margin of the segment. It extends posteriorly subsequently removing the surrounding shale, the from the intramandibular articulation, tapering to skull and additional portions of both jaws were a point below the glenoid fossa. This appears to be brought to light, so that there cannot now be a rea- the condition in Hesperornis also (the skull of Hesper- sonable doubt that all are parts of the same bird” ornis is presently being restudied). The surangular (Marsh 1873). This assignment was accepted until forms the lateral wall of the segment; it is broken 1952 when J. T. Gregory compared the jaws of Icthy- anteriorly and dorsally. The articular constitutes most ornis dispar to those of mosasaurs. Gregory (1952) of the posterior pomrtionof the fragment. A small me- concluded that the toothed jaws assigned to Ichthy- dial process of the articular is present; however, there ornis by Marsh were actually the jaws of a small was no well-developed retroarticular process in Ichthy- mosasaur which had accidently been deposited to- ornis. Two distinct oblique articular cotylii are present gether with the skeleton of a bird. The recent classi- in the mandibular glenoid fossa for articulation with a fications of fossil birds by Storer ( 1966), Romer double-keeled quadrate. The prearticular is fused pos- (1966, 1968), and Brodkorb (1967) all follow Greg- teriorly with the articular, anteriorly it forms the me- ory and removed Ichthyornis from the toothed birds. dial surface of the jaw segment. The prearticular is After careful study of the toothed jaws associated also broken anteriorly and dorsally. with the type specimen of Ichthyomis dispur (Yale The newly discovered jaw segment was compared Peabody Museum [YPMJ 1450) M. V. Walker directly with the jaws associated with the type specimen (1967) recently stated his opinion that until further of lchthyornis dispar (YPM 1450). The left mandible associated with the type specimen consists of a com- proof is available the toothed jaws should be con- plete dentary-splenial segment, but only the most ante- sidered to belong to Ichthyornis. Fortunately, Walker rior portion of the posterior mandibular segment. The has since found a second partial skeleton of Icthy- right mandible of the type specimen, including parts ornis, again with associated toothed jaws. The jaws of the posterior segment, is mounted on a plaque for associated with Walkers’ partial skeleton consist only exhibition. The quadrate articular region of this jaw of portions of the toothed dentaries; the quadrate was removed from the plaque and cleaned to allow articulations of both jaws are unfortunately missing direct comparison with the new specimen (fig 2). ( Walker, unpubl. ) . The two specimens agree closely in morphology al- I recently discovered an additional mandibular though the new specimen is from an individual fragment of Ichthyornis (I. cf. dispar) which was slightly larger than the type. Unfortunately, the an- catalogued together with bones of a specimen of the terior and lateral portion of the articular region of the much larger Cretaceous bird Hesperornis (YPM 1478) type specimen is badly distorted. By comparison with in the Yale Peabody Museum collection. This new the new segment it is clear, however, that the quad- mandibular fragment of Ichthyornis is the only known rate articular region of the type specimen also had specimen revealing the morphology of the quadrate the two oblique articular cotylii characteristic of birds. articulation. It presumably was collected together with YPM 1478, which was collected by B. F. Mudge DISCUSSION from the Upper Cretaceous Niobrara Chalk, Wallace County, Kansas in 1876. The new mandibular frag- As evidence of affinity with mosasaurs, Gregory ment of Ichthyornis is now catalogued as YPM 6264. (1952) stated that the jaws associated with the type of I. d&par have a suture on the upper edge of the DESCRIPTION OF MANDIBLE surangular for a distinct coronoid bone. I can find Mandibles of Ichthyomis are divided into anterior no trace of this suture on either the left or the right and posterior segments by an intramandibular articu- jaw. The surangular itself is well preserved in both lation, between the splenial and angular bones, per- jaws associated with the type specimen. This region mitting limited flexion similar to that in the mandibles of the new mandibular segment is not preserved. The of Hesperornis, mosasaurs, varanoid lizards, and following characters cited by Gregory as evidence of [4711 472 SHORT COMMUNICATIONS FIGURE 2. Comparison of the new left mandibular segment (left, YPM 6264) with the right mandibular segment associated with the type specimen of Ichthy- ornis di.spar (right, YPM 1450). The anterior and lateral margins of the type specimen (right) are broken and highly distorted. (mp = medial prncess of the articular, other abbreviations as in fig. 1). with the quadrate in the mandible of Zchthyornis (and in Hesperornis and most living birds) differ from the transverse articular cotylus of mosasaurs and other reptiles. The morphology of the quadrate articulation of the new specimen described above and of the right mandible associated with Ichthy- FIGURE 1. Newly discovered posterior segment of or& by Marsh is the best anatomical evidence that a left mandible of Ichthyornis (YPM 6264) in lateral these jaws belonged to a bird. The discoveries by (A), dorsal (B ), and medial ( C ) view. (a = angular, Marsh and Walker of toothed jaws associated with af = articular foramen, ar = articular, ima = intra- Ichthyomis skeletons further confirm that Ichthy- mandibular articulation, par = prearticular, sa = sur- ornis was a toothed bird. angular, ac and pc = anterior cotylus and posterior Bock (1969) has recently questioned whether the cotylns for articulation with the quadrate). teeth associated with Hesperornis actually belong to that bird, stating that no teeth have been found in place in the jaws. The left mandible of the most affinity with mosasaurs are also present in the bird complete Hesperornis skull (YPM 1206) does in fact Hesperornis, which Gregory ( 1951) has shown had have three teeth in place in the dental groove as in fact converged remarkably in skull morphology Marsh (1880) illustrated them in his original recon- with mosasaurs. In both Ichthyornis and Hesperomis struction of this skull. Both lchthyornis and Hesper- the angular tapers to a point below the articular ornis were toothed birds. They are the latest surviv- cotylii. In both, the anterior end of the angular has a ing birds known to have had true teeth. facet along its front edge for articulation with the I thank John Ostrom for his assistance in reviewing splenial. In both, the splenial is visible posteriorly the manuscript, Craig Black for the loan of the skull on the external edge of the jaw where it articulates of Hesperornis from University of Kansas, and C. E. with the angular. In both, the anterior ends of the Ray for the loan of the skull of Hesperornis from the dentaries apparently lack any symphysis. The pre- U.S. National Museum. Myrl V. Walker kindly sent articular is similar in both lchthyornis and Hesper- me a copy of his preliminary description of the newly ornis. It extends forward from the articular and is discovered lchthyornis skeleton. inserted between the splenial and dentary. Thus many of the characters shared by Ichthyornis and ADDENDUM mosasaurs are also present in Hesperomis and are a result of evolutionary convergence. They do not in- Dr. Hildegarde Howard has recently called my atten- dicate close relationship between khthyornis and tion to an additional left posterior mandibular seg- mosasaurs. ment of Ichthyornis (YPM 1761) among specimens The new mandibular segment cannot belong to presently on loan to Dr. Pierce Brodkorb. Dr. Brod- Hesperornis because of the great difference in size korb has kindly made this specimen available to me. (all known posterior mandibular segments of Hesper- The quadrate articular region of YPM 1761 is crushed ornis are approximately three times the length of this dorsoventrally and the specimen is slightly larger than segment). lchthyornis and Hesperornis further dif- YPM 6264; it is otherwise morphologically identical fer in that there is no retroarticular process and the to YPM 6264 described here. teeth are set individually in sockets in Ichthyornis, whereas in Hesperornis there is a large retroarticular LITERATURE CITED process and the teeth are set in grooves. These differ- ences are relatively minor and may be functionally BOCK, W. J. 1969. The origin and radiation of birds. related. Ann. New York Acad. Sci. 167:147-155. The paired, oblique articular cotylii for articulation BRODKORM, P. 1967. Catalogue of fossil birds: Part SHORT COMMUNICATIONS 473 3 (Ralliformes, Ichthyornithiformes, Charadrii- on the extinct toothed birds of North America.
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