Great Basin Naturalist
Volume 43 Number 4 Article 3
10-31-1983
A mammalian humerus from the Upper Jurassic of Colorado
Donald R. Prothero Knox College, Galesburg, Illinois
James A. Jensen Brigham Young University
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Recommended Citation Prothero, Donald R. and Jensen, James A. (1983) "A mammalian humerus from the Upper Jurassic of Colorado," Great Basin Naturalist: Vol. 43 : No. 4 , Article 3. Available at: https://scholarsarchive.byu.edu/gbn/vol43/iss4/3
This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. A MAMMALIAN HUMERUS FROM THE UPPER JURASSIC OF COLORADO
Donald R. Prothero' and James A. Jensen-
Abstract.— The first reported mammal fossil from Dry Mesa Quarry (Upper Jurassic Morrison Formation, Mesa
Countv, Colorado) is the distal end of a right humerus. It is very similar to humeri described by Jenkins (1973) from the Morrison Formation at Como Bluff, Wyoming. It has a distinct ulnar condyle and a spiral humero-ulnar joint, both features found in prototherian mammals but not in therians.
Postcranial remains of Jurassic mammals bones usually oriented at right angles to the are extremely rare. An articulated skeleton of stream flow. Sorting was biased by shape a dryolestid therian mammal has been report- rather than by size. ed from the Jurassic of Portugal (Henkel and Sediments overlying the bone layer are
Krebs 1977), but is still undescribed. Early predominantly light-colored, cross-bedded Jurassic mammalian postcranial fossils are sands with occasional lenses of clay and fine also known from India (Datta et al. 1978), gravel, the latter often containing clay peb- but are undescribed. A few fragmentary post- bles. Sediments underlying the bone layer are cranial remains of mammals have been de- principally a light, blue-green clay with oc- scribed from the Upper Jurassic of England casional traces of bright yellow zones of (Seeley 1879, Simpson 1928, Haines 1946) oxidation. and from the Upper Jurassic Morrison For- The bone layer consists of an imusual vari- mation of Wyoming (Jenkins 1973). Of the ety of disarticulated bones of all sizes, includ- five important mammal-producing localities ing specimens representing crocodilians, fish, in the Morrison Formation (listed in Clemens turtles, pterosaurs, four new theropods, an et al. 1979:23-26), two have produced mam- unknown variety of sauropods, some ornitho- malian postcranial fossils prior to this paper: pods, and the mammal described herein. Due Como Bluff, Wyoming (Jenkins 1973), and to the great variety of disarticulated bones in the Fruita Paleontological Area, Mesa Coun- the deposit, and the generic novelty of the ty, Colorado (Rasmussen and Callison 1981). fauna, descriptive work has been postponed In 1977, the distal portion of a right hu- until enough material is available. Field work merus of a mammal was found in Dry Mesa has been carried out for the last 10 years. The Quarry, Mesa County, Colorado. This speci- following have been identified so far: men (BYU 2026) was first mentioned by Cle- Lungfish tooth plate (probably Ceratodus— mens et al. (1979:24), and is described below. K. Thomson, pers. comm.) Pterodactyloid phalanx (Jensen and Ostrom Locality and Associated Fauna 1977) Torvosaurus tanneri, a megalosaur (Galton
Dry Mesa Quarry is located in the lower and Jensen 1979) section of the Brushy Basin Member of the Prototherian mammal humerus (this paper) Upper Jurassic Morrison Formation, 135 feet below its contact with the overlying Cre- Description taceous Cedar Mountain Formation. The quarry sediments include very fine to coarse BYU 2026 (Fig. 1) is the distal portion of a sands, grits, and fine gravels containing angu- right humerus of a mammal. It has been bro- lar to well-rounded clay and bone pebbles. ken at midshaft, but is otherwise well pre- Stream gradient was sufficient to move very served. There is relatively little evidence of large bones, with the long axes of all large crushing or distortion. The shaft cross-section
'Department of Geology, Knox College, Galesburg, Illinois 61401. Earth Science Museum, Brigham Young University, Prove, Utah 84602.
551 552 Great Basin Naturalist Vol. 43, No. 4
is a mediolaterally compressed triangle with the apex pointing anteriorly. Distally the shaft expands transversely to form the large medial and lateral epicondyles. The distal end of the hiunerus is naturally flattened an- teroposteriorly. The long axis of the distal end is not perpendicular to the major axis of the shaft cross-section, but is rotated about 20 degrees clockwise (viewed from the distal end). The shaft of the humerus has a strong anterior crest that is deflected anterolat- erally. This crest is probably the distal end of the deltopectoral crest. A faint posterolateral crest merges with the lateral epicondyle.
The medial epicondyle is considerably lateral epicondyle. more prominent than the Fig. 1. BYU 2026, distal end of a right humerus. A, It flares medially and is anteroposteriorly Anterior view. B, Posterior view. C, Distal view. Abbre- viations: dpc, deltopectoral crest; enf, entepicondylar compressed. The entepicondylar foramen is foramen; le, lateral epicondyle; me, medial epicondyle; visible in anterior view. It is broken at the of, olecranon fossa; re, radial condyle; rf, radial fossa; uc, anterior end, where it passes anteromedially. ulnar condvle. The lateral epicondyle merges with the radial condyle. It is connected to the shaft of the 18). The axis of the ulnar condyle as it crosses humerus by a thin posterolaterally arched over the distal end of the humerus is at an crest. The radial and olecranon fossae are in- approximately 60 degree angle to the trans- terconnected, forming an apparent supra- verse (interepicondylar) axis of the humerus trochlear foramen. This feature may be an ar- (seen in distal view). This compares with an- tifact of breakage, however. gles of 58-65 degrees reported by Jenkins In anterior view, the main body of the (1973:286) for several humeri from Como shaft bifurcates to form crests joining the Bluff, Wyoming. radial and ulnar condyles. These crests sur- round the radial fossa. In posterior view, the Discussion olecranon fossa is broadly concave and ex- tends partially up the shaft. From this view The Dry Mesa Quarry mammal very close- the apparent supratrochlear foramen has an ly resembles the Como Bluff humeri de-
irregular margin that is clearly enlarged by scribed by Jenkins (1973). It differs from breakage. them in having a more bulbous and broader In distal view, three main features are ulnar condyle. In this respect, it is more like seen: the medial epicondyle, the ulnar con- the humeri referred to the multituberculate dyle, and the radial condyle-lateral epicon- Catopsolis by Jenkins (1973, Fig. 19). The dyle. The latter two features are confluent multituberculates Tugribataar (Kielan-Jawo- and separated only by a shallow groove. The rowska and Dashzeveg 1978) and Ptilodus ulnar and radial condyles, on the other hand, (Gidley 1909), the triconodont Eozostrodon are separated by a narrow, deep inter- (Jenkins and Parrington 1976), and the mon-
condylar groove. The radial condyle is broad otremes (Howell 1937, Haines 1946) also and bulbous in anterior view. The spiral ul- have prominent bulbous ulnar condyles. BYU
nar condyle is very similar to that shown by 2026 clearly does not have a trochlear con-
Jenkins (1973, Fig. 13). It is wrapped around dyle, which Jenkins (1973) considers charac- the distal end of the humerus, with a prox- teristic of therian mammals. imolaterally oriented extensor surface and a The only other feature that distinguishes proximodistally oriented flexor surface. How- the Dry Mesa Quarry mammal from the ever, the anterior portion of the ulnar con- Como Bluff humeri is the apparent supra-
dyle is more bulbous than the same feature in trochlear foramen. As noted above, this fea- the humerus figured by Jenkins (1973, Fig. ture may be an artifact of breakage. a
October 1983 Prothero, Jensen: Jurassic Mammalian Humerus 553
The affinities of BYU 2026 are difficult to Literature Cited assess based on such hmited evidence. The Clemens, W. A., A. Lillec;raven, E. H. Lindsay, and presence of a distinct ulnar condyle with a J. G. G. Simpson. 1979. Where, when and what— spiral humero-ulnar joint is characteristic of survey of known Mesozoic mammal distribution. 7-58 prototherian mammals (Jenkins 1973). The Pages in J. A. Lillegraven, Z. Kielan-Jawo- rowska, and W. A. Clemens, eds., Mesozoic advanced therian trochlear condyle is known mammals: the first two-thirds of mammalian his- from rocks as old as the Lower Cretaceous tory. Univ. of California Press, Berkeley. (Jenkins 1973, footnote 3). The Dry Mesa p. M., p. B. Datta, Yadagiri, and R. J. Rao. 1978. Dis- Quarry mammal humerus could have be- covery of Early Jurassic micromammals from up- longed to a number of prototherian mammal per Gondwana sequence of Pranhita (iodavari Valley, India. Geol. Soc. India 19:64-68. taxa presently known from the Morrison For- J. p. A. Galton, M., and J. Jensen. 1979. A new large mation (Prothero Clemens et al. 1981, 1979). theropod dinosaur from the Upper Jurassic of It could also have come from some of the Colorado. Brigham Young Univ. Geology Studies. primitive Morrison therian mammals that 26(2):1-12. Gidley, W. 1909. Notes on the fossil mammalian may or may not have had a trochlear con- J. genus Ptilodus, with descriptions of new species: dyle. Until the Portuguese dryolestid therian Proc. U.S. National Museum .36:611-626. skeleton (Henkel and Krebs 1977) is fully de- Haines, R. VV. 1946. A revision of the movements of the in tetrapods. 80:1-11. scribed, we cannot rule out the possibility forearm J. Anatomy tliat the Dry Mesa Quarry humerus belonged Henkel, S., and B. Krebs. 1977. Der erste Fund eines Saugetier-Skelettes aus der Jura-Zeit. Umschau in to a very primitive therian mammal. Of the Wissen.schaft und Technik 77:217-218. possible candidates nontherian among mam- Howell, A. B. 1937. The swimming mechanism of the mals, BYU 2026 resembles the known humeri platypus. 18:217-222. J. Mammal. of multituberculates and triconodonts. The Jenkins, F. A., Jr. 1973. The fimctional anatomy and ev- olution of the mammalian humero-ulnar articu- skeleton of docodonts is presently unknown. lation. Amer. Anat. 137:281-298. The systematic affinities of the Dry Mesa J. Jenkins, F. A., and F. R. Parrington. 1976. The post- Quarry mammal cannot be determined more cranial skeletons of the Triassic mammals Eo- precisely at present than Mammalia incertae zostrodon, Megazostrodon, and Enjthrotherium. sedis. Roy. Soc. London Philos. Trans., B (Biol. Sci.) 27.3:387-431.
Jense.n, J. A., and J. H. Ostrom. 1977. A second Jurassic pterosaur from North .-America. Paleont. Acknowledgments J. 51:867-70. Kielan-Jaworowska, Z., and D. Dashzeveg. 1978. New The Dry Mesa Quarry has been worked for Late Cretaceous mammal locality in Mongolia 10 years by the Earth Science Museum of the and a description of a new multituberculate. Brigham Young University under permits Acta Palaeont. Polonica 23:115-130. Prothero, D. R. 1981. New Jurassic mammals from from the U.S. Forest Service, as authorized Como Bluff, Wyoming, and the interrelationships by the Antiquities Act of 1906. Funds for this of the non-tribosphenic Theria. Bull. Amer. Mus. work have been provided by research sup- Nat. Hist. 167 (art ,5):277-326. port grants from Brigham Young University, Rasmussen, T. E., and G. Callison. 1981. A new spe- cies of triconodont mammal from the upper with additional support from the National Ju- rassic of Colorado. Paleont. 55:628-6.34. Society, R. J. Geographic Kenneth Thomson, Seeley, H. G. 1879. Note on a femur and humerus of a and several other private donors. small mammal from the Stonesfield Slate. Quar- Measurements of angles were made with terly Journal (Teological Society of London an E.P.O.I. Shopscope. D.R. Prothero was 35:456-463. Simpson, G. G. 1928. A catalogue of the Mesozoic mam- supported by a Columbia Faculty Fellowship malia in the Geological Department of the Brit- diu-ing preparation of this paper. We thank ish Museum. British Museum (Natural History), Henry Galiano for advice on the illustration. London.