Diatryma - pegadas fósseis da ave gigante

Giant bird Diatryma footprint found in Whatcom County

By Dave Tucker

June 30, 2010

FOSSIL FOOT PRINT OF GIANT EOCENE BIRD, DIATRYMA, FOUND IN CHUCKANUT FORMATION

The slab bearing the Diatryma track measures 28 x 26 cm. Click to enlarge any image.

A 50-million-year-old fossilized footprint of Diatryma, a giant flightless bird, has been found in the Eocene Chuckanut Formation in Whatcom County, northwest Washington State.

The single three-toed footprint measures 25 x 27.5 cm (10 by 11 in) and appears to be the world’s only authenticated Diatryma track. Diatryma (Greek, ‘through a hole’, a reference to perforations in some of the foot bones) was a 2+ meter tall (6.5 feet), perhaps 175 kg (385 pound) flightless bird (Feduccia, 1999). The giant bird’s foot track is impressed 2 cm into a thin veneer of mudstone laminated over sandstone on a 1.5 m x 1 m x 20 cm (5 x 3 foot x 8”) slab. The bird walked across mud deposted on top of sand, probably on the bank of a stream in the subtropical floodplain environment of the Chuckanut Formation. Western Washington University’s geology department has removed the rock slab with the footprints via helicopter for display later in 2010. Subscribe to this website to obtain a post about the opening of the exhibit at WWU later this year.

Skeleton and foot bones at Museum of Natural History, New York.

The trace fossil was identified as a Diatryma track by Western Washington University paleontologist George Mustoe. George is well known for his published studies on Chuckanut Formation fossils, including tracks of smaller birds and mammals (see references below). There are other bird tracks on the same slab. Most of these are from unknown small wading birds, and measure only 2-3 cm across. There is a partial track of a heron-sized bird on one edge of the slab.

The Diatryma trace fossil was discovered in spring of 2009, as Mustoe and Keith Kemplin looked at plant fossils in the rubble of the January, 2009 Racehorse Creek landslide. The track was in plain view nearly a year, and was a well-known visitor attraction for those willing to make the climb through the slide debris. Recognizing the value of this track to science and as a public attraction, a volunteer committee materialized to keep watch over the track and to arrange for its protection and eventual removal for display. In spring of 2010, an incipient fracture began to separate the thin mudstone veneer holding the Diatryma track from the sandstone underlayment of the slab.

Chisel marks left after two tracks were illegally removed from the gray area to right of Diatryma track. Compare with first photo on this post.

Shortly after, someone chipped away and removed two of the smaller bird tracks on the margin of the slab. Given the friable nature of the rock, those tracks were probably destroyed in the process. To protect the slab from further damage or even from outright theft by commercial or amateur fossil hunters, the committee moved the ‘Big Bird’ slab on a purpose-built wooden sled a few meters away and covered it with about 1 ton of sandstone slabs.

Vegetarian, or carnivore?

The first thing that jumps out when you see a Diatryma reconstruction is the monstrous beak. At first glance, and probably the next several, you’ll think “Carnivore! Glad they are extinct!” But wait. Whether or not Diatryma was herbivorous or carnivorous is a matter of some controversy in the world of avian paleontologists. Papers have been published examining the evidence for carnivory. Some artist’s renderings (and here’s another) show fierce Diatrymas preying on the early horse Hyracotherium (once known as Eohippus); tracks of this or a similar small mammal, perhaps early tapirs, are found in the Chuckanut Formation. Whitmer and Rose (1991) wrote a highly readable and nicely illustrated study of Diatryma jaw mechanics, and concluded that this gigantic bird was a meat eater, specializing in bone-crushing, though they could say whether via predation or scavenging. But, that study did not demonstrate carnivory, only that the gigantic bird possessed a very strong mandible and beak. Other studies (e.g. Andors, 1991, 1992) are equally convincing for folivory or leaf eating. Andors asserts that a heavy beak would be very useful for cutting tough vegetation, and flightlessness is generally associated with vegetable diet rather than carnivory. That is because leaves take time and considerable energy to digest in the bird’s stomach, energy that would otherwise be used for flight. Andors also states the morphology of the pelvis and femur to indicate a pedestrian (walking or striding), rather than a cursory (running) gait.

Artist depiction of a Diatryma family scene. Note the giant beak; wings are vestigial.

The Chuckanut foot track sheds further light on the subject. What? How does a footprint do that? Most reconstructions of the feet of Diatryma show long, grasping talons, similar to modern raptors. Such toes are almost universally present in carnivorous birds, and are used to snag and hold prey. Skeletal foot remains, however, do not indicate the amount of flesh covering the bones of the foot. The newly found track clearly shows only very short, small, pointy claws at the front of the toes, not long talons. This condition is more commonly associated with plant-eating birds.

This skeleton is in Helsinki, but came from Wyoming

The combination of huge size, flightlessness, and the talon-free toes probably swings the balance in favor of a plant-eating Diatryma, certainly to the dismay of many hoping for a more fearsome beast . We’ll see how well interpretations based on the foot track pass muster when the paper (Mustoe, Tucker and Kemplin) is eventually peer reviewed.

The original anatomical description of a largely complete fossilized Diatryma, with numerous photos of skeletal parts, is found in Mathew and Granger (1917).

The Rescue

The fossil bird track was flown out with the help of Columbia Helicopters (of Portland Oregon) on Monday July 12. The giant Boeing/Kawasaki Vertol 107-II. Despite the thin drizzle and light breeze, the heavy wooden sled constructed to support the ~1300 pound (585 kg) slab was flown with little drama to the waiting WWU flatbed. After a safe landing, interviews and photos were taken for the Bellingham Herald story (to be published Tuesday, July 13) written by Dean Kahn. There were lots of happy and relieved volunteers once the slab was firmly secured to the truck.

The rock slab and its wooden skid rests safely on the truck in the rain.

Diatryma? Or Gastornis?

Very similar fossil remains of the same age in Europe have been placed in the genus Gastornis. Reevaluation of the taxonomy may yet show that Diatryma may be congenetic with this other huge bird (Buffetaut, 1997). If that is the case, then the genus Gastornis will take precedence, as it was used first in taxonomic descriptions. In the meantime, ‘Diatryma’ is usually used for the North American birds. In the early Eocene, North American and Europe were still connected by a polar land bridge through Greenland and Scandinavia, so the birds could have spread one way or the other; most specialists will agree that radiation was probably east to west.

There is enormous popular interest on the internet for Diatryma / Gastornis. Search images on the internet for ‘Diatryma’ to see for yourself. Diatryama/Gastornis is a character in an online game, ‘Carnivores: Ice Age’(even though they lived in the Eocene, not Pleistocene, but what’s a few million years, anyway?), and in something called ‘Final Fantasy XI’. There are Facebook pages for these birds, too (of course). And then there is this Diatryma toy, this Diatryma postage stamp from Tanzania, this one from Palau, and this one from Oman. Oh, and these cleverly disguised bank robbers. And last, these French rockers from 1973, who named there band ‘Diatryma’.

References:

• Andors, A. V., 1991, Paleobiology and relationships of the giant groundbird Diatryma (Aves: Gastornithiformes): Acta XX congressus internationalis ornithologi
• Andors, A.V., 1992, Reappraisal of the Eocene groundbird Diatryma (Aves: Anserimophae): In K. E. Campbell, Jr. (ed.), Papers in avian paleontology honoring Pierce Brodkorb, Science Series No.36: p 109-125. Los Angeles: Natural History Museum of Los Angeles County.
• Buffetaut, E., 1997, New remains of the giant bird Gastornis from the Upper Paleocene of the eastern Paris Basin and the relationships between Gastornis and Diatryma: Neues Jahrbuen fur Geologie und Palaontologie, v. 1997, n. 3 p 179-90. (in English)
• Feduccia, Alan, 1999, The origin and evolution of birds (2^nd Edition): New Haven : Yale University Press, 1999. 466 pages. The section on Diatryma is partially available online at Google Books. A copy is available at the Western Washington University library.
• Mathew, W.D., and Granger, W., 1917, The skeleton of Diatryma, a gigantic bird from the Lower Eocene of Wyoming: Bulletin of the American Museum of Natural History, v. 37, p. 307-336.
• Mustoe, G., Dilhoff, R., and Dillhoff, T., 2007, Geology and paleontology of the early Tertiary Chuckanut Formation: in Stelling and Tucker, eds., , Floods, Faults, and Fire: Geological Field Trips in Washington State and Southwest British Columbia (Cordilleran GSA Field Guide). A full bibliography of other Chuckanut paleontology papers by Mustoe and various colleagues is in the references at the back of the above listed paper.
• Witmer, L.M., and Rose, K.D., 1997, Biomechanics of the jaw apparatus of the gigantic Eocene bird Diatryma: implications for diet and mode of life: Paleobiology, v. 17, n. 2, p. 95-120.

Fonte: http://nwgeology.wordpress.com/the-fieldtrips/the-chuckanut-formation/giant-bird-diatryma-footprint-found-in-whatcom%C2%A0county/