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These two (partial) bones were collected two summers ago (2021) on private land in Weld County, Colorado. They’re from a super fossiliferous bit of exposure which, based on the Oreodont fauna, is likely to be part of the Poleslide Member, which dates to the early Oligocene. These bones have puzzled me since I collected them, and I had for quite a while hypothesized that they must be some sort of ungulate metatarsal. I decided to pull them out again tonight, and having more experience with osteology generally I have decided that these look decidedly avian - specifically, the proximal ends of a right tarsometatarsus. The White River Formation has a rather diverse formally described avifauna: Benton et al. (2015) lists six genera in five families along with several different morphologies of bird eggs in The White River Badlands: Geology and Paleontology, however only one family (the Bathornithidae) has more than one described genus. Bird fossils are well-documented from the WRF, and incidentally quite a few avian holotypes have been described from the White River Formation in Colorado, so perhaps the preservational bias favors them more in that general region. However, bird fossils are simply generally rare in the fossil record due to their delicate nature, so I thought I’d post on the forum before I make a (relatively) extraordinary claim. Now, these two bones aren’t in any way associated, but they’re the exact same sort of bone. They’re identical (with the exception of the shorter one being slightly larger) - same bone, same taxon (as far as I can tell, the shorter one has some weathering). This is what puts me off even more - what are the odds that not only did I come across bird fossils, but that I independently found the exact same end of the exact same bone from the exact same avian taxon in the same week? Tarsometatarsus bones do seem to be a fairly commonly preserved isolated element for fossil birds, many genera are described based of a single tarsometatarsus and the same summer I found these I happened to find an avian tarsometatarsus in the Hell Creek Formation. But it would still be odd. Here are the pictures: I should also add that these bones are in fact thin-walled and hollow, though getting the camera to focus on the broken ends is difficult. But that doesn’t automatically make them avian, bones are fairly frequently hollowed out by simple decay. Below is an image from Benton et al. (2015), with an illustration of the holotype tarsometatarsus of Badistornis aramus, an extinct relative of the living limpkin, for comparison: My bones actually compare quite well to this illustration, though there are differences so I wouldn’t assign them to this genus/species even if I could confirm that they are bird tarsometatarsi. I also don’t think that they’re Bathornithid bones, even though those are the most famous birds from this rock unit. Of the genera from the WRF only Bathornis has a known tarsometatarsus, and it’s both morphologically very different and much larger than my bones. I’ve also ruled out the Galliformes and Falconiformes (two bird orders with representatives in the WRF), again assuming these are avian bones. I think that it compares best with wading birds such as herons and, indeed, limpkins. But this is mostly informed speculation I admit. So does anyone have any thoughts? Is there a more obvious answer for what this bone could be, or does it truly have avian affinities in your opinion? Looking forward to hearing everyone’s thoughts. References: Benton, R. C., Terry, D. O., Evanoff, E. and McDonald, H. G. (2015). The White River Badlands: Geology and Paleontology. Indiana University Press.