Featured Fossil: Chimaeroid egg capsule from Oregon
This fossil specimen was found on a NARG (North America Research Group) field trip in August 2009. It is from the Keasey Formation (Oligocene) near Vernonia, OR. The formation is marine sediment, but we have found leaf fossils in the formation. We knew it was something unique, but what was it? We thought it might be a sea pen; another thought was a large seed pod. We asked some of the local experts, but they weren’t sure. It was later identified as a Chimaeroid Egg Capsule. These fossils have been found in deposits as old as the Devonian outside of the Pacific Northwest. As far as we know, it’s the first one of its kind found in Oregon. It will be donated to the University of Oregon Condon Collection where the hope is that research will reveal a new species.
Aaron Currier, North American Research Group, Salem, Oregon
The paleontologist’s perspective
Sediments of the Keasey Formation crop out in the extreme northwestern part of Oregon. This rock unit consists of mudstones, silts and sandstones that are of early Oligocene age (about 30 million years old). The Keasey Formation contains exceptionally well-preserved fossils, including those of crinoids, corals and the chimaeroid egg capsule depicted here. Fossil localities consisting of exceptional preservation are referred to as Lagerstätten, a term of German origin, loosely translated to mean “mother lode.” Konservat-Lagerstätten is a term used to describe localities where soft tissues are preserved. These deposits are exceedingly rare, but are of great scientific value to paleontologists.
Chimaeras are fishes with cartilaginous skeletons and they have a spotty fossil record extending back 400 million years into the Paleozoic Era. Fossils of chimeras are mainly represented by isolated tooth plates, fin spines and egg cases. Fossilized egg capsules are very rare, having been reported from only a handful of localities of Mesozoic and Cenozoic age (Brown 1946). Obruchev (1967), who reviewed the fossil record of egg capsules, reported only 31 occurrences worldwide, and only one record from the Cenozoic. This record from the Oligocene of Alaska was given the name Chimaerotheca alaskana by Brown (1946), but was referred to the genus Harriotta (spookfish) in Obruchev (1967). To our knowledge, the fossil from the Keasey Formation is the only other example of a chimeroid egg case reported from the Cenozoic.
They share the cartilaginous (not bony) skeletons with sharks, and thus are classified together in the group called Chondrichthyes. Modern representatives of the chimaeras include rat fish, rabbit fish and the elephant “shark” depicted below. The elephant shark (Callorhinchus milii) is native to the temperate waters off southern Australia and New Zealand, living at depths of 200 to 500 meters and also migrating into shallower waters. Studies published just this year (Venkateash et al. 2014) of the genome of the modern chimaera (Callorhinchus milii) indicate that they are the most primitive vertebrates with jaws. Both from paleontological and genomic points of view, chimaeras and their close relatives (called holocephalians) have remained relatively unchanged for millions of years through evolutionary history. They are examples of what paleontologists call “living fossils.”
The chimaeroid egg capsule from Oregon is a rare find, one of potential interest in unraveling the embryonic characteristics of these enigmatic ancient vertebrates.
Bruce MacFadden, Florida Museum of Natural History, University of Florida
Austin Hendy, Florida Museum of Natural History, University of Florida
For further reading
Brown, R. 1946. Journal of Paleontology, Vol. 20, No. 3, pp. 261-266 http://www.tandfonline.com/doi/abs/10.1080/.U5sN5MacQSY
Obruchev, D. V. “Fossil chimaera egg capsules.” International Geology Review 9.4 (1967): 567-573.
Venkatesh, B. et al. 2014. Elephant shark genome provides insights into gnathostome evolution. Nature, volume 505, pages 174-179, http://www.nature.com/nature/journal/v505/n7482/pdf/nature12826.pdf