Friday, March 7, 2014

Megachasma applegatei: A new megamouth shark from the Oligocene and Miocene of California and Oregon



In 1976, a strange large bodied shark with a wide mouth and a multitude of tiny, unicuspate teeth was discovered after being entangled in an anchor of a US Navy ship off the coast of Hawaii. Preliminary examination indicated it was an entirely new genus and species of filter feeding shark, not similar or closely related to basking sharks (Cetorhinus) and whale sharks (Rhincodon). It was named several years later as Megachasma pelagios – the megamouth shark. Megachasma is approximately 4-6 meters in length, inhabits temperate waters of the Atlantic, Pacific, and Indian Oceans, and extraordinarily rare – only 55 specimens have been observed since its discovery, explaining why this shark took so long to discover (in contrast, most other large bodied sharks at temperate latitudes have been known to science since the 18th century).






The modern megamouth shark, Megachasma pelagios.




This week, a new species of fossil megamouth – named Megachasma applegatei after the late paleoichthyologist Shelton Applegate – was described by Kenshu Shimada, Bruce Welton, and Doug Long. Fossil teeth of M. applegatei occur in the late Oligocene-early Miocene Pyramid Hill member of the Jewett Sand near Bakersfield (California), the Skooner Gulch Formation in Mendocino County (California), and the Yaquina Formation and Nye Mudstone of coastal Oregon. Oddly enough, despite being named recently, the first fossils of this new species were discovered (at the Pyramid Hill locality) fifteen years prior to the discovery of the modern megamouth shark – which sort of makes the modern megamouth shark a living fossil.



The holotype and some paratypes of Megachasma applegatei.

Shimada et al. (2014) describe in total a series of 67 teeth (see above) – virtually every specimen present in museum collections. Many more specimens are present in private collections, but are useless to paleontologists interested in publishing as they are not publishable specimens. Private specimens include many published in an earlier study by de Schutter (2009), who unfortunately published photographs and descriptions of specimens in private collections. The 67 specimens reported by Shimada et al. (2014) include all publishable specimens, and constitutes a fairly large sample set. Other Cenozoic sharks are represented by tens of thousands of specimens – but a fair amount of variation is recorded in this sample. This large sample demonstrates two primary morphological differences between Megachasma applegatei and extant Megachasma pelagios: relatively shorter crowns (relative to root size) and the primitive retention of lateral cusplets in M. applegatei. The lateral cusplets and overall morphology of the teeth of M. applegatei are reminiscent of sand tigers (Odontaspididae), and appear to retain some primitive lamniform tooth morphology.





The rather large sample size of Megachasma applegatei. Serious kudos to the authors for figuring every single specimen!



The authors also review the rest of the published fossil record of Megachasma, and demonstrate that most Cenozoic teeth fall into two categories: Megachasma applegatei and similar teeth from Belgium from Mio-Pliocene deposits, and younger Pliocene specimens much more similar to extant Megachasma pelagios (e.g., Pliocene Yorktown Formation, Lee Creek Mine, North Carolina). The third species, Megachasma comanchensis, was described earlier by Shimada (2007) from the Cretaceous of the western interior (USA) but has been challenged by other authors as not genuinely representing a Cretaceous megamouth shark.


Proportional differences between M. applegatei and M. pelagios. Note the overlap between the two. From Shimada et al. (2014).


This study and two recent papers on fossil basking sharks mark the return of paleoichthyologist Bruce Welton, who published quite a bit during the 1970’s and 1980’s, but was less productive prior to his retirement from the petroleum industry. I’m truly pleased that this paper is finally out, and am eagerly looking forward to more papers on fossil sharks from the North Pacific. On that note, I will conclude that I have just submitted my own paper on fossil sharks from the region – with Dana Ehret, Doug Long, Evan Martin, and my wife Sarah – so, there will be more to read in the somewhat distant future!


References

De Schutter, P. 2009. The presence of Megachasma (Chondrichthyes: Lamniformes) in the Neogene of Belgium, first occurrence in Europe. Geologica Belgica, 12: 179–203.

Shimada, K. 2007. Mesozoic origin for megamouth shark (Lamniformes: Megachasmidae). Journal of Vertebrate Paleontology, 27: 512–516.

Shimada, K., Welton, B.J., and Long, D.J. 2014. A new fossil megamouth shark (Lamniformes, Megachasmidae) from the Oligocene-Miocene of the western United States. Journal of Vertebrate Paleontology 34:281-290.
 

1 comment:

ethan keeler said...

This is a very cool blog