So uncommon are the extant sharks that they are referred to by number. Megamouth #1 was caught (this 4.5m mature male was actually snagged by a U.S.N. research vessel) off Hawaii in 1976, and described in 1983. In November 1994, Megamouth #7 (the first female, adolescent, 4.7m) washed ashore in Japan. Scientists worldwide collaborated on its study, the results published in a hard-bound obit/autopsy. Number 11 washed up in the Philippines during the summer of 1998, and in August that year, #13 was seen being attacked by Sperm whales. Number 14 was netted off California in November 1999. By July 2007, reported occurrences had reached thirty-eight.
The rarity factor has had a similar effect on the fossil teeth. At Aurora, a newly found Megachasma tooth can draw more attention than most megalodon teeth.
M. pelagios TAYLOR, COMPAGNO & STRUHSAKER, 1983
Extant Megamouth Shark
The Megamouth shark is a very large filter-feeder which is best characterized by its size, large head and huge terminal mouth. It has been found in Atlantic and Pacific Oceans. Reported stomach contents suggest that this shark feeds largely on euphausid shrimp, but also partakes in the ever-succulent copepods and jellyfish (large, slow moving, pelagic vacuum cleaners aren't proud).
Yabumoto, et al (1997) provided excellent descriptions and illustrations of Megamouth #7's dentition and teeth. The dentition included 83 upper and 97 lower files, each with three functional rows of teeth. (The authors noted that the male holotype has 30% more teeth in each row.) Upper teeth tended to be smaller than lowers, and each tooth band lacks symphyseal teeth.
Moving distally from the symphysis, the teeth in the first five or six files rapidly increase in size, and those in files five through seven tend to be the largest in the tooth band. Starting around file eight, the teeth decrease in size gradually as they approach the commisure. All portions of the tooth grow smaller, but the crown diminishes in size much more rapidly than the root. In Megamouth #7, the largest lower teeth were approximately 8.5 mm in height (upper 6.5 mm) and the posteriors, 3.0 mm (upper 2.5 mm). Overall, the dentition can be said to have gradient monognathic heterodonty.
The megamouth tooth can be broadly described as having a high, porous, triangular root, lacking a clearly defined nutrient groove and a lingually directed cusp which is broad and deep basally and narrow apically. Referring to the accompanying illustration (Fig. ) several features are worth expanding upon.
The Megamouth Tooth
Although the root is roughly triangular, D-shaped may be a more appropriate description. The illustrated tooth is redrawn from Yabumoto, et al (1997) and represents a tooth from the fifth file of a female megamouth. In more distally located files, the root becomes relatively broader (laterally expanded) and less deep.
Viewed basally, a medial depression in the root exists but could not be referred to as a nutrient groove. The two root lobes cause the labial edge of the root to be weakly concave. The basal face has scattered pores of varying sizes. From a lateral perspective, the root is higher lingually and the margins have numerous scattered pores.
The crown is lingually directed with a broad, deep base which slenders apically. The cutting-edge is incomplete, and on some teeth there is a medial depression in the labial face of the crown. A relatively wide and deep dental band extends around the tooth between the cusp and root. Viewing the crown from a labial/occlusal perspective, the smooth, un-notched shoulders have an alopid appearance.
Lee Creek Fossil Teeth
During the summer of 1998, the Lee Creek (North Carolina) Megamouth teeth in the Smithsonian collection were studied. The collection included three Yorktown (Pliocene) specimens and two from the Pungo River (Miocene) formation. I had viewed these specimens on several occasions during the prior years, but never created a viable collecting image. The only commonly available source which pictured this tooth was the N.C.F.C. publication by Chandler & Timmerman (1994).
Figure depicts a Yorktown specimen. Compared with the teeth of the living megamouth,
these teeth are much larger, have remarkably erect cusps and more clearly differentiated root lobes. The lingual protuberance of the lobes standout as does the foramina on the labial face of the root. The thresher-like design of the crown is more apparent in the fossil teeth, however there are significant similarities between the Lee Creek teeth and those of the extant species. The Pungo River teeth ascribed to Megachasma were significantly smaller. The condition of these specimens did not lend themselves to an extended description and struck the author as symphyseals of Physogaleus contortus.
Other Fossil Megamouths
In addition to North Carolina, Megachasma teeth have been found/reported from:
Florida (Fig. -),
Virginia (Fig. ),
Chile (Fig. -),
Europe (Fig. ),
California (Fig. -)
and Oregon. Except for size, the specimens from Chile, Europe and Florida are virtually indistinguishable form the extant taxon M. pelagios.
In 2002 Lutz Andres sent me a copy of Keupp & Bellas (2002), an article which included a megamouth type tooth from Miocene of Crete that had been identified as Hexanchus. This matter dropped-off the radar until 2006 when megamouth teeth began to turn-up in collections from Antwerp Harbour, BE. Pieter DeSchutter (2009) began to pursue these teeth which were found in a transgressive lag at the base of the Pliocene. He notes that although recognizable as Megachasma, these teeth were smaller than their Western Atlantic counterparts and appeared transitional to the Kern Co., CA specimens.
Kern County, California (and Oregon) produce teeth that have been deemed by some to be megachasmid (sometimes as Megascyliorhinus CAPPETTA & WARD 1977).
These teeth are clearly different from M. pelagios and their fossil Western Atlantic counterparts. According to Jess Duran (pers. com. 2008), Kern Co. has yielded teeth from the Pyramid Hill Sand and Freeman Silt (both Early Miocene); it's not known elsewhere from more recent deposits (although sometimes reported as from Sharktooth Hill).
A Cretaceous Megamouth?
Shimada (2007) erected Megachasma comanchensis for a megamouth-type tooth-design from the Greenhorn Fm (Lincoln Limestone Mbr, Middle Cenomanian) of Colorado. The depicted (Fig. ) tooth actually looked too good; however, the absence of multiple complete tooth-positions and a Paleogene fossil record left this writer highly suspect of his determination (maybe a Johnlongia SIVERSON, 1996 symphyseal or something?). Queried on his position, Kenshu (pers com July 2007) provided collateral data and images that better argued his conclusion -- I can only say I could not assemble arguments to challenge his determination. The primary difference between the extant taxon and M. comanchensis is the more salient nutrient groove. Shimada includes numerous papers that support the possibility of a Cretaceous origin for this genus. Not included in his paper but cited by Kenshu was the molecular research of Martin et al (2002) which also points to a middle Cretaceous origin for Megachasma.
Shimadaís conclusions are not without critics; they point out the very poor state of preservation of the material and argue it most likely represents a Johnlongia anterior with eroded roots.
Compagno, L. J. V., 1990. Relationships of the megamouth shark, Megachasma pelagios (Lamniformes: Megachasmidae), with comments in its feeding habits. Elasmobranchs as Living Resources: Advances in the Biology, Ecology, Systematics, and the Status of the Fisheries. NOAA Tech Rpt, no 90. pp. 357-379.
De Schutter. P., 2009. The presence of Megachasma (Chondrichthyes: Lamniformes) in the Neogene of Belgium, first occurrence in Europe. Geologica Belgica 12/3-4: pp 179-203. PDF (5 MB)
Keupp, H. and Bellas, S., 2002. Miozšn-Fossilien aus NW-Kreta III Die Beckenfazies, Fossilien, 19(1): 34-40.
Martin, A., Pardini, A., Noble, L. and Jones, C., 2002. Conservation of a dinucleotide simple sequence repeat locus in sharks. Molecular Phylogenetics and Evolution 23 pages 205-213.
Shimada, K., 2007. Mesozoic Origin for Megamouth shark (Lamniforms: Megachasnidae), Journal of Vertebrate Paleontology 27(2):512-516.
Siverson, M. 1996. Lamniform sharks of the mid Cretaceous Alinga Formation and Beedagong Claystone, western Australia. Palaeontology 39:813-849.
Yabumoto, Y., Goto, M., Yano, K., and Uyeno, T., 1997. Dentition of a female megamouth, Megachasma pelagios, collected from Hakata Bay, Japan, Biology of the Megamouth Shark. Tokai University Press. pp. 63-75.
Yabumoto, K., Toda, M., Uchida, S. and Yasuzumi, F, 1997. Gross anatomy of the viscera and stomach contents of a megamouth shark, Megachasma pelagios, from Hakata Bay, Japan, with a comparison of the intestinal structure of other planktivorous elasmobranchii, Biology of the Megamouth Shark. Tokai University Press. pp. 105-113.
Florida Museum of Natural History, a has a series of webpages on this genus including a
worldwide distribution map.