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Gli squali fossili

Megachasma

TAYLOR, COMPAGNO & STRUHSAKER, 1983 
Squalo Megamouth
Miocene - Recente
Ordine Famiglia
Lamniformes BERG 1958 Megachasmidae TAYLOR, COMPAGNO & STRUHSAKER, 1983

If it wasn't for rarity, both the living shark and the fossil teeth of the genus could serve as an icon for dull. The shark itself is quite large with an oversized mouth, ugly by most dictionary definitions. The teeth are denticle-like and generate the horrific image of an oversized grooming brush bristle.

However, rare is rare. So uncommon are these 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.

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.


Megachasma pelagios TAYLOR, COMPAGNO & STRUHSAKER, 1983

Megamouth vivente
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. 2) 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 includes three Yorktown (Pliocene) specimens and two from the Pungo River (Miocene) formation. I had viewed these specimens on several occasions during the last few 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).
 
Fig. 3 Megachasma sp. - Aurora (Pliocene) USNM 388,967
height 15, width 13, depth 12.5 mm [Donated by Frank & Becky Hyne]

Figures 3 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.


Other Fossil Megamouths
Besides the Western Atlantic locales, Kern County, California (and Oregon) produce teeth that have been deemed by some to be megachasmid (sometimes asMegascyliorhinus CAPPETTA & WARD 1977. These Pacific teeth are clearly different from their Atlantic counterparts (refer to figure 4). Kern Co. has yielded teeth from Pyramid Hill and Sharktooth Hill (Alter pers. comm. 1998) which cover the Lower and lower Middle Miocene. (The Pyramid Hill teeth are smaller with more robust roots.)
Fig. 4 ?Megachasma sp. - Sharktooth Hill (Middle Miocene)
Width 10 mm; From the Collection of Walter Hunt

Differentiating these teeth from the Lee Creek specimens are the cusplets, more lingually directed cusps and the C-shaped root (viewed basally). Keeping in mind that the American continents were still not joined, this would result in two geographically separated species during the Lower/Middle Miocene, only one leading to the extant species. Two other possibilities exist: the Pungo River teeth are not Megachasma (misinterpreted symphyseals), or the Pacific material represents another species or genus entirely. Whichever scenario proves correct, "interesting" must be added to the property set of Megachasma and its teeth.


Specific References
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.

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.

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