Fossils and Fish: Extinctions Past and Present
Picture a variety of camels, hippopotamus-like rhinoceroses and giant sloths wandering around the
Mexican countryside perhaps grazing by a river. Along with many other now extinct mammals, remains of these animals have been found in digs from 1837 to the present in the floodplains, point bar deposits and arroyos between Rancho Viejo and Los Galvanes and east to Rancho El Ocote. These dig sites are only 12 to 25 kilometers from San Miguel. As you drive along Highway 51 from San Miguel to Dolores Hidalgo, look to the east and picture what might have been there during the late Miocene and Pliocene (many millions of years ago) and in some cases on into the Pleistocene and the early Holocene (thousands of years ago). More bones are buried out there somewhere.
Bones and teeth from several extinct species of horses have been discovered in the digs. Remember, the horse became completely extinct in the western hemisphere and was reintroduced only in historic times.
But, my favorite is the giant ground sloth in the genus Glossotherium. It was a heavily built animal with a length of about 4 meters (13 ft.) snout to tail-tip; a weight estimated at 1002.6 kg (2210.4 lb.) and could potentially assume an upright stance. Yes, I think a sloth could be scary. Another one to picture, in your nightmares, is the Stegomastodon sp. which quite like modern elephants had two upward curving tusks that were about 3.5 meters (11.5 ft.) long.
Sometimes when Linda is away and we, therefore, aren’t painting, I post informational pieces. Fossils and Fish: Extinctions Past and Present is the first of many explorations along the Laja.
Leaving fossils, but not extinctions, another tidbit of information gleaned from the Internet is about the Laja River and its fish population. Species loss in the Mexican freshwater ecosystem is among the most dramatic anywhere due to habitat deterioration, over-exploitation, and the introduction of invasive (usually exotic) species. Extinction is a definite possibility because 70% of the native freshwater fish species in Mexico are found nowhere else on Earth and sometimes in a very limited geographic area of Mexico. Thus, species loss in a particular place can mean extinction for that species.
The River Laja is unusual among Mexican rivers in that its fish fauna has been documented regularly since the 1960’s, allowing the study of long-term changes in the composition of its fish community. The Laja basin covers 3476 km2 (1342 mi2) of the State of Guanajuato. The main stem of the Laja is 154 km (96 mi) in length.
Twenty-two fish species (natives and exotics) were known from the Laja River based on historical samples. One additional exotic was found in the 2003-2004 sampling. Sadly, 6 native and 2 exotic species that had historically been collected were not found in this sampling. Perhaps some of the fish not collected in the 2003-2004 sampling are on their way to extinction.
On the positive side, three native species were the most prevalent and abundant species in the basin: Charal (Chirostoma jordani), Carpa Blanca (Yuriria alta), and Pintada (Xenotoca variata).
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