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Mancalla sp. Time Place In Our Area Description
By far the most common bird fossils recovered from the San Diego Formation are wing bones of flightless auks in the genus Mancalla. At least three different species have been described from the local Pliocene sandstones including Mancalla milleri, M. diegensis, and M. emlongi. Like modern penguins, species of Mancalla are grossly distinguished on size with Mancalla milleri being the smallest at ~30 cm tall and weighing ~1.65 kg, and M. emlongi being the largest at ~55 cm tall and weighing ~3.8 kg. Although these birds could not fly, they were adept as wing-propelled swimmers, with very flattened, paddle-like wing bones that helped push them through the water. Like other members of the family Alcidae, they would have "flown" underwater with partly folded wings. Paleontologists also believe that the rigidity of the vertebrae may have helped them adapt to swimming. Ecology It is likely that, here in the Northern Hemisphere, the flightless auks occupied a similar ecological niche that the penguins fill in the Southern Hemisphere. Species of Mancalla may have lived in large groups, much as modern penguins do today. Both alcids and penguins are extremely proficient divers; some alcids are known to dive to depths of 180 meters (590 feet!) Scientists postulate that flightless birds most likely evolved in conditions where there was an abundant and regular supply of food, and a lack of predators, or at least where predators were rarely encountered. These seemingly ideal conditions may have been present in San Diego's Pliocene embayment. There was apparently an abundance of fish for these birds to feed on, and there may have been islets and islands off shore that allowed them to avoid the majority of terrestrial mammalian predators. These islands could have served as rookeries for young birds, also. Why would it be favorable for survival for a bird to lose the ability to fly? Large flight wing muscles and bones require much energy to maintain. In a safe supportive environment, Mancalla and many other birds conserved energy by becoming flightless over time. Examples include penguins of the southern hemisphere, the flightless cormorant of the Galapagos Islands, the flightless grebes of Peru, and the recently extinct flightless Great Auk of the North Atlantic. The physical mechanism for this transformation to flightlessness may have simply involved the alteration of a few genes that control development, so that the adult bird has many features of the flightless young chick. In flightless birds feathers typically are more open and less tightly constructed and the bird's overall size tends to become larger, with a bigger pelvic region and hind legs. In many ways the adult flightless Mancalla would have looked like a large, overgrown chick. Another member of the Alcidae that has become extinct did so only during very recent times. The Great Auk, or Pinguinis impennis, a large black and white flightless seabird found in the North Atlantic, was often seen by fishermen offshore. Standing as tall as 2.5 feet (75 centimeters), these birds were slow and awkward on land. During its nesting period of a few weeks each summer the birds waddled onshore to lay their eggs in huge colonies on the islands off Greenland, Iceland, Labrador, Great Britain, and Europe. Laying only a single egg and having little or no defense against human predation, the birds were slaughtered by the thousands by hunters and their eggs collected as well, with the sad result that they became extinct by 1844. Part of their vulnerability could be attributed to the fact that as large, plump birds they, and their large eggs, made more tempting targets for hunters than other smaller birds. Like modern penguins, their colonial nesting habits also put them at risk. Great Auk images figure in several prehistoric cave paintings in France and Spain. Suggested Reading
Text: Margaret Dykens and Lynett Gillette Search | Site Index | Home | Museum Guide (PDF) © San Diego Natural History Museum |
         
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