Live reconstruction of Santanachely's gaffneyi
|Lower Cretaceous ( Albium )|
|110 million years|
|Scientific name of the genus|
|Hirayama , 1998|
|Scientific name of the species|
Fossils of this type were found in the deposits of the highest Lower Cretaceous ( Albium ) and are about 110 million years old. They are therefore of great importance for the characterization and research of the early evolution of the real sea turtles and leatherback turtles . Santanachelys gaffneyi is placed by the first descriptor Ren Hirayama in the now extinct family of Protostegidae , the most prominent representative of which is Archelon from the Upper Cretaceous North America, the largest known marine turtle in geological history. Was found santanachelys in eastern Brazil in the Santana Formation , some interesting turtles were found in addition to a variety of fish fossils. Besides santanachelys these are about the taxon Araripemydidae associated type Araripemys barretoi (Meylan, 1996) and in the taxon Bothremydidae -ranked Cearachelys placidoi (2001, see literature). The holotype (THUg1386), i.e. the specimen on which the first description is based, is in the Teikyo Heisei University in Ichihara , Chiba , Japan . It is an almost complete and well-preserved skeleton ; no other finds of Santanachely's gaffneyi are known to date.
The author of the first description derives the scientific name from the site Santana in Brazil and the Greek name chelys for turtle. The epithet gaffneyi is a tribute to Eugene S. Gaffney , who is considered an authority on fossil research in turtles.
Characteristics of Santanachelys gaffneyi
The fossil of the Santanachelys gaffneyi is about 20 centimeters long, the carapace reaches a length of 14.5 centimeters on its central axis. The most important features of Santanachelys gaffneyi are the paddle-like limbs and the structure of the skull with today's representatives of sea turtles. The paddles are not as pronounced and were probably only useful for short swimming distances. The skull has some clear similarities with those of today's representatives, such as the large interorbital skull window (a gap in the bones between the eyes), which suggest that this species also already had a salt excretion system with salt glands. Presumably these were already there before the turtles finally moved to the marine habitat. The salt glands may have been found useful in the lagoon that the Santana Formation likely was 110 million years ago.
Santanachelys gaffneyi differs from all other previously known representatives of sea turtles by the special, curved shape of the first rib and the flexible connection of the metacarpal bones (metacarpals) and the finger bones and a significantly different shape of the humerus . In the obviously very closely related genus Rhinochelys , of which only one skull is known, it is primarily the significantly narrower nasal bones that distinguish it from Santanachelys gaffneyi .
Importance of the fossil record
The discovery of the fossil in the Santana Formation shifted the fossil record for sea turtles from the Upper Cretaceous, from which already known fossils of the genera Rhinochelys and Notochelone originate, to the Lower Cretaceous by about ten million years. The features of Santanachelys gaffneyi characterize them as the most primitive representative of the previously known Protostegidae. Furthermore, this find can prove that a separation of the three sea turtle families, the still living real sea turtles (Cheloniidae) and leatherback turtles (Dermochelyidae) and the extinct Protostegidae, must have already taken place in the early Cretaceous or before. The emergence of the sea turtle relatives (Chelonioidea) probably took place much earlier than previously assumed. The third peculiarity of the fossil can be pointed out that Santanachelys gaffneyi is the first find of a Protostegidae in South America. In the case of the animal found, it can be assumed that it is a fully grown specimen; the high degree of ossification of the skeleton, especially of the skull and limbs, can be seen as an indication of this.
One of the most important questions about the adaptation of sea turtles is about when the limbs have developed into paddle-like swimming legs. A separate movement of the fingers is no longer possible after this point in time. A more primitive form of the paddle limbs with still movable individual fingers was found among the representatives of the genera Toxochelys and Tasbacka within the real sea turtles from the late Paleocene . The discovery of the relatively primitive paddle-like limbs in Santanachelys gaffneyi means that the paddles in the three groups of sea turtles likely evolved independently of one another.
The evolution of the salt glands, which are important in the representatives of marine turtles living today, also represents an important question in the history of the development of sea turtles. These glands help the turtles to excrete excess salt that enters the body through osmosis , as the kidneys alone cannot do this have grown. The salt glands themselves are large glands on the inner edge of the eyes (they are larger than the animal's brain) and accordingly take up space in the turtle's skull. For this reason, the sea turtles have a large skull window known as the "interorbital foramen" . It is limited by a bone brace ("Inferior Processus parietalis"). In leatherback turtles, this clasp is completely absent, and the animals have correspondingly large salt glands. The real sea turtles as well as Santanachelys gaffneyi and also other representatives of the sea turtle relatives ( Toxochelys , Corsochelys ) still have this process, but as a narrow clasp. In species that are even further back, it is wider. It is assumed that the species with a narrow bone brace already had well-developed salt glands and must have lived in a marine habitat; they used it well before the development of swimming paddles.
On the basis of a comparison of a total of 104 characteristics of the skeletons of various extinct and still living sea turtle relatives and some species outside of this taxon (outgroup comparison), a cladistic family tree analysis was carried out in order to classify Santanachelys gaffneyi within the sea turtle system (Hirayama 1998). The analysis was carried out using software known as PAUP (phylogenetic analysis using parsimony) and came to the following result (simplified):
|Sea turtle relatives (Chelonioidea)||
- Ren Hirayama: Oldest known sea turtle. In: Nature . Vol. 392, No. 6677, 1998, pp. 705-708, doi : 10.1038 / 33669 .
- Eugene S. Gaffney, Diogenes de Almeida Campos, Ren Hirayama: Cearachelys, a New Side-Necked Turtle (Pelomedusoides: Bothremydidae) from the Early Cretaceous of Brazil (= American Museum Novitates. No. 3319, ). American Museum of Natural History, New York NY 2001, doi : 10.1206 / 0003-0082 (2001) 319 <0001: CANSNT> 2.0.CO; 2 , (PDF; 586 kB).
- John G. Maisey (Ed.): Santana Fossils. An Illustrated Atlas. TFH Publications, Neptune City NJ 1991, ISBN 0-86622-549-8 .
- Judith Jördens: Oldest fossil sea turtle discovered - turtle is at least 120 million years old , Senckenberg press office, September 7, 2015.