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Fossils of Small Species Discovered in Rio Grande Do Sul Could Be the Closest Ancestors to the Larger Animals of Today

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Fossils of Small Species Discovered in Rio Grande Do Sul Could Be the Closest Ancestors to the Larger Animals of Today SCIENCE PALEONTOLOGY he T origin of mammals Fossils of small species discovered in Rio Grande do Sul could be the closest ancestors to the larger animals of today MARCOS PIVETTA 28 I PESQUISA FAPESP I SPECIAL ISSUE 2002 MIGUEL BOYAYAN Rocks from the Triassic period, with fossils of animals that lived around 210 million years ago: relatives of the mammals PESQUISA FAPESP I SPECIAL ISSUE 2002 I 29 FOTOS MIGUEL BOYAYAN Brasilodonte: above, left mandible and, at the side, left jaw bone with teeth ny paleontologist will tell you that the mammals des- cended from some form of cynodont, a vast group of animals – extinct, just likeA the dinosaurs – characterized by having craniums and teeth similar to those of our current dogs. The cyno- donts belong to the group of synapsids, which served as the transition between the reptiles and the mammals. What has been missing is to know what line- age of the cynodont generated, around 210 million years ago (more or less du- ring the time that the dinosaurs came into being), the first beings with the bone characteristics that define a mam- mal today: four types of teeth functio- nally and anatomically differentiated, middle ear with three sound conduc- ting ossicles and a larger more protru- SIRIO J. B. CANÇADO ding cranium. As pioneers of bringing together these bone traits and probably tion of mammals, but paleontologists or wild squirrels – recovered over the being warm blooded, as well as having from the Federal University of Rio last two years in the region of Santa a hairy body and glands for the pro- Grande do Sul (UFRGS) and the Ar- Maria, at around 200 kilometers from duction of milk for their offspring, the- gentinean Museum of Natural Sciences the city of Porto Alegre, the capital of se animals were the primordial exam- believe that they have found important state of Rio Grande do Sul, the resear- ples of the future mammals, among pieces to this phylogenetic puzzle. chers have identified two new cyno- them man. Perhaps it will never be pre- Starting from the analysis of vari- donts – which they have provisionally cisely determined which species of cy- ous fossil fragments of small reptiles – named Brasiliterio and Brasilodonte – nodont took the final step in the direc- of the size of a pen and similar to rats with anatomical characteristics that 30 I PESQUISA FAPESP I SPECIAL ISSUE 2002 Cranium and mandible of Brasiliterio: the second species to have been discovered Rock with the teeth of the Brasiliterio: white marks to the right MARK A. KLINGER/CMNH Hadrocodium and would place them as the ancestors clo- its cranium: sest to the primordial mammals. “The- the oldest se pre-mammalian reptiles from Brazil representative of could be a sister group to the mammals”, today’s mammals says the renowned Argentinean paleon- tologist José Bonaparte, 73 years of age, who discovered the petrified bones of from the Argentinean museum retur- mal Morganucodon and to the Brasilo- the two species together with resear- ned to the region of Santa Maria in se- donte, an animal with a greater number chers from the Institute of Geosciences arch of more fossils. of characteristics of a mammal. In other of UFRGS, at which the researcher is a words, the lineage of the Brasiliterio visitor with a scholarship from the Na- he comparison of the forms could have been the ancestor both of tional Council for Scientific and Tech- of the cranium, mandible synapsids very close to the mammals, nological Development (CNPq). “The- and teeth of these animals and to the first mammals themselves. se animals, removed from the rocks of with those of the Morga- the Upper Triassic period – with a geo- nucodon – one of the ol- Brother group - For now, the fossils of logical age estimated at between 230 Tdest known mammals (close to 200 the Brasiliterio do not appear to be sen- and 205 million of years ago –, were not million years) with fossil material of sitive to the analyses as profound as tho- mammals, but their evolution was cle- good quality and that had fifteen centi- se carried out on the Brasilodonte, and arly in that direction.” For this reason meters in length and looked like a wild the researchers have not gone as far as they received names that place them in rat – has turned the southern Brazili- labeling the mini cynodonts from Rio this animal group. Brasiliterio means an findings into strong candidates of de Grande do Sul as the father of all the “mammal from Brazil”, whilst the being the best representatives of the mammals. Instead of emphasizing the Brasilodonte has the teeth (odon) of a cynodonts lineage that ended up in notion of a paternal and filial lineage, mammal and its post-canines, used for mammals. “The cynodonts of Rio they prefer a concept more precise from crushing, form four right angles. Grande do Sul show a type of carnivo- the scientific point of view: that of a bro- The majority of the fossils found re-insectivore dentition, above all in ther group. If their theories prove to be are complete or partial craniums, with the mandible, comparable to the first correct, the Brasilodonte and the Brasili- preserved mandibles and teeth. Other mammals”, says Bonaparte. He believes terio should take the place of two fami- bones have been rescued, but not an that the Brasiliterio produced descen- lies of small cynodonts, the tritylodon- entire skeleton. Just this month, rese- dants which, generation later, could tids and the tritheledontids (also called archers from UFRGS and technicians have led both to the primordial mam- ictidosaurs), found in various parts of PESQUISA FAPESP I SPECIAL ISSUE 2002 I 31 The tecodino of Santa Maria ton that seems to be a young example of an enigmatic animal – belonging to the thecodonts, ancestors of the dino- The region of Santa Maria, in the seum of Natural Sciences of the Zoo- saurs or to the dinosaurs themselves. center of the State of Rio Grande do botany Foundation of Rio Grande do Or perhaps of an animal that was a Sul, does not only provide interesting Sul –, which in 1998 had already par- transition between two types of repti- fossils of mini cynodonts that can ticipated in the discovery of the first les, a hypothesis that inspired the pro- help to explain the origin of the mam- pre-mammal mini cynodonts in the visional name of tecodino. “We still mals. From there, there has also come hinterland of Santa Maria, made ano- have to prepare the majority of the vestiges of the bones of extinct ani- ther startling discovery: they found fossils – to clean and to separate out mals of the Triassic period that inha- rich fossil material of a mysterious the petrified bones encrusted in the bited the earth between 250 and 205 animal, informally called for now the rock – and to study them with care”, million years ago. A famous fossil re- tecodino. advised Ferigolo. moved from the rocks of the region, What could this creature have Though little material has so far for example, is named Staurikosaurus been? The paleontologist Jorge Feri- been prepared, the fossil fragments of pricei, one of the oldest dinosaurs re- golo, from the Foundation, still does the tecodino that have already been covered in the world, whose skeleton, not know precisely if the petrified bo- cleaned, reinforce the hypothesis that discovered in 1937, is on exhibition at nes found in the municipality of Dona they are dealing with a hybrid species, the Museum of Comparable Zoology Francisca – a complete skeleton, two with some characteristics of the the- of Harvard in the United States. entire craniums, two partially preser- codont and others of the dinosaur. The More recently, paleontologists ved craniums and four backbones al- mysterious animal – that lived some from a state institution – the Mu- most intact, as well as parts of a skele- 235 million years ago and should be a Schultz (left) and Bonaparte (with tie) with UFRGS paleontologists; fertile land MIGUEL BOYAYAN the world and that have battled for some time the elite position of being the brother group closest to animals with hair and warm blood. “Our findings could assist in better understanding the ori- gin of the mammals and to show which lineages of the cynodonts participated in the process of transition of the synapsids in the direction towards this type of animal”, comments the paleon- tologist Cesar Schultz, of UFRGS, co-author of Bonaparte’s studies on the Gaucho pre-mammal mi- ni synapsids. With an appearance and di- mensions similar to those of the Morganucodon, the new cynodonts me- and other smaller animals, lived in bur- were removed from rocks in paleonto- asured between 9 and 15 centimeters rows and had night habits”,says logist sites in the Gaucho municipaliti- from head to the end of their tail and Schultz. They must already have had its es of Faxinal do Soturno and Candelá- the length of its cranium varied betwe- body covered with hair, and its paws ria, belonging to the Santa Maria en 18 and 22 millimeters. Their weight were strong enough for it to open up Formation, one of the areas in the Sta- would not have been more than a few burrows in the ground where it shelte- te with Triassic sedimentary rocks – an hundred grams. The Brasilodonte was red from predators such as the first di- agitated geological period of little more about 40% larger than the Brasiliterio. nosaurs and other reptiles. The remains than 40 million years, that occurred “These animals probably ate insects of this as yet poorly known mini-fauna between 250 and 205 million years ago.
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