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 PESQUISA FAPESP 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 SPECIAL ISSUE 2002 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 cynodonts 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 lineage of the cynodont generated, around 210 million years ago (more or less during the time that the dinosaurs came into being), the first beings with the bone characteristics that define a mammal 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 PESQUISA FAPESP 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 paleontologist 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 SPECIAL ISSUE 2002 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 origin 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 paleontologist Cesar Schultz, of UFRGS, co-author of Bonaparte’s studies on the Gaucho pre-mammal mini 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.

32 PESQUISA FAPESP SPECIAL ISSUE 2002 biped of around 2.5 Bone plates of the meters in total length – backbone of the tecodino: reveals lots of characte- a transition species ristics of having had a MIGUEL BOYAYAN backbone that is more or less typical of the di- The region of Santa nosaurs. However, they Maria is so rich in fossil have not as yet been remains that the go- able to determine if its vernment of Rio Gran- articulation with the de do Sul, by way of the femur, in the pelvis, is Pro-Guaíba Program, perforated, a trait cha- which is carrying out a racteristic of the dino- survey of the potential saurs. of this hydrographic The thecodont side basin, has already pro- of the animal shows it- posed the creation of a self in the bone plates that seem to the evolutionary scale close to the Paleontology Park in the surroun- have existed below its skin, more or thecodonts, the oldest types of dino- dings of the town of Candelária. “The less as it occurs today in the crocodiles, saurs, of the age of the tecodino,ap- chosen area is already in the phase of called osteoderms. Although it had parently don’t have them. The osteo- being bought over”, explains the pale- been hoped that they would show derms became more common in later ontologist Ana Maria Ribeiro from the these bone plates, so as to place in on dinosaurs. Foundation.

Besides making a comparison of the es of echidna, similar to a hedgehog; do Sul are the closest parents known to anatomy of the Rio de Grande do Sul the marsupials, such as the possum, the the mammals might appear daring, but cynodonts with the primordial mam- kangaroo and the koala bear, which Bonaparte – who for the last seventeen mal Morganucodon, the group confron- house and feed their young in a skin years has had the financial support of ted their findings with the morphology pouch; and the placental, more nume- the North American National Geogra- of the Chinese mammal Hadrocodium rous, which include the rodents and phic Society for his field work – has the wui, which lived some 195 million ye- elephants, passing through the prima- credentials to defend the hypothesis. ars ago and was also similar to a small tes, amongst which there is man. To With more than four decades of re- rat. Result: the upper teeth of the Bra- compare the new Rio Grande do Sul search, Bonaparte is one of the best silodonte and of the Brasiliterio resem- fossils with the Hadrocodium, the most specialists on dinosaurs in the world. ble those of the Chinese fossil, whose distant descendant of all of the living Thanks to his work, above all in the Ar- lowermost teeth measure a total length mammals up until now localized, is a gentinean Patagonia, more than twenty of two centimeters. The feeling from manner of asserting if the Brasiliterio species of dinosaurs typical of the sou- the comparison comes from the fact and the Brasilodonte also do not guard thern hemisphere have been discove- that, though animals of the type of some relationship, however remote, red, among them the Amargasaurus, Morganucodon are considered to be the with the current animals of hair and the Argentinosaurus (perhaps the lar- first mammals, one does not know with warm blood. gest in the world with up to twelve me- certainty if its descendants generated ters in height), the Saltasaurus and the the current existing lineages. This ori- he discoveries have not as Carnotaurus. This last one, a carnivore gin, it would appear, is more probably yet been formally publis- of 3.5 meters in height, 7.5 meters in related to a mammal of the type such hed, which should occur length and with two horns, transfor- as Hadrocodium. during this year. In April med itself into an animated movie per- The object of a cover article of the of 2001, at an informal sonality – in the children’s film Dino- North American magazine Science in Tmeeting with researchers from North saur, launched in 2000 by Disney. May of last year, this Asian little animal America at the Museum of Comparati- More recently, and without aban- is considered the potential common ve Zoology of Harvard, Bonaparte de- doning the dinosaurs, Bonaparte be- ancestor of the three current groups of monstrated some of the fossils and dis- gan to study the ancestors of the mam- mammals (a fourth group, that of the cussed their peculiarities and the mals, which appeared on Earth at the multituberculates, has been extinct for importance of these new species. He time as these large reptiles and that li- 40 million years): the monotremes, stated that the receptivity of his fellow ved discretely under their shadow for which lay eggs and are represented by colleagues was good. The proposal that 160 million years. In the Argentine, he the duckbill platypus and by two speci- the mini cynodonts from Rio Grande found fossils of small synapsids with

PESQUISA FAPESP SPECIAL ISSUE 2002 33 mammiferous traces, that group of the thecodonts. could be labeled as dis- Until the end of the Triassic tant cousins of Morganu- era, in a movement of the codon. “However, these MIGUEL BOYAYAN renovation of the fauna al- fossils were of the Lower most simultaneous to the to Middle Triassic era, bet- appearance the dinosaurs, ween 250 and 230 million the first mammals, crocodi- years ago”, he ponders. His les and turtles appeared as hope of finding advanced well as the pterosaurs, flying cynodonts, which would reptiles also extinct. At this make the link between re- point in time, the fauna and ptiles and mammals, con- flora were very similar in all sequently made him mo- places, since there was only ve to the South of Brazil, one continent. “Animals si- more specifically to the Rio milar to the mini-cyno- Grande do Sul rocks of the Cleaning a Fossil: donts of Rio Grande do Sul Upper Triassic period. care and patience must have existed in other In the middle of 1997, parts of the planet”,com- Bonaparte came to work ments Bonaparte. at the Museum of Natural Sciences of rock. “We were used to looking for fos- It is within this lost world of the Tri- the Zoo-botany Foundation of Rio sils of larger reptiles such as dinosaurs assic period, more than 200 million ye- Grande do Sul, in Porto Alegre, fa- and even cynodonts of larger size”,tells ars ago, which the researchers are ma- mous by its research in the area of pa- Cesar Schultz. “We had to learn to look king their efforts to find the most leontology. One year later, at the side at the rock in a different way in the se- adequate place for the Brasiliterio and of two researchers from the foundati- arch for these small pre-mammals.” As the Brasilodonte in the evolutionary on, Jorge Ferigolo and Ana Maria Ri- it is the only place in the country with tree. How close to the first mammals to beiro, he gathered together numerous sedimentary rocks containing fossils of these reptiles that remind one of rats? fossil fragments of a new advanced terrestrial vertebrates from the Triassic There are two anatomical parameters cynodont, named Riograndia guai- period, when both the dinosaurs and in the bones that help to demonstrate bensis, in rocky outcrops of Candelá- the mammals came into being, Rio the distance of one animal in relation ria, and described the animal in a sci- Grande do Sul is a privileged land for to the mammals: the construction of entific paper published in 2001 on prospecting. the teeth and of the mandible and the the British magazine Palaeontology. bone composition of the middle ear. A possible ancestor of Brasiliterio ne of the largest difficul- Mammals change their teeth only once and Brasilodonte, the Riograndia could ties for studying animals during their life and have four types of not have mammals as its descendants. of the transition era is well defined teeth: incisors to bite or to “Its teeth are different”, explains the pa- exactly how to classify gnaw, canine to tear and molars and leontologist Marina Bento Soares from each fossil into a catego- pre-molars to chew and to crush. In the UFRGS. Nevertheless, the localization Orized date – cynodont or mammal, di- reptiles, the substitution of teeth is con- of lots of petrified remains of this spe- nosaur or bird. It is common to find tinuous and lasts the life time and it is cies confirmed the clues that it was nothing more than teeth or fragments impossible to differentiate between pre- worth while exploring the soil of Can- of cranium, of difficult identification. Brasilodonte and the Brasiliterio, the delária and of its surroundings. “The In the case of the pre-mammalian rep- distinction between molars and pre- fossil potential of the region is extreme- tiles from Rio Grande do Sul, until the molars is as yet not clear and the chan- ly high”, comments Ana Maria Ribeiro, fossils found are reasonable abundant ging of teeth seems to obey the stan- who continues to study the pre-mam- and rich in detail. Even though there are dard of the reptiles”, comments mals of the Rio Grande do Sul Triassic well accepted conventions for dealing Bonaparte. In spite of this, the sharp period parallel to the work of Bonaparte, with the dividing line between species, points of the canines resemble those of today based at UFRGS. Encrusted and so- it is difficult to have unanimity.“The clas- the primordial mammals, which rein- metimes found in the middle of the red- sifications reflect more or less the pre- forces the thesis that they have the po- dened rocky sediments of the Santa Ma- ferences of each author and are, in some tential to generate, in lineages yet to ria Formation, the bony remains of these measures, arbitrary”, Schultz says. come, some form of mammal. diminutive pre-mammal reptiles are During the Triassic era, the solid The hearing system shows that they difficult to locate by the untrained eye. earth was unified in the super conti- were on the correct path, but there were nent named Pangaea, whose center was not yet mammals. This is because in Privileged location - The clue to a pe- an immense desert. The ice of the polar the mammals the middle ear, an inter- trified bone generally does not go be- caps had been melted. In this arid en- nal cavity full of air, is composed of yond a white point – as it contains cal- vironment, around about 230 to 220 three small interconnecting bones: the cium which is of this color – of the size million years ago, the dinosaurs came hammer, the anvil and the stirrup. The of a match head on the surface of the about, descendants of reptiles of the middle ear of the cynodonts, among

34 PESQUISA FAPESP SPECIAL ISSUE 2002 Frontier dinosaurs

A new archeological site is gai- ropod, a large herbivore with a long In Uruguay, fossils of fish and di- ning fame in the state of Rio Grande neck that may well have been more nosaurs have been found in the same do Sul: the municipality of Santana than 13 meters in length”, says Cesar rocky layer in which now flourish the do Livramento, in the west, on the Schultz from UFRGS, who is stud- remains of reptiles on the Brazilian si- frontier with Uruguay. Here, some ying the recently discovered vestiges de of the division. For this reason the 500 kilometers from the city of Porto near the frontier. Another type of fo- researchers are going to make explo- Alegre, researchers from the Federal otprint was made by paws with three ratory expeditions into the region, in University of Rio Grande do Sul toes, which could have belonged to the hope of rescuing the bones of the (UFRGS) and from the University of the carnivorous dinosaurs or to a authors of the footprints. The rocky the Vale do Rio dos Sinos (Unisinos) herbivorous group of the hadrosau- layer that preserves the tracks is below found during 2001 and in January of rus: in either of the cases, by the size the type of rock that forms the Botu- this year various types of dinosaurs of the footprints, they must have catu sandstone, whose top layer is da- at the side of a road where the earth been biped dinosaurs of around ted at 134 million years ago, back in was being moved and leveled. By three meters in height. the Cretaceous period, nonetheless, their format, they belong whose base could still be to two forms that must placed at the end of the have lived between 144 Jurassic era. It is possible and 137 millions of years that we are dealing with ago, at the end of the Ju- a sedimentary block of

rassic era the beginning CLAITON SCHERER the last stage of the Ju- of the Cretaceous era, rassic period or the first when the dinosaurs do- stage of the Cretaceous minated the scenario. era, a period that did A type of footprint, not have known layers which went as far as lea- in Rio Grande do Sul. ving trails, resembling elephant tracks, each one Footprint with marks of 40 centimeters on ave- of three toes: probably a rage. “These tracks must teropod (carnivore) or have come from a sau- a hadrosaurus (herbivore)

them the Brasiliterio and the Grande do Sul, Bonaparte Brasilodonte, do not have disagrees: the fauna of the these bones. Through evolu- cynodonts, even before the tion, the hammer, anvil and origin of the Morganucodon stirrup of the middle ear of and other primordial mam- the mammals were construc- mals, were already of vari- ted beginning with the bones ous sizes, some with more originally situated in the than 1.5 meters in length and mandible and in the region of the articu- mmal”, compares Schultz. The fossils others with only a few centimeters as in lation of the cranium of ancestral rep- from Rio de Grande do Sul still have the case of the Brasilodonte and the tiles. The mammals always have only a more than one bone in the mandible, Brasiliterio. Therefore, there was no need large bone in the mandible and three in denoting their pre-mammal character. to diminish in size in order to become the middle ear. It so happens that the The discovery of the new brothers mammals. For him, something dis- reptiles have an articulated mandible on of the warm blooded animals also puts tinctive occurred: there were large and top of various bones and the middle ear in check a common idea about the ani- small cynodonts, but only one of these is without a hammer, anvil and stirrup. mals which made up the bridge betwe- small ones evolved into the mammals. The cynodonts show exactly a transiti- en the cynodonts and the mammals: “With these Gaucho fossils, we can bet- onal standard between these two extre- that there had been a miniaturization ter understand this process of transi- mes.“The smaller the number of bones of this group of reptiles before the first tion”, emphasizes Bonaparte. “And, in in the mandible of an animal, the closer mammals were derived. Based on the many cases, better understand and this animal is to the condition of a ma- evidence of the fossils from Rio de substitute theories.” •

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