The Golden Age of the Cynodonts

PALEONTOLOGY y The golden age of the cynodonts Species first discovered in Africa, and now in Brazil, lived during the peak of diversity of mammal precursors Triassic landscape: By the water, the Igor Zolnerkevic cynodont Menadon besairiei with offspring, followed by a band of PUblISHED IN DeCEMbeR 2015 Santacruzodon hopsoni. At left, Dagasuchus santacruzensis, a carnivorous reptile like the Chanaresuchus bonapartei (behind the tree) 64 z JULY 2017 ens of millions of years before dino- and one of them, the mammaliamorphs, gave rise saurs ruled the Earth, a peculiar kind to the mammals. of animal reigned on land. A large, di- Paleontologists Tomaz Melo and Marina Soares verse group with a striking similarity of the Federal University of Rio Grande do Sul to present-day mammals. These primi- (UFRGS), working with Argentine paleontolo- Ttive animals were the cynodonts. The cynodonts gist Fernando Abdala of the University of the had begun to develop the characteristics that to- Witwatersrand, South Africa, discovered that day are exclusive to mammals: they were warm- Menadon besairiei, the first fossil of which was blooded and had hairy bodies and various types found in rocks on the island of Madagascar (off of teeth. In Latin, cynodont means “dog teeth”. the eastern coast of Africa), also lived during that For a long time, all the continents were inhab- same period, 230 million years ago, in an area ited by carnivorous and herbivorous cynodonts, that is now the interior of the Brazilian state of such as Menadon besairiei, pictured watching Rio Grande do Sul. Therefore, Menadon lived in over its young in the illustration above. Today, the middle of the Triassic period—250 million we know that Menadon also lived in what is now to 200 million years ago—when South America, eto N southern Brazil. Menadon, which was about one Africa (including Madagascar) and the other con- aes P meter in length—the size of a large dog—may tinents were all part of a single supercontinent E IR have looked like a descendant of an impossible known as Pangaea. olta V cross between an alligator and a capybara, a large The study, published online in September 2015 ON rodent. It belonged to the lineage of transverso- in the Journal of Vertebrate Paleontology, con- ati R dontids, the most diverse of the cynodonts, which firms that transversodontids such as Menadon ust ILL is now extinct. There were many other lineages, populated Pangaea end to end. PESQUISAPESQUISA FAPES FAPP 238ESP z 65 A Menadon besairiei skull found in Rio Grande do Sul state: the animal was about one meter in length and belonged to the transversodontid lineage age, between 232 million and 228 million years old—the only sedimentary rocks of that age preserved in South America 1 and Africa. “Each new finding enables us to strengthen the temporal correla- tions between rock layers in different parts of the world.” There are gaps in the Triassic history of every continent. In southern Af- “Most of the transversodontid fos- “It could have been a very closely relat- rica, for example, paleontologists have sils have been found in South America ed species, but in the final analysis, we now identified sedimentary rocks that and southern Africa, but there are also haven’t found any difference between it were formed from mud or sand at the records in North America and Europe,” and Menadon besairiei,” says Melo, who beginning and the end of the Triassic, explains Soares, who advised Melo for is in the midst of his doctoral research on but there are no preserved rocks from his Master’s research on Menadon at transversodontids under Soares’ advisor- the middle of the Triassic, as occur in UFRGS. “Since there weren’t any big ship. “It is likely to be the same species Argentina and Brazil. geographic barriers to fauna on Pangaea, found in Madagascar.” Melo explains that researchers are the transversodontids and other animal The discovery helps piece together rarely able to date the age of sedimentary groups at the time had a cosmopolitan two parts of the geological puzzle that rocks from the Triassic with absolute distribution.” paleontologists need to assemble in or- certainty. Some layers in Argentina, for der to reconstruct the history of life in example, have been dated by means of FROM SANta CRuz TO MaDAGascaR the Triassic. “Not every place in the radioactive isotope decay in volcanic ash. Abdala, regarded as one of the world’s world has preserved rocks of the same “But generally speaking, we depend on leading cynodont experts, had previ- age,” Soares explains. The similarity be- comparison of fossils found in different ously (in 2001) noted a similarity between the fossil fauna from Rio Grande layers to find out whether they are the tween the fossilized fauna in a layer of do Sul and from Madagascar—great same age,” Melo says. rocks that form a sandstone outcrop- enough to share a species—confirms To determine the relative ages of rock ping in the municipality of Santa Cruz that the sandstone layers in Santa Cruz layers, researchers generally use com- do Sul, Rio Grande do Sul, and the fossil do Sul and Isalo II are likely the same parisons between microscopic fossils fauna found in a rocky layer of the Isalo II Formation, found in Madagascar and 2 3 described by American paleontologists in 2000. One of the transversodontids discovered in Isalo II, Dadadon isaloi, looked very much like Santacruzodon hopsoni, found in Santa Cruz do Sul (shown behind Menadon in the illustration on pages 64 and 65). Likewise, the skull of Menadon besairiei exhibited similarities with the skull of a species that had been found in Santa Cruz but not yet identified. Botucaraitherium It fell to Melo to shed light on the mat- belarminoi: a ter in his Master’s research, in which he carnivorous cynodont slightly larger than a compared the skull described by Abdala rat, with sharp, pointed to other materials—more skulls, pieces of teeth (above), jaw and a few fragments of bones from adapted for an the body—from the unidentified spe- insect-based diet cies, which had been collected later at the same site and were preserved by researchers from UFRGS and the Zoobo- tanical Foundation of Rio Grande do Sul. 66 z JULY 2017 The carnivore Trucidocynodon riograndensis, which grew as large as 1.2 meters: one of the most complete cynodont skeletons ever found in Brazil 2 4 5 such as grains of pollen and pteridophyte spores, which are abundant in all pe- riods. “Our problem is that the rocks from the Triassic in Rio Grande do Sul were deposited on the banks of rivers and floodplains, which are well-aerated environments,” Melo explains. “Oxygen destroyed the pollen and spores. The only way to date the rocks is through vertebrate fossils.” The group of cynodonts that became most extensively diversified in the Middle Triassic was the transversodontids. Un- T like other cynodonts, which were gener- UR O C ally carnivorous or omnivorous, transver- tten sodontids were herbivores. Their teeth The layers of Triassic rock younger serrated teeth, useful for an insect-based BI were specialized for eating roots, leaves or than those in Santa Cruz do Sul, how- diet. “There is no other cynodont group olfo AD any other plant matter that was available ever, suggest that all the known transver- in the world as similar to a mammal as 5 in the hot, semiarid climate that predomi- sodontids were extinct long before the the ones discovered in Rio Grande do EFS U / / nated in the interior of Pangaea. end of that period. Nevertheless, other A Sul,” Soares notes. Of these small cyn- IR ve cynodont species lived on until the Late I odonts, known as mammaliamorphs, L O COMPetitiON AMONG HERBivORes O Triassic, exhibiting extraordinary forms the species she and her colleagues most TÉ 4 The transversodontids had to compete such as those of Trucidocynodon riogran- recently described is Botucaraitherium opes for food with the other large herbivores densis, a cynodont that had protruding belarminoi, found in 2014 in the munici- L O I V of the period—the dicynodonts, which canines, was 1.2 meters in length and pality of Candelária, in Rio Grande do Á L F may have been carnivorous. Found in S were related to the cynodonts but did Sul. “We have three more new species LUÍ 3 not have mammalian features, and the 2009 in 220-million-year-old rocks in being examined,” says Soares, who hopes O C rhynchosaurs, which were reptiles. The the municipality of Agudo, also in Rio to discover another species of the first blan large predators of these herbivores were Grande do Sul, the fossilized skeleton of mammals, which emerged in the Late ge R O the Pseudosuchia, reptiles similar to gi- Trucidocynodon is one of the most com- Triassic and probably coexisted with J 2 n gs ant crocodiles. One of the Pseudosuchia plete cynodont skeletons ever discovered. the cynodonts. “We’ll find it one day.” R F that lived in Rio Grande do Sul and was Another interesting cynodont group, / U also discovered in rocks in Santa Cruz found only in rocks from the Upper Tri- Melo Z Scientific article A do Sul is Dagasuchus santacruzensis, de- assic—230 million to 200 million years M O MELO, T. P.; ABDALA, F.; SOARES, M. B. The Malagasy T scribed by UFRGS’ Marcel Lacerda and ago—in the Santa Maria Formation, Rio 1 cynodont (Cynodontia; Traversodon- Menadon besairiei s O T colleagues in an article published in 2015 Grande do Sul, consists of small animals tidae) in the Middle-Upper Triassic of Brazil.

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