A new non-mammaliaform eucynodont from the Carnian-Norian Ischigualasto Formation, Northwestern Argentina

Abstract

-The record of non-mammaliaformes eucynodonts from the Carnian-Norian Ischigualasto Formation is diverse and abundant, including a medium to large size herbivore and small carnivores. Here is described a new small eucynodont from the Ischigualasto Formation, on the basis of a partial skull. The new taxon is characterized by palatal process of the premaxilla extending posterior to the level of the fi rst postcanine; deep and large maxillary laterodorsal fossa that opens at the level of the root of the upper canine; and postorbital bar diverging posterolaterally at very low angle (35.6°) from the anteroposterior axis of the skull. Results from a phylogenetic analysis supports the new genus placement as a probainognathian eucynodont, more derived than Probainognathus Romer, and more closely related to Ecteninion Martinez, May & Forster and Trucidocynodon Oliveira, Soares & Schultz than to any other eucynodont. Ecteniniidae is proposed as a new clade including the new genus, Ecteninion and Trucidocynodon, and in the phylogenetic hypothesis represents the sister-group of Prozostrodontia (Prozostrodon Bonaparte & Barberena, Tritylodontidae and Mammaliaformes). Additionally, the new taxon from the Ischigualasto Formation shows that the Scaphonyx-ExaeretodonHerrerasaurus biozone has similar cynodont diversity than the supposedly contemporaneous Hyperodapedon Assemblage Zone of Santa Maria 2 Sequence, in Southern Brazil.Key words: Triassic, Argentina, Ischigualasto Formation, Cynodontia, Eucynodontia, Ecteniniidae. RESUMO -O registro dos eucinodontes não-mamaliformes do Carniano-Noriano da FormaçãoIschigualasto é diversifi cado e abundante, incluindo herbívoros de médio a grande porte e pequenos carnívoros. Descreve-se aqui um pequeno novo eucinodonte da Formação Ischigualasto, com base em um crânio parcial. O novo táxon é caracterizado pelo processo palatal da pré-maxila estendendo-se posteriormente até o nível do primeiro dente pós-canino; uma fossa profunda e ampla abrindo-se látero-dorsalmente na maxila ao nível da raiz do canino superior; e uma barra pós-orbital divergindo póstero-lateralmente em ângulo muito baixo (35,6°) em relação ao eixo ântero-posterior do crânio. Resultado da análise fi logenética sugere que o novo gênero é um eucinodonte Probainognathia, mais derivado que Probainognathus Romer e estreitamente relacionado a Ecteninion Martinez, May & Forster e Trucidocynodon Oliveira, Soares & Schultz do que qualquer outro eucinodonte. O clado novo gênero + Ecteninion + Trucidocynodon é aqui reivindicado como uma nova família chamada Ectininiidae, que é considerada como o clado-irmão de Prozostrodontia (Prozostrodon Bonaparte & Barberena, Tritylodontidae e Mammaliaformes). Além disso, a nova espécie da Formação Ischigualasto mostra que a biozona Scaphonyx-Exaeretodon-Herrerasaurus tem uma diversidade de cinodontes semelhante à da supostamente contemporânea Zona de Associação de Hyperodapedon, da Sequência 2 da Supersequência Santa Maria no sul do Brasil. Palavras-chave:

Two New Cynodonts (Therapsida) from the Middle-Early Late Triassic of Brazil and Comments on South American Probainognathians

“…Conspicuous denticles in the upper canines, as those present in Bonacynodon , are uncommon in other small-sized probainognathians (e.g., Lumkuia , [ 63 ]; Prozostrodon , UFRGS-PV-0248-T; Botucaraitherium , MMACR-PV-003-T; Brasilitherium , UFRGS-PV-0929-T; Riograndia , UFRGS-PV-0596-T; Irajatherium , UFRGS-PV-0599-T). Among probainognathians, conspicuous serrations are only present in the labiolingually compressed canines of Trucidocynodon (UFRGS-PV-1051-T) ( Fig 12D ), Ecteninion [ 21 ], and Diegocanis [ 22 ]. However, the denticles in the upper and lower canines of ecteniniids (even in juvenile specimens) are small and delicate in comparison to those of Bonacynodon ( Fig 12 ).…”

“…The most conspicuous probainognathian record from Argentina comes from the Ischigualasto-Villa Unión Basin. It includes Chiniquodon theotonicus [ 15 , 40 ] and Probainognathus jenseni [ 41 ] from the lower Upper Triassic Chañares Formation; Chiniquodon sanjuanensis ([ 15 ], = Probelesodon sanjuanensis sensu [ 6 , 23 ]), Ecteninion lunensis [ 21 ], Diegocanis elegans [ 22 ], cf. Probainognathus sp.…”

A new specimen provides insights into the anatomy of Irajatherium hernandezi, a poorly known probainognathian cynodont from the Late Triassic of southern Brazil

“…Here, to analyze the phylogenetic relationships of I. hernandezi as well as other ictidosaurians in the context of Eucynodontia, we employed the data matrix of Wallace et al (2019), which was based on Liu and Olsen (2010), Soares et al (2014), Martinelli et al (2016), Martinelli, Soares, et al (2017), and Martinelli, Eltink et al (2017), plus their modifications. To this data matrix, we added seven operational terminal units (OTUs): the tritheledontids I. hernandezi (scored based on previously published material and the one here presented; Table 1), C. musteloides (specimens PVL 3857, PULR 081; Bonaparte, 1980; Martinelli & Rougier, 2007), D. broomi (specimens BP/1/4882, NM C249; Crompton, 1958; Crompton & Parkyn, 1963; Gow, 1980, 1994), Elliotherium kersteni (Sidor & Hancox, 2006), Trithelodon riconoi (Broom, 1912; Gow, 1980); and the ecteniniids Trucidocynodon riograndensis (specimens UFRGS‐PV‐1051‐T, CAPPA/UFSM 0029; Oliveira et al, 2010; Stefanello et al, 2018) and Diegocanis elegans (Martínez et al, 2013). Pachygenelus monus was previously included in the matrix, but we further reviewed the codification based on the specimens NHMUK PV R 4091 holotype, BP‐1‐4741, BP‐1‐5623, BP‐1‐5691, and Hopson Lab uncatalogued specimens.…”

A proposed terminology for the dentition of gomphodont cynodonts and dental morphology in Diademodontidae and Trirachodontidae

“…(A) Tree topology based on the results of the cladistic analyses of Liu & Abdala (2014) for cynognathians and Martínez, Fernández & Alcober (2013) and Martinelli, Soares & Schwanke (2016) for probainognathians. Titanogomphodon , considered to be a close relative of Diademodon by Keyser (1973) and Martinelli, De La Fuente & Abdala (2009), is placed among Diademodontidae.…”