Mesozoic Mammals from South America and Their Forerunners
Abstract
This book summarizes the most relevant published paleontological information, supplemented by our own original work, on the record of Mesozoic mammals’ evolution, their close ancestors and their immediate descendants. Mammals evolved in a systematically diverse world, amidst a dynamic geography that is at the root of the 6,500 species living today. Fossils of Mesozoic mammals, while rare and often incomplete, are key to understanding how mammals have evolved over more than 200 million years. Mesozoic mammals and their close relatives occur in a few dozen localities from Argentina, Brazil, Chile, Bolivia, and Peru spanning from the Mid- Triassic to the Late Cretaceous, with some lineages surviving the cataclysmic end of the Cretaceous period, into the Cenozoic of Argentina. There are roughly 25 recognized mammalian species distributed in several distinctive lineages, including australosphenidans, multituberculates, gondwanatherians, eutriconodonts, amphilestids and dryolestoids, among others. With its focus on diversity, systematics, phylogeny, and their impact on the evolution of mammals, there is no similar book currently available.
... INTRODUCTION Dryolestoideans (or "dryolestidans") represent a diverse mammalian group known mostly from the Jurassic and Cretaceous. They are a part of the cladotherian clade on the stem leading to crown Theria (Marsupialia and Placentalia) (Kielan-Jaworowska et al., 2004;Martin, 2018;McKenna, 1975;Prothero, 1981;Rougier et al., 2021a). Dryolestoideans or Dryolestoidea of Butler (1939) historically included dryolestids, "paurodontids" that are now a paraphyletic grouping according to Rougier et al. (2012), and the putative relatives of these groups. ...
... Re-evaluation of the taxonomic relationships within the dryolestoideans has been prompted by new fossil discoveries within the last 15 years (Martinelli et al., 2021;Rougier et al., 2021a). Similarly, the hypothesized relationships of family-level clades within dryolestoideans to other Mesozoic mammals have also changed. ...
... Similarly, the hypothesized relationships of family-level clades within dryolestoideans to other Mesozoic mammals have also changed. The discovery of the Cretaceous meridiolestid Cronopio led to a new hypothesis proposing that the South American meridiolestidans were nested in a more inclusive Dryolestoidea (equivalent to "Dryolestida" of some authors) (Chimento et al., 2015;Martinelli et al., 2021;Rougier et al., 2011Rougier et al., , 2021aRougier et al., , 2021b. The inclusion of Necrolestes in their phylogeny reinforced the interpretation that dryolestoideans (including meridiolestidans) are a clade that crossed the Cretaceous-Paleogene (K/Pg) boundary (Chimento et al., 2012;Martinelli et al., 2021;Rougier et al., 2012;Wible & Rougier, 2017). ...
... Eutriconodontans are a diverse group of crown mammals that were distributed from the Jurassic through Cretaceous on the northern and southern landmasses (Kielan-Jaworowska et al., 2004;Rougier et al., 2021). They are characterized by molariforms with three main cusps mesiodistally aligned on a laterally compressed crown (i.e., the "triconodont" pattern; Jenkins and Crompton, 1979). ...
... On the Indian subcontinent, two "triconodont" taxa, Dyskritodon indicus and Paikasigudodon yadagirii, were reported on the basis of isolated teeth from the Jurassic Kota Formation (Prasad and Manhas, 2002). Their affinities with eutriconodontans, "amphilestids," or more basally branching mammaliaforms (e.g., morganucodontans) are still uncertain (Kielan-Jaworowska et al., 2004;Rougier et al., 2021). Here, we report a mammalian tooth from the latest Cretaceous (late Maastrichtian) of peninsular India, which represents not only the first confirmed record of Eutriconodonta from the Cretaceous of India but also the globally youngest record of the group, postdating the early Campanian Alticonodon lindoei from Canada by ∼16 Ma (Fox, 1969(Fox, , 1976Rogers et al., 2023). ...
... If the Moroccan eutriconodontans are indeed Bathonian, Indotriconodon would be the only eutriconodontan mammal known from the Cretaceous of Gondwana. Austrotriconodontids from the Campanian-Maastrichtian of Argentina were initially described as "triconodonts" but recent studies have shown them to be premolariform teeth of meridiolestids (Averianov et al., 2013;Gaetano et al., 2013;Rougier et al., 2011Rougier et al., , 2021. ...
Eutriconodonta is a diverse group of crown mammals that are known from the Jurassic and Cretaceous, mainly on the northern landmasses. Here we report a single lower molariform from the "intertrappean" deposits exposed near Anjar, in the Kutch district of the state of Gujarat, India that date to the last 259,000 years of the Cretaceous. The lower molariform of this new genus of eutriconodontan is relatively large, has three subequal, erect, lanceolated cusps, and lacks both a buccal and lingual cingulum, accessory cusps (d, e), and a vertical groove mesial to cusp b. It represents the first eutriconodontan mammal from the Cretaceous of India and the Late Cretaceous of Gondwana, and is the globally youngest record of this group. The results of a cladistic analysis support placement of the new Indian taxon among the Eutriconodonta, possibly more closely allied to triconodontids than to other groups. This new genus of eutriconodontan is the tenth named mammalian species known from the Late Cretaceous of India and the first relatively large-bodied faunivore. This record shows that the Eutriconodonta had a broader temporal and geographic distribution than previously thought and highlights the importance of continued paleontological exploration on the Indian subcontinent.
... Based on the tree topology, gondwanatherians would have originated by the Middle Jurassic, as already noted by Hoffmann et al. (2020). The high-crowned and multiple enamel-ridged tooth patterns in many gondwanatherians (Krause et al. 2014, Rougier et al. 2021) are more reflective of the allotherian tooth pattern than the triconodontan, symmetrodontan, or tribosphenic pattern. The unique molar morphologies of gondwanatherians may be interpreted as a specialization of the groups in the southern continents, and much of the morphological diversity is yet to be discovered (Hoffmann et al. 2020). ...
... If the upper and lower dentitions are subequal in tooth number and length, it stands as evidence for a very short, if any, palinal component in lower jaw movement during chewing in Haramiyavia. The general molar morphology and palinal jaw movement are the main features uniting gondwanatherians (Rougier et al. 2021). The molar morphology of gondwanatherians is complex and peculiar, but it seems more derivable from the allotherian tooth pattern than from the triconodont molar or its derivatives. ...
... Gondwanatherians would have originated by the Middle Jurassic, as depicted by Hoffmann et al. (2020). The highcrowned and multiple-ridged tooth patterns in many gondwanatherians (Krause et al. 2014, 2020a, Rougier et al. 2021) are more reflective of the allotherian tooth pattern than the triconodont, symmetrodont, or tribosphenic pattern. The unique molar morphologies of gondwanatherians may be interpreted as specialization of the groups in the southern continents and much of the morphological diversity is yet to be discovered (Hoffmann et al. 2020). ...
Allotheria are an extinct group of mammaliaforms that originally comprised multituberculates, to which ‘haramiyidans’ and gondwanatherians were later added. Phylogenetic relationships of allotherians have remained controversial since the first reports of the allotherian fossils in the 1800s. Here we report a new euharamiyidan based on a skeletal specimen from the Jurassic Daohugou phase of the Yanliao Biota, China. The superb specimen preserves impressions of differentiated hairs. The pes with impressions of toe skin and claw sheath represents the first such evidence in Mesozoic mammaliamorphs. The remarkable tooth morphologies surpass in complexity any Mesozoic mammals previously known. Phylogenetic analyses place the new species within euharamiyidans from Eurasia and support euharamiyidans, multituberculates, and gondwanatherians as a tripartite monophyletic Allotheria within mammals. This clade is supported not only by their similar molar pattern, but also by many shared derived features of the dental system recognized from discoveries during the last two decades. Allotherians and therians represent the two most successful groups of mammals that are characterized by distinct dental systems. They originated at the latest in the Late Triassic and became diversified in the Middle Jurassic, representing two directions of ecomorphological adaptation during early evolution of mammals.
... The sulcus is absent in the dryolestid Beckumia (Martin et al. 2022b). Meckel's sulcus is also well developed in the Late Cretaceous meridiolestid Cronopio (Rougier et al. 2011), but can be variable among other meridiolestidans, an endemic clade of South America (Rougier et al. 2021a). ...
... Henkelotherium differs from dryolestids in that the angular process of the latter group is not medially inflected, and has a straight and leveled ventral margin (Krebs 1971;Martin 1999). Henkelotherium is similar to meridiolestidans in the inflection of the angular process (Rougier et al. 2011(Rougier et al. , 2021a. Both the meridiolestidan Cronopio, and Necrolestes that is hypothesized to be related to meridiolestidans, have a prominent, medially inflected angular process (Asher et al. 2007;Rougier et al. 2012), although the degree of inflection and size of the angular process in Henkelotherium are much less than those of meridiolestidans. ...
... Peramus has three molars (McKenna 1975;Butler and Clemens 2001;Davis 2012). Vincelestesa mammal possibly closer to boreosphenidans (including crown Theria)-has three molars (Rougier et al. 2021a). Among zatherians closer to crown therians than dryolestidans, Amphitherium is an outlier with seven molars (Prothero 1981;Butler and Clemens 2001), presumably representing a convergent case in evolution of supernumerary molars as Amphitherium is phylogenetically distinct from dryolestidans in the latest phylogenetic studies (Close et al. 2015;Panciroli et al. 2018). ...
Henkelotherium guimarotae Krebs 1991 is an important Jurassic mammal for understanding therian evolution. We are presenting a new study of extensive, previously undescribed, mandibles and dentitions. The revised dental formula is: I4? or 5?/i4, C1/c1, P4/p4, M6/m7. The canine and premolars show an alternate replacement that ends with M4/m4 eruption, and is followed by a late sequential eruption of the last three lower (m5-7) and last two upper (M5-6) molars. The lower premolars erupted in the following order: p1 → p3 → p2 → p4, and the canine erupted most probably shortly before p4. The timing of the premolar replacement before the late molar eruption is similar to that of Dryolestes leiriensis, and is a characteristic of dryolestidans. Henkelotherium lower molars have subequal roots, a plesiomorphy of non-dryolestidan mammals, and the upper molars are supported by a strong, curved lingual root; a derived character. In the upper molars, the postvallum wear surface is contiguous to the parastyle wear surface of the succeeding molar, which differs from dryolestids. The parastylar lobe of the succeeding molar, and the postvallum of the preceding molar, are imbricated, and can develop strong, continuous wear surfaces, matching the prevallid crest of the lower molar. Henkelotherium differs from dryolestids in having an inflected, shelf-like mandibular angular process with a foramen. This large sample of Henkelotherium shows a significant variation gradient along the molar series, with the strongest wear occurring only in two to three consecutive molars. The extraordinarily long molar row is correlated with the late growth of jaws; and the jaw with late addition of molars sustained an effective mastication, much longer in older adults of dryolestidans than in other Mesozoic stem therians. The late eruption of several more molars after completion of antemolar replacement suggests that dryolestidans had either a longer-lived life, or slower life-history traits, or a combination of both, than crown therians.
... The site is renowned for its collection of other early mammals, including three possible eutherians/australosphenidans (Ausktribosphenos nyktos, Bishops whitmorei, and Kryoparvus gerriti), and the monotreme Teinolophos trusleri, as well as a host of fungi, plants, palynomorphs, invertebrates, and other vertebrates (see overview in Poropat et al. 2018). Rich et al. (2009) suggested that NMV P216655 was the first record of Multituberculata in Australia and one of the very few records of that clade from the entire southern supercontinent of Gondwana, the other three being at the time questionable records from the Early Cretaceous of Morocco and the Late Cretaceous of Argentina and Madagascar (see reviews in Krause et al. 2017, Rougier et al. 2021, and "Biogeographic implications" section below). ...
... The specimens of cf. Sudamerica ameghinoi and Gondwanatherium pata-gonicum are too fragmentary to be particularly informative in the current context, and identification of the latter as pertaining to Gondwanatherium has been questioned (Pascual et al. 1999;Rougier et al. 2021). Our comparisons of the lower jaw morphology of Corriebaatar marywaltersae are, therefore, restricted to the dentaries of Adalatherium hui (UA 9030), Sudamerica ameghinoi (MPEFCH 534), and Galulatherium jenkinsi (RRBP 02067). ...
... Ferugliotheriidae.-Ferugliotherium windhauseni from the latest Cretaceous (Maastrichtian) of Argentina includes two junior synonyms, Vucetichia gracilis (see Krause 1993) and Argentodites coloniensis (see Gurovich and Beck 2009;Brown et al. 2020;Rougier et al. 2021). Whether or not the similarly sized Trapalcotherium matuastensis, known only from a single incomplete m1 from the Allen Formation (Rougier et al. 2009), is a distinct genus and species is debatable and awaits the recovery of more complete material. ...
A second specimen of the Australian cimolodontan multituberculate Corriebaatar marywaltersae from the same loca�lity (Flat Rocks) as the holotype and previously only known specimen, reveals far more anatomical information about
the species. The new specimen, composed of most of a dentary containing a complete p4 and alveoli for the lower
incisor and the lower first and second molars, exhibits a suite of features consistent with allocation of Corriebaatar to
Cimolodonta and further confirms the presence of multituberculates on Gondwana during the Mesozoic. The revised
(older) age of the Flat Rocks locality to latest Barremian (mid-Early Cretaceous) establishes C. marywaltersae as the
oldest currently known cimolodontan. This has profound biogeographic implications for the distribution of multituber�culates on Gondwana as well as globally, particularly in light of the fact that Corriebaatar appears to be a relatively
derived member of Cimolodonta.
... Our knowledge of the Mesozoic mammalian faunas from South America (SA) is still in its infancy and is limited both temporally and geographically. The bulk of the record comes from the Late Cretaceous of southern South America, Argentina in particular (see Rougier et al. 2021 for a review). The discovery of new specimens of the meridiolestid Reigitherium from the Late Cretaceous La Colonia Formation provides the impetus to reassess the dryolestoid (in a wide sense) diversity in SA. ...
... (Rougier et al. 2011(Rougier et al. , 2012Wible and Rougier 2017). Cronopio dentiacutus from the Cenomanian Candeleros Formation (Rougier et al. 2011) is the oldest and best known taxon among the sharp-toothed meridiolestidans; together with its relatives, it is grouped in Cronopioidea, defined as the last common ancestor of Cronopio, Necrolestes, and all its descendants (Rougier et al. 2021). The other major group of meridiolestidans, Mesungulatoidea (Rougier et al. 2011), includes taxa with bunodont (or at least not very sharp) dentitions, prominent cingula, and subequal molar roots that are mesiodistally compressed. ...
... The other major group of meridiolestidans, Mesungulatoidea (Rougier et al. 2011), includes taxa with bunodont (or at least not very sharp) dentitions, prominent cingula, and subequal molar roots that are mesiodistally compressed. Mesungulatum, Coloniatherium, and Reigitherium from the Campanian-Maastrichtian of Patagonia and Peligrotherium from the Paleocene of the Salamanca Formation are the best represented taxa of mesungulatoids; however, a few other potential taxa are known (see Rougier et al. 2021) that potentially date back to the Coniacian (Forasiepi et al. 2012). ...
We describe the first maxillae and additional new specimens of Reigitherium bunodontum, a small meridiolestidan from the Late Cretaceous La Colonia Formation, Patagonia, Argentina. The new material supports a dental formula of I?, C1, P4, M3, resolves postcanine positional uncertainty and corrects previous interpretations. Our phylogeny recovers Reigitherium as a meridiolestidan allied to other bunodont Mesungulatoidea, as the sister group of the Paleocene Peligrotherium. Posterior premolars/molars of Reigitherium, and to a smaller degree Peligrotherium, are dominated by an incomplete transverse ridge running between the protoconid-metaconid in the lowers and the paracone-stylocone in the uppers, semi-symmetrical basins developing mesially and distally from these central ridges. The trigonid-derived single transverse crest results from a mesial shift of the robust metaconid, an enhancement of the basin crest stretching from the protoconid/metaconid, and a shallower trigonid basin. The mesungulatoid condition, with its complete absence of talonid, contrasts sharply with that of therians with lophs, or transverse ridges, which involved at least one talonid-derived loph resulting in two transverse crests per tooth. Mesungulatoid meridiolestidans achieved complex tooth-on-tooth occlusion with a predicted increase in herbivory/omnivory, departing from the traditional sharp-cusp insectivores plesiomorphic for meridiolestidans and Mesozoic mammals in general. Reigitherium’s dramatic remodeling of the primitive meridiolestidan molar morphology, extensive continuous occlusal surface, accessory cuspules, and highly textured crenulated enamel illustrates one of most distinctive adaptations to herbivory among Mesozoic mammals.
... They had a specialized dentition for herbivorous feeding OPEN Patagonia 14,30,31 . In addition to gondwanatherians and meridiolestidans, the current record of Late Cretaceous mammals in South America includes two species of dryolestidans from Argentina closely related to Laurasian forms (i.e., Groebertetherium stipanicici and G. allenensis 11,15 ), as well as a handful of other records of uncertain affinities from Peru, Bolivia and Brazil 32-35 . ...
... By that time, intensive fieldwork led by Dr. José F. Bonaparte resulted in the discovery of a diverse mammal assemblage from the Campanian-Maastrichtian Los Alamitos Formation (northern Patagonia, Argentina), comprising as many as 15 new species of non-therian mammals (excluding gondwanatherians), mostly based upon isolated dental elements. Further materials coming from other localities and ages (e.g., Candeleros, Allen, La Colonia and Salamanca formations) indicated that this diversity was overestimated; in some cases, taxa were recognized on the basis of isolated teeth representing different loci of dentition in the same taxon [10][11][12][13]15,38,42,[51][52][53] . The foundational stone for the recognition of the South American clade Meridiolestida was based on the large number of mammalian discoveries at the roughly coeval Los Alamitos, Allen and La Colonia formations, plus significant records in the mid-Cretaceous Candeleros Formation and lower Paleocene Salamanca Formation 13 . ...
... The foundational stone for the recognition of the South American clade Meridiolestida was based on the large number of mammalian discoveries at the roughly coeval Los Alamitos, Allen and La Colonia formations, plus significant records in the mid-Cretaceous Candeleros Formation and lower Paleocene Salamanca Formation 13 . The whole evidence proves that meridiolestidans evolved more disparate dental and craniomandibular morphotypes than their relatives the dryolestid and paurodontid dryolestoids [8][9][10][13][14][15]31 . Contrary to this line of evidence, Averianov et al. 49 proposed an alternative hypothesis in which meridiolestidans were nested as non-cladotherian trechnotherians related to spalacotheroid "symmetrodontans". ...
In the last decades, several discoveries have uncovered the complexity of mammalian evolution during the Mesozoic Era, including important Gondwanan lineages: the australosphenidans, gondwanatherians, and meridiolestidans (Dryolestoidea). Most often, their presence and diversity is documented by isolated teeth and jaws. Here, we describe a new meridiolestidan mammal, Orretherium tzen gen. et sp. nov., from the Late Cretaceous of southern Chile, based on a partial jaw with five cheek teeth in locis and an isolated upper premolar. Phylogenetic analysis places Orretherium as the earliest divergence within Mesungulatidae, before other forms such as the Late Cretaceous Mesungulatum and Coloniatherium , and the early Paleocene Peligrotherium . The in loco tooth sequence (last two premolars and three molars) is the first recovered for a Cretaceous taxon in this family and suggests that reconstructed tooth sequences for other Mesozoic mesungulatids may include more than one species. Tooth eruption and replacement show that molar eruption in mesungulatids is heterochronically delayed with regard to basal dryolestoids, with therian-like simultaneous eruption of the last premolar and last molar. Meridiolestidans seem endemic to Patagonia, but given their diversity and abundance, and the similarity of vertebrate faunas in other regions of Gondwana, they may yet be discovered in other continents.
... It is thus likely that the endemic Cenozoic lizard fauna of South America could result from the radiation of groups already present by the Late Cretaceous, and which survived the K/Pg mass extinction event. The latter was less severe in South America than in Laurasia, as evidenced by plants (De Benedetti et al. 2023), mammals (Rougier et al. 2011(Rougier et al. , 2012(Rougier et al. , 2021, and the survival of different sphenodontian groups and of madtsoiid snakes among other lepidosaurian clades into the Cenozoic (Albino and Brizuela 2014;Apesteguía et al. 2014), contrasting with the devastating nature of this event as recorded in the northern continents (Longrich et al. 2012;Lyson et al. 2019;Chiarenza et al. 2020). The presence of teiioids in the Upper Cretaceous of Patagonia is consistent with this scenario, making it unnecessary to invoke their immigration from North America around the early Late Cretaceous, as it has been previously conceived (Gao and Fox 1991;Nydam and Voci 2007). ...
... The presence of pleurodontan iguanians and 'anilioid' snakes in the Upper Cretaceous of Patagonia (Gómez et al. 2008;Albino and Brizuela 2014;Head 2021) reinforces that several endemic, or almost exclusive, South American squamate lineages were already established during the Mesozoic. A similar pattern is observed in the case of Cretaceous mammals from South America, with the presence of endemic groups that, as highlevel taxonomic units, survived the disturbances of the end-Cretaceous mass extinction (Gelfo and Pascual 2001;Rougier et al. 2011Rougier et al. , 2012Rougier et al. , 2021. ...
... Considering that deciduous mammalian teeth tend to have a slightly different morphology than permanent teeth (Kielan-Jaworowska et al., 2005), the deciduous nature of the new tooth, if confirmed, could explain the differences in size, talonid morphology and position of accessory cusp concerning the holotype of B. stardusti, and their association cannot be ruled out. South American Mesozoic mammals have been known since the Early Jurassic from localities in Argentina, Bolivia, Brazil, Chile, and Peru, comprising about 30 species of several lineages (see the recent review of Rougier et al., 2021). However, concerning the Late Cretaceous, biogeographic differences are recognized between southern and northern South America, as illustrated by the lack of tribosphenidans' records in the fossil-rich sites of Patagonia (Castro et al., 2018). ...
... The comparison was focused on Eunotosuchia teeth from the Adamantina Formation, which represents the most extreme case of multicusped teeth among Crocodyliformes and with abundant records in this unit (Montefeltro et al., 2009;Melstrom and Irmis 2019). In this sense, Rougier et al. (2021) mention that the affinity of B. stardusti to juvenile sphagesaurid notosuchian cannot be ruled out. However, we observed that the teeth of multicusped eunotosuchians bear multiple apico-basal ridges, tend to have thicker enamel, and usually have three or four cusps (Montefeltro et al., 2009;Melstrom and Irmis 2019). ...
The Adamantina Formation is the most extensive and taxonomically rich unity within the Bauru Group (Upper Cretaceous). This unity is considered one of the complete fossil records of vertebrate continental communities during the Cretaceous Gondwanan landmasses. However, this important fossil record is mainly based on large and articulated remains, and few works attempted to better understand its microvertebrate assemblages. This study reports a diverse microvertebrate assemblage from a new site in the Adamantina Formation in São Paulo, Brazil. The geological context in which the fossils were collected was analyzed, and all fossils were recovered in channel facies of the outcrop. The fossil assemblage described here includes elements commonly found in previous works in the Adamantina Formation, such as lepisosteiforms, amiids, dinosaurs, and crocodyliforms. But in this assemblage were also identified elements rarely recovered from Adamantina Formation, such as siluriforms, anurans, and a putative mammal. Furthermore, the taxa identified here indicate a humid environment, contributing to recognizing a diversity of paleoenvironments in the Adamantina Formation and highlighting the importance of microvertebrate assemblages to understanding the palaeoecological aspects of fossil communities.
... It was defined as the least inclusive clade containing Prozostrodon brasiliensis, Tritylodon longaevus (Tritylodontidae), Pachygenelus monus ("Trithelodontidae"), and Mus musculus (Mammalia). During the Late Triassic/Early Jurassic, the clade Mammaliaformes emerged and diversified (Hunttenlocker, Grossnicke, Kirkland, Schultz, & Luo, 2018;Liu & Olsen, 2010;Luo, Gates, Jenkins Jr., Amaral, & Shubin, 2015;Rougier, Martinelli, & Forasiepi, 2021;Rowe, 1988). Besides mammaliaforms, "tritheledontids" and tritylodontids were cynodont lineages that survived the Triassic/Early Jurassic boundary, with the former disappearing by the Early Jurassic and the latter in the Early Cretaceous (Matsuoka, Kusuhashi, & Corfe, 2016;Kerber, Martinelli, Müller, & Pretto, 2022 and references cited therein). ...
... The rims of the fenestra vestibuli form an osseous ring, as described for Pro. jenseni, D. broomi, and R. guaibensis, among others (Luo, 1994;Rougier et al., 2021;Soares et al., 2014), but not as well-developed as in traversodontids, in which this structure forms a tube enclosing the fenestra (e.g., Kemp, 1980, figure 1; Abdala, Barberena, & Dornelles, 2002, figure 6). In contrast, mammaliaforms do not present an osseous ring (Luo, 1994). ...
Prozostrodon brasiliensis and Therioherpeton cargnini are non-mammaliaform cynodonts that lived ~233 million years ago (late Carnian, Late Triassic) in western Gondwana. They represent some of the earliest divergent members of the clade Prozostrodontia, which includes "tritheledontids", tritylodontids, "brasilodontids", and mammaliaforms (including Mammalia as crown group). Here, we studied the endocranial anatomy (cranial endocast, nerves, vessels, ducts, ear region, and nasal cavity) of these two species. Our findings suggest that during the Carnian, early prozostrodonts had a brain with well-developed olfactory bulbs, expanded cerebral hemispheres divided by the interhemispheric sulcus, and absence of an unossified zone and pineal body. The morphology of the maxillary canal represents the necessary condition for the presence of facial vibrissae. A slight decrease in encephalization is observed at the origin of the clade Prozostrodontia. This new anatomical information provides evidence for the evolution of endocranial traits of the first prozotrodonts, a Late Triassic lineage that culminated in the origin of mammals.
... Este ejemplar incluye la parte posterior del cráneo y la secuencia articulada de vértebras hasta el inicio de la cola. Bonaparte, 1986a(e.g., Bonaparte, 1986aRougier et al., 2021). ...
... El estudio integral de estos materiales, sumado a los diversos ejemplares de arcosaurios atesorados en el IML (descubiertos e interpretados por J. F. Bonaparte en la década de 1970), resultaron en trabajos que sustentaron la monofilia de Dinosauria (Novas, 1996), temas que fueron ahondados a partir de allí por otros paleontólogos, varios de ellos argentinos. Amarga, Lohan-Curá, Bajo de la Carpa, Angostura Colorada, Allen y Loncoche, entre otras (e.g., Bonaparte, 1996aBonaparte, , 1996bMartinelli y Forasiepi, 2004 (Bonaparte et al., 2003;Bonaparte y Migale, 2015;Rougier et al., 2021). ...
... The presence of docodontans in South America has not been corroborated so far. Reigitherium bunodontum, formerly attributed to Docodonta (Pascual et al., 2000), was later referred to meridiolestidans (Harper et al., 2019;(Rougier et al., 2021a)Rougier et al., 2021b. The interpretation of the two tooth fragments (MPM-PV-22878 and MPM-PV-22881) from Tres Lagos is hampered by their fragmentary nature. ...
... 7)and with the penultimate upper premolar MACN-RN-171, attributed to Quirogatherium major byBonaparte (1990); the two Barberenia species were synonymized with Groebertherium stipanici and Quirogatherium major is regarded as Meridiolestida indet. byRougier et al., 2021a. However, MPM-PV-22879 differs from these by the absence of an ectoflexus and a more mesially placed paracone. ...
The Upper Cretaceous (Cenomanian) Mata Amarilla Formation in western Central Santa Cruz Province of Argentina has yielded fragmentary teeth of a large ?docodontan, an australosphenidan, a meridiolestidan (Amarillodon meridionalis gen. et sp. nov.), and a stem dryolestid (Treslagosodon shehuensis gen. et sp. nov.). These represent the first possible records for docodontans, Cretaceous australosphenidans, and stem dryolestids in South America. Both ?docodontan tooth fragments are unusually large and exhibit potential durophagous adaptation. The Amarillodon gen. nov. lower (?deciduous) posterior premolar has a trigonid angulation of 100° and is autapomorphic by a large and exoedaenodont distolabial accessory cusp on the distal cingulid. Both the mesial and distal cingulid are shelf-like. The mesiolabial upper molar fragment of the ausktribosphenid australosphenidan is similar to an upper M1 of aff. ?Bishops from the lower Albian (Lower Cretaceous) of Australia by its large stylar cusp ?C and breached paracone. The lower molar of the new stem dryolestid is characterized by a large talonid cusp d and a mesio-distally strongly compressed mesial root that is weaker than the distal one. The australosphenidan, if corroborated, suggests faunal interrelationships between Australia and South America by the late Early/early Late Cretaceous.
... Dryolestids and meridiolestidans.-In the broadest sense, three groups can be considered as potential members of the Dryolestoidea: the Dryolestidae, Paurodontidae, and Meridiolestida (Butler 1939;Prothero 1981;Bonaparte 1992;Martin 1999;Rougier et al. 2011Rougier et al. , 2012Rougier et al. , 2021aAverianov et al. 2013;Averianov and Martin 2015). The interrelationships among these three distinct groups are poorly resolved and alternatives range from all three forming a single monophyletic group to each one of them being successive sister groups to each other as stem lineages to Theria (Rougier et al. 2011(Rougier et al. , 2012Martin et al. 2021). ...
... Vincelestes.-The cranial anatomy of Vincelestes is relatively well known (Bonaparte and Rougier 1987;Hopson and Rougier 1993;Rougier et al. 2021a), with the petrosal/ braincase having been described in detail (Rougier et al. 1992). Most phylogenies recover Vincelestes as a stem therian closer to Theria than basal cladotheres like dryolestoids and meridiolestidans, but alternative views exist (Bonaparte 2008;Averianov et al. 2013;Bonaparte and Migale 2015). ...
The Morrison Formation bears one of the most diverse assemblages of Late Jurassic terrestrial vertebrates worldwide. A recently discovered site in eastern Utah, the Cisco Mammal Quarry (CMQ), shows excellent preservation and small vertebrates (particularly mammals) collected thus far are very diverse. Two isolated petrosals from the CMQ, representing the same taxon, are described here based on CT data. Several plesiomorphic mammalian characters are present, including a horizontal crista interfenestralis, unfloored cavum supracochleare, a perilymphatic foramen, and an open perilymphatic groove. By contrast, a well-developed tractus foraminosus is present for distribution of cochlear nerve fibers and the cochlear endocast makes one full turn, as in early therians such as Prokennalestes. This latter derived condition is unrecorded in the Jurassic; the mammal fauna characteristic of the Morrison is dominated by docodontans, eutriconodontans, dryolestoids (dryolestids and paurodontids), and multituberculates. All currently-known examples of these high-level taxa possess a relatively short, straight or curved cochlea. Therians are not known from the Morrison Formation, but these petrosals invite comparison with some stem therians of South America, the meridiolestidans, which also bear a full coil, but differ from the Morrison petrosals in aspects of external morphology. While the taxonomic affinity of the specimens we describe here remains uncertain, the unique combination of petrosal characters is evidence that our current view of petrosal/inner ear evolution is oversimplified. With the presence of derived and plesiomorphic features character conflict is unavoidable, pointing to a complex interplay of external petrosal osteology under the influence of neurovascular and middle ear evolution and the (perhaps more independent) biophysical demands of inner ear function.
... Recent discoveries of Mesozoic vertebrates from Gondwanan landmasses have begun to yield fundamental new insights into their biogeographic histories. Numerous discoveries of Cretaceous and even Jurassic materials in PAT (see a review in Rougier et al. 2021) give a new panorama on mammal and mammaliform evolution, starting not with the latest Early Cretaceous isolation from Africa, but with the late Middle to earliest Late Jurassic mammaliform and mammal fossils from Chubut (central PAT): Asfaltomylos and Henosferus (Rauhut et al. 2002;Martin and Rauhut, 2005;Rougier et al. 2007a), Argentoconodon (Rougier et al. 2007b) and Condorodon (Gaetano and Rougier, 2012), and the Middle Jurassic ichnofauna from Santa Cruz (southern PAT) (Casamiquela, 1961) all of which predate the seafloor spreading between Africa and SAM (Dalziel and Lawver, 2001). ...
... Late Cretaceous mammal faunas in PAT have been grouped under the Alamitan SALMA. Bonaparte (1986cBonaparte ( , 1990 has greatly improved our knowledge of the land mammal fauna of this interval (see a review in Rougier et al. 2021), but the continued absence of therians has not modified the basic dispersal situation reviewed by Woodburne and Case (1996) and Case et al. (2005). The Alamitan fauna contains numerous dryolestoids, and the less prominent 'triconodonts,' 'symmetrodonts,' and gondwanatheres, which flourished in a variety of ecologies within subtropical to tropical aquatic to woodland settings reflected by a diverse framework of gymnosperms, angiosperms, ferns, monocots, conifers, and grasses (Stiles et al., 2020). ...
The Mesozoic plate tectonic and paleogeographic history of the final break up of West Gondwana had a profound effect on the distribution of terrestrial vertebrates in South America. As the supercontinent fragmented into a series of large landmasses (South America, Antarctica, Australia, New Zealand, the Indian subcontinent, and
Madagascar), particularly during the Late Jurassic and Cretaceous, its terrestrial vertebrates became progressively isolated, evolving into unique faunal assemblages. The episodic nature of South American mammalian Cenozoic faunas became apparent in its modern formulation after George Gaylord Simpson’s seminal works on this topic. Two aspects add complexity to this generally accepted scheme: first, the fact that South America is not (and was not) a biogeographic unit, as the Neotropical Region does not include its southernmost tip (the Andean Region, including Patagonia and the southern Andes). Second, and intimately linked with the first one, that South America was not an island continent during the Late Cretaceous and the beginning of the Cenozoic, being
its southernmost portion closely linked with West Antarctica up to the late Paleocene at least. Here we stress on this second aspect; we summarize a series of recent, detailed paleogeographical analyses of the continental breakup between Patagonia (including the Magallanes Region) and the Antarctic Peninsula crustal block, beginning with the opening of the Atlantic Ocean in the Early Cretaceous and running up to the Early Paleogene
with the expansion of the Scotia Basin. In second place, we comment on the implications of these distinct paleogeographic and paleobiogeographic scenarios (before and after their geographic and faunistic isolation) for the evolution of South American terrestrial mammalian faunas. Summarizing, (1) we recognize a West Weddellian terrestrial biogeographic unit with the assemblage of the southern part of South America (Patagonia and the Magallanes Region) and the Antarctic Peninsula (and probably Thurston Island) crustal block of West Antarctica, spanning from the Late Cretaceous (Campanian) through the Early Paleogene (Paleocene); (2) we suggest that the Antarctic Peninsula acted as a double "Noah’s Ark” regarding, first, the probable migration of some non-therian lineages into southern South America; later, the migration of metatherians to Australasia.
... The assumption that most Mesozoic mammals were generalized small animals with generalized feeding habits and a ground-dwelling (i.e., terrestrial locomotor mode) lifestyle has been refuted by spectacular new findings from the fossil record in the past 25 years (e.g., Luo, 2007;Martin, 2018;Rougier et al., 2011Rougier et al., , 2021. Our current view is that the Jurassic and Cretaceous mammal assemblages were ecomorphologically diverse, approaching the diversity of the Cenozoic assemblages of terrestrial, smallbodied mammals in many aspects (Chen et al., 2019;Luo, 2007;Martin, 2018). ...
The complex evolutionary history behind modern mammalian chewing performance and hearing function is a result of several changes in the entire skeletomuscular system of the skull and lower jaw. Lately, exciting multifunctional 3D analytical methods and kinematic simulations of feeding functions in both modern and fossil mammals and their cynodont relatives approach this topic, giving fresh insights into the history of mammalian masticatory behaviors and their evolutionary trends. One crucial transformation in this context is the segregation of postdentary bones (becoming the mammalian middle ear) from the lower jaw, which is posited to have led to the important functional decoupling of the hearing and feeding systems. Evolution of the middle ear is regarded as the key transition that enhanced both mammalian chewing performance and hearing capacity. Three major functional parts undergo substantial evolutionary changes in this process that are anatomically linked to each other: the lower jaw and dentition, middle ear, and inner ear. Sound, transmitted via vibrations of the bony middle ear elements to the inner ear, is converted into movements of the endolymph fluid that shift hair cells of the organ of Corti, triggering neural stimuli perceived as hearing. Structural changes in one part of the system influence the function of the other two. In this review, I highlight recent advances in research focusing on the enhancement of both chewing performance and hearing ability in mammalian history to feature the mechanisms that led to the decoupling of the hearing system (i.e., middle and inner ear) from the feeding system.
... Synapsida, one of the major groups of terrestrial amniotes, comprises both living mammals and a diverse group of stem-taxa (Luo et al., 2002;Kemp, 2005;Benton, 2015;Angielczyk and Kammerer, 2018;Rougier et al., 2021). During the Permian Period, the clade Anomodontia was the most species-rich clade of non-mammalian synapsids (Fröbisch, 2009;Angielczyk and Kammerer, 2018) and was dominant in terms of specimen abundance, geographic range, species richness, and ecological diversity (Kemp, 2005;Fröbisch and Reisz, 2008;Fröbisch, 2009;Nicolas and Rubidge, 2010;Smith et al., 2012). ...
The Dicynodontia (Therapsida: Anomodontia) is one of the most successful Permo-Triassic terrestrial tetrapod clades and the oldest specimens are recorded from the middle Permian Eodicynodon Assemblage Zone of South Africa. Their fossil record is abundant and species-rich across Pangea. By contrast, the fossil record of the basal-most anomodonts, which includes non-dicynodont anomodonts and early forms of dicynodonts, is patchy and their morphology and phylogeny are deduced from relatively few specimens. Discovered in 1982 and described in 1990, the holotype of Eodicynodon oelofseni (NMQR 2913) is one of the better-preserved early anomodont specimens. However, it has been suggested that E. oelofseni does not belong to the genus Eodicynodon. Here, using CT-scanning and 3D modeling, the skull of Eodicynodon oelofseni, Patranomodon nyaphulii and Eodicynodon oosthuizeni are redescribed. In the framework of this study, the application of 3D scanning technology to describe anatomical structures which were previously inaccessible in these fossils has enabled detailed redescription of the cranial morphology of the basal anomodonts Patranomodon, Eodicynodon oelofseni and E. oosthuizeni and led to a greater understanding of their cranial morphology and phylogenetic relationships. Based on an anatomical comparison and phylogenetic analyses (Bayesian and cladistics) the phylogenetic relationships of basal anomodonts are reassessed and it is suggested that NMQR 2913 does not belong to the genus Eodicynodon but likely represents a separate genus basal to other dicynodonts. A new genus is erected for NMQR 2913. This presents one of the first applications of Bayesian Inference of phylogeny on Therapsida.
... The Late Cretaceous Los Alamitos Formation (Maastrichtian; Franchi and Sepúlveda, 1983;Bonaparte et al., 1984;Andreis, 1987;Franchi et al., 2001) is a well-known stratigraphical unit, mostly because of the abundant vertebrate record (Bonaparte et al., 1984;Bonaparte, 1987Bonaparte, , 2002. These include pipoid and calyptocephalellid anurans, diverse madtsoiid snakes and chelid turtles, meiolaniid tortoises, sphenodontians, sauropod, non-avian theropods and birds, abundant specimens of the hadrosaurid dinosaur Huallasaurus australis, and particularly diverse mammals, including a dozen of species belonging to dryolestoids, including meridiolestidans, gondwanatherians, and taxa of uncertain affinities (Bonaparte et al., 1984;Bonaparte, 1986Bonaparte, , 1987Bonaparte, , 1994Bonaparte, , 2002Báez, 1987;Broin, 1987;Powell, 1987Powell, , 2003Albino, 1987Albino, , 2000Apesteguía, 2005;Agnolin and Martinelli, 2009;Gómez, 2016;Rougier et al., 2021;Rozadilla et al., 2021). ...
... Some basal mammals displayed a sprawling posture, in some cases related to arboreal habits 34,35 , which can be thus discarded. Most of the remains of Cretaceous mammals in South America correspond to cranial fragments and isolated teeth [36][37][38] , for which is difficult to estimate their body mass (Table S3). A single well-documented mammal taxon is Vincelestes nuequenianus (Mammalia, Cladotheria) from Barremian-Aptian deposits of Patagonia, but it can be discarded as producer because of larger size (body mass ranging from 619 to 1228 g) and an inferred scansorial-arboreal habit 39,40 . ...
Scarce fossil tetrapod burrows have been recorded in Cretaceous rocks, which is probably linked to the dominant equable climates that existed for most of this period. The occurrence of Cretaceous tetrapod burrows from Patagonia (Chubut Province, Argentina) dated between 118 and 115 million years ago, gives insights into their paleoecology and paleoenvironment. The rocks containing the tetrapod burrows are of pyroclastic origin and represent eolian dunes and ash-fall deposits, some reworked by fluvial currents and others showing soil development. Fossil burrow casts preserved in a paleosol are composed by a ramp with a slightly curved or straight path in plan-view and lacking bifurcation, a rounded termination with no enlargement, showing a reniform cross-section, and are assigned to the ichnospecies Reniformichnus katikatii. The strongly flattened cross-sectional shape of the burrow casts and comparison with modern lizard burrows suggest that the producers were lepidosaurs (body mass = 50–323 g). Among Cretaceous fossorial lepidosaurs from Patagonia, the best candidate is an eilenodontine sphenodontian. Sphenodontians burrowed in the fossil soils where also arthropods, earthworms and shrubby plants thrived. The rare occurrence of tetrapod burrows in Cretaceous rocks is linked to stressing conditions related to frequent arrival of volcanic ash and a semiarid seasonal climate.
... Mammaliamorph cynodonts diverged in Late Triassic and true mammals become diversified and cosmopolite in Middle Jurassic (e.g., Grossnickle et al., 2019). Though the evolution of true mammal characters (such as typical mammalian middle ear, diphyodonty, and craniomandibular suspension composed just of the squamosal-dentary articulation) is well known due to a sequence of transitional forms, the mammalian Mesozoic record is still incomplete and fragmentary when compared to the Cenozoic, especially considering postcranial elements (Chimento et al., 2016;Abdala et al., 2020;Rougier et al., 2021, and references cited by them). With respect to this, trace fossils provide important and reliable data, not only complementing the osteological record, but also introducing new and unique information regarding the evolution of the locomotion patterns (Casamiquela, 1961;Rainforth & Lockley, 1996;de Valais, 2009;Buck et al., 2017), paleoecology (Kuznetsov & Panyutina, 2018), and geographic distribution (Leonardi, 1981;Leonardi & Carvalho, 2021a, b) of mammals in the Mesozoic. ...
Mammalian lineage is deep rooted in Mesozoic, with several taxa and ichnotaxa described worldwide. One of the most prolific mammaliaform ichnospecies is Brasilichnium elusivum, which is extremely abundant in the Lower Cretaceous Botucatu Formation of the Brazilian Paraná Basin, as well as in other Mesozoic ichnofaunas of North America and Africa. In this contribution, we revisited the proposed southernmost occurrence of B. elusivum using both classical and photogrammetric (3D digitization) approaches. The flagstone containing the studied material was found ex situ, in a sidewalk at Santa Cruz do Sul Municipality (the Rio Grande do Sul State, Brazil), and the results of the performed analysis showed that the tracks cannot be assigned to any ichnotaxon. Lastly, some significant issues related to ichnological research of ex situ track-bearing flagstones and large eolian deposits are discussed. Keywords: Early Cretaceous, Botucatu Formation, mesozoic mammals, South America, vertebrate ichnology.
... About both Brasiles. stardusti and the Santo Anastácio specimen, ROUGIER et al. (2021) stated that premolars are poorly diagnostic and that alternative taxonomic affinities, such as notosuchian crocodyliforms, could not be ruled out. In fact, based on anatomical and histological characters, these authors questioned the mammalian affinity of Brasiles. ...
The Bauru Basin bears one of the best sampled tetrapod paleofaunas of Brazil, with about 70% of this diversity collected from its deposits in São Paulo. Its fossils are known since the beginning of the 20th century, coming from all stratigraphic units of the Basin cropping-out in the state, i.e., Santo Anastácio, Araçatuba, Adamantina (alternatively divided into Vale do Rio do Peixe, Presidente Prudente, and São José do Rio Preto formations), and Marília formations. Identified taxa include rare anurans, mammals, and squamates, an important set of testudines, theropods (including birds), and sauropods, in addition to one of the most diverse crocodyliform faunas known worldwide. This congregates more than fifty unique taxonomic entities, including 42 formally described species. Based on biostratigraphic correlations (including tetrapods), on few absolute ages, and other sources of evidence, the Bauru Basin deposits in São Paulo seem to be chronologically restricted to the Late Cretaceous, but further investigation is much needed. Finally, the history of research with such fossils highlights the importance of public funding for research and decentralization of university education for the advancement of science.
... The fossil record of Argentine non-mammaliaform cynodonts is relatively rich and includes several forms that are important to understand the early evolution of mammalian features (Bonaparte and Migale 2010;Martinelli et al. 2016;Rougier et al. 2021). One of the most well-known fossiliferous beds that has yielded a large number of cynodonts is the uppermost Middle to early Late Triassic Chañares Formation at the Chañares site, located in Talampaya National Park, La Rioja province, Argentina (Bonaparte 1966(Bonaparte , 1969Romer 1972aRomer , 1972b. ...
The aim of the present contribution is to examine the bone histology of two non-mammaliaform cynodonts recovered from the Late Triassic (Carnian) Chañares Formation, at Los Chañares locality, La Rioja province, Northwestern Argentina. The taxa analysed here are the Traversodontidae Massetognathus pascuali and the Probainognathidae Probainognathus jenseni, both forms known from multiple skeletons. Our osteohistological results reveal an uninterrupted growth pattern and high intraspecific histovariability in Massetognathus. On the other hand, Probainognathus presents cyclical growth, a cortex dominated by parallel-fibred bone tissue and scarce vascularisation. In spite of being derived non-mammaliaform eucynodonts, Massetognathus and Probainognathus show high growth plasticity, a feature usually regarded as plesiomorphic for cynodonts. Both taxa show a thick cortex in the long bones, a feature that could be linked to counteract the strong flexion forces during digging.
... In Magallanodon, the anterior margin of the fossa is not delimited by a ridge, but by a smooth surface. In Sudamerica, the anterior margin of this fossa is not marked (see Rougier et al., 2021 : fig 8.3d), and Pascual et al. (1999) not recognized the anterior edge of the masseteric fossa. An anteriorly extended masseteric fossa is shared between haramiyidans and multituberculates (Luo et al., 2015) in which the fossa barely reaches the level of p4 o earlier (e.g., Arborohamiya, Shenshou, Qishou, Xianshou; Bi et al., 2014;Mao and Meng, 2019). ...
The fossil record of gondwanatherian mammaliaforms from Patagonia is represented by several species known on the basis of isolated teeth, with the single exception of a partial dentary with two molariforms of the Paleocene species Sudamerica ameghinoi. The aim of the present contribution is to describe both a fragmentary dentary (with the base of the lower incisive) and a partial upper incisor coming from the Campanian–Maastrichtian Chorrillo Formation, at the La Anita Farm, SW Santa Cruz province, Argentina. The specimens are referred to Magallanodon baikashkenke, a species previously known by isolated teeth from the Dorotea (Chile) and Chorrillo (Argentina) beds. The present discovery expands our knowledge of this mammaliaform clade.
This paper analyzes the paleoneurology (cranial endocast and maxillary canal) of Massetognathus pascuali, an iconic non-mammaliaform cynodont from the early Late Triassic of South America, using Neutron Tomography. The application of neutron tomography holds the potential for uncovering more refined anatomical and quantitative data. The newly examined cranial endocast shows a forebrain with a tubular shape without an interhemispheric fissure, presence of a pineal body (with a closed parietal foramen), and a marked unossified zone. In comparison with a smaller, putatively juvenile specimen previously studied (PVL 4016), the new endocast exhibits a similar degree of encephalization, indicating little change in relative brain size between both ontogenetic stages. In the context of cynognathian brain evolution, M. pascuali maintained a low encephalization quotient, typical of early cynognathians, contrasting with the higher values of some Late Triassic taxa. The maxillary canal of M. pascuali is described here for the first time. It is considerably ramified, although slightly less than in some early cynognathians, following the general pattern of non-probainognathians and suggesting the absence of a flexible rhinarium or mobile vibrissae. By integrating endocast data with the maxillary canal, this study offers enhanced insights into the neurosensory ecology of M. pascuali, thereby deepening our understanding of its biology and ecological interactions.
Graphical Abstract
Dryolestoid mammals are classical members of the Jurassic faunas of Laurasia but mostly absent during the Cretaceous. The reverse is true in Gondwana in general and South America in particular, where meridiolestid dryolestoids are dominant in the Late Cretaceous. We describe here 21 new mammalian specimens from the Upper Cretaceous locality Cerro Tortuga (Allen Formation, Patagonia, Argentina) collected via screenwashing, which we identify as meridiolestid dryolestoids. We recognize a new species of meridiolestid and reassign a previously described specimen to the new taxon. The morphology of these new remains represents a new morphotype in the spectrum of meridiolestid diversity, recording a broadening of trophic adaptations from the ancestral insectivory to the more derived herbivory observed among the later and more derived members of the group. The novel dental morphology helps bridge the anatomy of the plesiomorphic sharp-toothed meridiolestidans with that of the more derived and bunodont mesungulatoids. The new taxon suggests that development of both broad cingulids and complex crown morphology precede the development of the wide compressed roots, bunodonty, and thickened enamel characteristic of derived mesungulatids. Other specimens from the collection are referable to taxa previously known from the same locality. These provide new information about tooth positioning, dental formula, and overall dental morphology. The new material suggests that Groebertherium, previously regarded as a dryolestid taxon, is in fact a likely member of Meridiolestida.
Body mass is a pivotal quantity in palaeobiology but must be inferred from an imperfect fossil record. We analyse the performance of regression models derived from various dentoskeletal predictors in mammals to inform fossils from the early, Mesozoic history of this radiation. Our focus is on the critical small end of the size spectrum; critical because the earliest mammals were small, because small size persisted onto the stems of the major extant radiations, and because small mammals compose a large proportion of crown diversity. The sampling strategy is diverse in terms of both phylogeny and skeletal predictors: the former allows a general application, while the latter enables comparison of various models. Linear regressions based on extant small mammals indicate a universal correlation of body mass with observed measurements, but with clear differences in precision. Postcranial predictors outperform jaw and dental metrics, with certain femoral joint dimensions providing surprisingly precise predictions. Our results indicate complex patterns of size evolution within the small‐bodied category, including the possibility that multiple Mesozoic species approached the theoretical lower limit of mammalian body size. The ability to study such dynamics only becomes possible when predicting body mass within a strict, highly focused phylogenetic context. The heuristic value of the models we provide here is not limited to the Mesozoic but is applicable to small‐bodied mammals of any geologic age.
The mammalian crown originated during the Mesozoic and subsequently radiated into the substantial array of forms now extant. However, for about 100 million years before the crown's origin, a diverse array of stem mammalian lineages dominated terrestrial ecosystems. Several of these stem lineages overlapped temporally and geographically with the crown mammals during the Mesozoic, but by the end of the Cretaceous crown mammals make up the overwhelming majority of the fossil record. The progress of this transition between ecosystems dominated by stem mammals and those dominated by crown mammals is not entirely clear, in part due to a distinct separation of analyses and datasets. Analyses of macroevolutionary patterns tend to focus on either the Mammaliaformes or the non-mammalian cynodonts, with little overlap in the datasets, preventing direct comparison of the diversification trends. Here I analyse species richness and biogeography of Synapsida as a whole during the Mesozoic, allowing comparison of the patterns in the mammalian crown and stem within a single framework. The analysis reveals the decline of the stem mammals occurred in two discrete phases. The first phase occurred between the Triassic and Middle Jurassic, during which the stem mammals were more restricted in their geographic range than the crown mammals, although within localities their species richness remained at levels seen previously. The second phase was a decline in species richness, which occurred during the Lower Cretaceous. The results show the decline of stem mammals, including tritylodontids and several mammaliaform groups, was not tied to a specific event, nor a gradual decline, but was instead a multiphase transition.
Theria represent an extant clade that comprises placental and marsupial mammals. Here we report on the discovery of a new Late Cretaceous mammal from southern Patagonia, Patagomaia chainko gen. et sp. nov., represented by hindlimb and pelvic elements with unambiguous therian features. We estimate Patagomaia chainko attained a body mass of 14 kg, which is considerably greater than the 5 kg maximum body mass of coeval Laurasian therians. This new discovery demonstrates that Gondwanan therian mammals acquired large body size by the Late Cretaceous, preceding their Laurasian relatives, which remained small-bodied until the beginning of the Cenozoic. Patagomaia supports the view that the Southern Hemisphere was a cradle for the evolution of modern mammalian clades, alongside non-therian extinct groups such as meridiolestidans, gondwanatherians and monotremes.
The Chañares Formation (Ischigualasto‐Villa Unión Basin) is worldwide known by its exquisitely preserved fossil record of latest Middle‐to‐early Late Triassic tetrapods, including erpetosuchids, “rauisuchians,” proterochampsids, gracilisuchids, dinosauromorphs, pterosauromorphs, kannemeyeriiform dicynodonts, and traversodontid, chiniquodontid and probainognathid cynodonts, coming from the Tarjadia (bottom) and Massetognathus ‐ Chanaresuchus (top) Assemblage Zones of its lower member. Regarding cynodonts, most of its profuse knowledge comes from the traditional layers discovered by Alfred Romer and his team in the 1960s that are now enclosed in the Massetognathus ‐ Chanaresuchus Assemblage Zone (AZ). In this contribution we focus our study on the probainognathian cynodonts discovered in levels of the Tarjadia Assemblage Zone. We describe a new chiniquodontid cynodont with transversely broad postcanine teeth ( Riojanodon nenoi gen. et sp. nov.) which is related to the genus Aleodon . In addition, the specimen CRILAR‐Pv 567 previously referred to cf. Aleodon is here described, compared, and included in a phylogenetic analysis. It is considered as an indeterminate Aleodontinae nov., a clade here proposed to included chiniquodontids with transversely broad upper and lower postcanines, by having a cuspidated sectorial labial margin and a lingual platform that is twice broader than a lingual cingulum. Cromptodon mamiferoides , from the Cerro de Las Cabras Formation (Cuyo Basin), was also included in the phylogenetic analysis and recovered as an Aleodontinae. The new cynodont and the record of Aleodontinae indet. reinforce the faunal differentiation between the Tarjadia and Massetognathus ‐ Chanaresuchus Assemblage Zones, in the lower member of the Chañares Formation, and inform on the diverse chiniquodontid clade with both sectorial and transversely broad postcanine teeth.
The fossil metatherian assemblage from La Venta (Middle Miocene, Colombia) is one of the most diverse in South America, and it is critical to understand the Neogene radiation of this group in this continent. La Venta contains the northernmost record of Thylacosmilidae Riggs, 1933 (Metatheria, Sparassodonta): Anachlysictis gracilis Goin, 1997, the first thylacosmilid species named for the Neotropics. This taxon was described mostly based on mandibular remains. Recent fieldwork and work in collections led to the discovery of new materials for this species, including the most complete skeleton ever found for this Sparassodonta Ameghino, 1894. Here, we present a detailed description of the cranial osteology and dentition of A. gracilis, which elucidates anatomical aspects previously inferred but hitherto unconfirmed. We investigate the phylogeny, and ecomorphological parameters of this taxon (diet and body mass) to set the evolutionary context of the species, understand its paleobiology, and evaluate palaeoecological implications. Additionally, we revise the phylogeny of the thylacosmilids, recovering the traditional classification of the group, differentiated from the proborhyaenids and borhyaenids. This work also proposes a new reconstruction of the external morphology of the head of A. gracilis based on 3D scans of the new referred materials.
Although several well-preserved crania are known for the Mesozoic Eutriconodonta, three-dimensional reconstructions of the character-rich inner ear and basicranial region based on high-resolution computed tomography scans have previously only been published for the Late Jurassic Priacodon. Here we present a description of the petrosal and inner ear morphology of a triconodontid eutriconodontan from the Lower Cretaceous Cloverly Formation, which we provisionally assign to Astroconodon. The bony labyrinth of Astroconodon is plesiomorphic for mammaliaforms in lacking a primary osseous lamina, cribriform plate, and osseous cochlear ganglion canal. However, as in Priacodon and the zhangheotheriid Origolestes, Astroconodon has a secondary osseous lamina base that extends nearly the complete length of the cochlear canal. The cochlear canal is straighter in Astroconodon and other eutriconodontans compared to several basal mammaliaform clades (e.g., morganucodontans, docodontans), that exhibit varying degrees of cochlear canal curvature. The pars cochlearis of the petrosal was well vascularized in Astroconodon, exhibiting a network of venous canals that crossed the cochlea transversely on its ventral and dorsal aspects. Of particular note are several canals that passed along the base of the secondary osseous lamina. As in Priacodon and Origolestes, those canals do not show the extensive connections to the cochlear labyrinth as seen in the basal mammaliaforms Morganucodon and Borealestes. The inner ear of Astroconodon thus highlights the complex history of the mammaliaform cochlear canal, in which different clades appear to follow independent evolutionary trajectories and various key morphological features (e.g., cochlear canal length, curvature, vascularization and osseous supports for the basilar membrane) exhibit considerable homoplasy.
A Bacia Bauru congrega um dos mais ricos conjuntos de somatofósseis de tetrá- podes do território brasileiro, sendo cerca de 70% dessa paleodiversidade procedente de seus depósitos em São Paulo. Com registros conhecidos desde o início do século XX, tais fósseis foram coletados em todas as unidades estratigrá cas da Bacia que a oram no estado, i.e., formações Santo Anastácio, Araçatuba, Adamantina (alterna- tivamente dividida em formações Vale do Rio do Peixe, Presidente Prudente e São José do Rio Preto) e Marília. Os grupos registrados incluem raros anuros, mamíferos e escamados, um importante conjunto de testudinos, dinossauros terópodes (incluindo aves) e saurópodes, além de uma das mais diversas faunas de crocodiliformes conhe- cidas para o registro fóssil global. Tal conjunto congrega mais de cinquenta entidades taxonômicas distintas, incluindo 42 espécies formalmente descritas. A partir de dados de cunho bioestratigrá co, incluindo correlação com base em tetrápodes, e poucas da- tações absolutas, a totalidade dos depósitos da Bacia Bauru em São Paulo parece estar cronologicamente restrita ao Neocretáceo, mas um maior detalhamento de tais inferên- cias se faz extremamente necessário. Por m, o histórico das pesquisas com tais fósseis evidencia a importância da interiorização do ensino universitário e do nanciamento público à pesquisa para o avanço da ciência.
A review of the Southern Hemisphere Mesozoic tribosphenic mammal fossil record supports the hypothesis that Tribosphenida arose in the Southern Hemisphere during the Early Jurassic, around 50 million years prior to the clade’s reliably dated first appearance in the Northern Hemisphere. Mesozoic Southern Hemisphere tribosphenic mammals are known from Australia, Madagascar, South America and the Indian subcontinent, and are classified into three families: Bishopidae (fam. nov.), Ausktribosphenidae and Henosferidae. These are stem therians, and considerable morphological evolution occurred within the lineage between the Jurassic and late Early Cretaceous. Important dental modifications include a graduated transition between premolars and molars, development of molar wear facets V and VI, loss of facets for postdentary bones, reduction in the Meckelian groove and development of a true dentary angle. Previous classifications of Southern Hemisphere tribosphenic mammals are ambiguous because information from the upper dentition has been lacking. Upper molars attributed to the late Early Cretaceous (Albian) Southern Hemisphere group Bishopidae fam. nov. are now known to possess a prominent protocone and stylar cusp C. We thus consider bishopids to be the sister group to Theria.
Timothy F. Flannery [tim.flannery@textpublishing.com.au], Kristofer M. Helgen [Kris.Helgen@Australian.Museum], Australian Museum, 1 William St Sydney 2000, Australia; Thomas H. Rich [trich@museum.vic.gov.au], Museums Victoria, PO Box 666, Q28 Melbourne, Victoria 3001, Australia; Patricia Vickers-Rich [pat.rich@monash.edu; prich@swin.edu.au], School of Earth, Atmosphere and Environment, Monash University, Victoria 3800, Australia; Swinburne University of Science and Technology, Department of Chemistry and Biotechnology, Hawthorn, Victoria 3122, Australia; Elizabeth Grace Veatch [elizabeth.veatch@gmail.com], National Museum of Natural History, Smithsonian Institution, Washington, DC 20013, USA.
Taeniolabis taoensis is an iconic multituberculate mammal of early Paleocene (Puercan 3) age from the Western Interior of North America. Here we report the discovery of significant new skull material (one nearly complete cranium, two partial crania, one nearly complete dentary) of T. taoensis in phosphatic concretions from the Corral Bluffs study area, Denver Formation (Danian portion), Denver Basin, Colorado. The new skull material provides the first record of the species from the Denver Basin, where the lowest in situ specimen occurs in river channel deposits ~730,000 years after the Cretaceous-Paleogene boundary, roughly coincident with the first appearance of legumes in the basin. The new material, in combination with several previously described and undescribed specimens from the Nacimiento Formation of the San Juan Basin, New Mexico, is the subject of detailed anatomical study, aided by micro-computed tomography. Our analyses reveal many previously unknown aspects of skull anatomy. Several regions (e.g., anterior portions of premaxilla, orbit, cranial roof, occiput) preserved in the Corral Bluffs specimens allow considerable revision of previous reconstructions of the external cranial morphology of T. taoensis. Similarly, anatomical details of the ascending process of the dentary are altered in light of the new material. Although details of internal cranial anatomy (e.g., nasal and endocranial cavities) are difficult to discern in the available specimens, we provide, based on UCMP 98083 and DMNH.EPV 95284, the best evidence to date for inner ear structure in a taeniolabidoid multituberculate. The cochlear canal of T. taoensis is elongate and gently curved and the vestibule is enlarged, although to a lesser degree than in Lambdopsalis.
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