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New mammals from the Upper Cretaceous Allen Formation (Patagonia, Argentina) and reassessment of meridiolestidan diversity
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.
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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.
Monotremata is a clade of egg-lying mammals, represented by the living platypus and echidnas, which is endemic to Australia, and adjacent islands. Occurrence of basal monotremes in the Early Cretaceous of Australia has led to the consensus that this clade originated on that continent, arriving later to South America. Here we report on the discovery of a Late Cretaceous monotreme from southern Argentina, demonstrating that monotremes were present in circumpolar regions by the end of the Mesozoic, and that their distinctive anatomical features were probably present in these ancient forms as well.
The Ksar Metlili Formation, from the Jurassic – Cretaceous transition (Tithonian – Berriasian) of eastern High Atlas Mountains, Morocco, has yielded one of the richest microvertebrate assemblages from the Mesozoic of Gondwana. With at least 19 species, mammaliaforms are particularly diverse. ‘Dryolestoidea’ are the most abundant and the most diverse; nevertheless, only one species, Donodon perscriptoris Sigogneau-Russell, 1991a, of the monotypic Donodontidae, had been described so far. Here, we describe four new species and three new genera of ‘dryolestoids’ from the Ksar Metlili deposits: Donodon minor sp. nov., Stylodens amerrukensis gen. et sp. nov., Anoualestes incidens gen. et sp. nov., and Amazighodon orbis gen. et sp. nov, all of which are included in the Donodontidae based on their dental morphology. We present the first phylogenetic analysis that incorporates the five donodontid species from the Ksar Metlili site into the cladotherian phylogeny. Our new analysis provides evidence for the paraphyly of the ‘Dryolestoidea’: Dryolestidae, ‘Paurodontidae’, Donodontidae, and Zatheria form a clade to the exclusion of the South American Meridiolestida. Donodontids are found to be closer to prototribosphenidans and zatherians than to any other ‘dryolestoid’ family and are more derived than meridiolestidans. This topology refutes a previous hypothesis that meridiolestidans are phylogenetically rooted among donodontids. Our phylogenetic analysis also supports Donodontidae as an endemic African monophyletic group. The close relationships of the donodontids and zatherians suggest that zatherians had a geographical origin possibly from Africa or Gondwana instead of Laurasia.
The terrestrial mammal diversity recovered from levels of Banco Negro Inferior (Lower Black Bank) at Punta Peligro and surrounding areas, in southeastern Chubut Province (Argentina) is detailed. Most of the metatherian taxa here described are new for this mammalian assemblage, which in turn is the base of the late Danian (early Paleocene, Peligran Land-mammal-age). Differences between this association and that of Tiupampa, in Bolivia (also of Paleocene age) are remarkable. The former represent a mixture of Gondwanan and Holarctic lineages: monotremes, gondwanatherians, dryolestoids,
metatherians, and eutherians, while those of Tiupampa are strictly therian in nature and with Laurasic affinities. More important than the differences in age between these faunas, we infer a decisive biogeographic influence in the composition of each one of them: the Punta Peligro fauna would be part of an Andean Region (Austral Kingdom) radiation, while that of Tiupampa seems to be more typically Neotropical (Holotropical Kingdom). Metatherians and eutherians from Punta Peligro are more derived than those of Tiupampa, and anticipate several lineages recorded in Eocene levels at other South American localities. The updated list of mammals from the Banco Negro Inferior includes
the following taxa: Prototheria, Monotremata, Ornithorhynchidae: Monotrematum sudamericanum; Allotheria, Gondwanatheria, Sudamericidae: Sudamerica ameghinoi; Cladotheria, Dryolestoidea Peligrotheriidae: Peligrotherium tropicalis; Metatheria, "Ameridelphia", Derorhynchidae: Derorhynchus aff. D. minutus; family indet.: Didelphopsis primaeva sp. nov.; Polydolopimorphia, Bonapartheriidae: Salamancatherium lacerum gen. et sp. nov.; family indet. Alfredocarlinia cratera gen. et sp. nov.; and eventually ?Sparassodonta, family, genus and species indet.Metatheria, order, family, genus and species indet.; Eutheria, Panperissodactyla, Litopterna, Notonychopidae: Requisia vidmari;
Didolodontidae: Raulvaccia peligrensis, Escribania chubutensis, E. talonicuspis.
Keywords. Mammalia, Danian, early Paleocene, Patagonia, Punta Peligro
We describe two skull fragments of a new morganucodontan from the Cisco Mammal Quarry (Upper Jurassic Morrison
Formation), preserving portions of the palate and snout in excellent 3D detail as well as the complete upper postcanine
dentition. Morganucodontans are best known by isolated elements and relatively complete skulls of several species of
Morganucodon from the Lower Jurassic of Wales and China; this group is fundamental to our understanding of the
early evolution of mammals. Cifellilestes ciscoensis gen. et sp. nov. possesses derived features of the snout paired with
plesiomorphic construction of the molars; the distal premolars are complex and there is an unusually low count (two) of
strongly imbricated molars. This character combination expands craniodental variation for the group. We sampled mud�stone from the Cisco Mammal Quarry for ash-fall zircon analysis and obtained a date of 151.50 ± 0.28 Ma. This dates the
locality to the earliest Tithonian and slightly younger than other major dated mammal-bearing localities in the Morrison
Formation. Cifellilestes represents one of the youngest members of this group and extends the record of morganucodon�tans in North America by more than 30 Ma. Morganucodontans are a rare component of Late Jurassic faunas but display
surprising dental diversity through variations in a tooth count and cusp morphology of a deeply conserved, generalized
mammalian tooth pattern, which was fully established in brasilodontid (non-mammalian) ancestors at least 80 my prior.
The large, bunodont, mammal Peligrotherium tropicalis is an enigmatic member of the earliest Paleocene fauna of Punta
Peligro, Argentina. While being a contemporary of many of the earliest large-bodied “archaic ungulates” in the Northern
Hemisphere, P. tropicalis is a remnant of an endemic Mesozoic non-therian lineage. The interpretation of P. tropicalis as
an omnivore/herbivore has therefore been difficult to evaluate, given its phylogenetic placement outside of the therian
clade, and lack of many of the molar characteristics thought to be essential for the forms of mastication seen in mar�supials and placentals. Here we present a three-dimensional generalization of the classical “bifulcral” biomechanical
model of bite force and joint force estimation, which is capable of accommodating the wide range of mediolateral force
orientations generated by the muscles of mastication, as estimated by the geometry of their rigid attachment surfaces.
Using this analysis, we demonstrate that P. tropicalis is more herbivorously adapted (viz. shows a greater Group 2 relative
to Group 1 jaw adductor advantage for producing postcanine orthal bite forces) than even the hypocarnivorous carniv�orans Procyon lotor and Ursus arctos, and is similar to the ungulates Sus scrofa and Diceros bicornis. This similarity
also extends to the mediolateral distribution of relative muscle group advantage, with Group 1 muscles (responsible
for effecting the initial adduction of the working-side hemimandible into centric occlusion) having greater orthal bite
forces labially; and Group 2 muscles (those responsible for producing occlusal grinding motions) being more powerful
lingually. Finally, we show that P. tropicalis preserves relatively little of its orthal bite force magnitude at high gape,
suggesting that large-object durophagy would not have been a likely feeding strategy.
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.
Biodiversity today has the unusual property that 85% of plant and animal species live on land rather than in the sea, and half of these live in tropical rainforests. An explosive boost to terrestrial diversity occurred from c. 100–50 million years ago, the Late Cretaceous and early Palaeogene. During this interval, the Earth‐life system on land was reset, and the biosphere expanded to a new level of productivity, enhancing the capacity and species diversity of terrestrial environments. This boost in terrestrial biodiversity coincided with innovations in flowering plant biology and evolutionary ecology, including their flowers and efficiencies in reproduction; coevolution with animals, especially pollinators and herbivores; photosynthetic capacities; adaptability; and ability to modify habitats. The rise of angiosperms triggered a macroecological revolution on land and drove modern biodiversity in a secular, prolonged shift to new, high levels, a series of processes we name here the Angiosperm Terrestrial Revolution.
The Cretaceous-Paleogene (K-Pg) mass extinction 66 million years ago was characterized by a worldwide ecological catastrophe and rapid species turnover. Large-scale devastation of forested environments resulting from the Chicxulub asteroid impact likely influenced the evolutionary trajectories of multiple clades in terrestrial environments, and it has been hypothesized to have biased survivorship in favour of nonarboreal lineages across the K-Pg boundary. Here, we evaluate patterns of sub-strate preferences across the K-Pg boundary among crown group mammals, a group that underwent rapid diversification following the mass extinction. Using Bayesian, likelihood, and parsimony reconstructions, we identify patterns of mammalian ecological selectivity that are broadly similar to those previously hypothesized for birds. Models based on extant taxa indicate predominant K-Pg survivorship among semi-or nonarboreal taxa, followed by numerous independent transitions to arboreality in the early Cenozoic. However, contrary to the predominant signal, some or all members of total-clade Euarchonta (Primates + Dermoptera + Scandentia) appear to have maintained arboreal habits across the K-Pg boundary, suggesting ecological flexibility during an interval of global habitat instability. We further observe a pronounced shift in character state transitions away from plesiomorphic arboreality associated with the K–Pg
transition. Our findings are consistent with the hypothesis that predominantly
nonarboreal taxa preferentially survived the end-Cretaceous mass extinction, and emphasize the pivotal influence of the K-Pg transition in shaping the early evolutionary trajectories of extant terrestrial vertebrates.
The La Colonia Formation is an Upper Cretaceous to Paleogene shallow marine sedimentary unit in Chubut Province (Patagonia), Argentina that preserves important vertebrate and plant fossils. Despite good exposures and significant paleontological investigation, the precise age of the La Colonia Formation and its stratigraphic relationship to the Cretaceous–Paleogene (K–Pg) boundary are not well understood. The K-Pg boundary is associated with one of the largest mass extinctions in Earth’s history yet there are few continental stratigraphic records of the boundary in South America, resulting in poor understanding of its effects there. This study combines magnetostratigraphy, detrital zircon uranium–lead (U–Pb) geochronology, and preliminary palynological results to constrain the age of the La Colonia Formation and its fossils more precisely and determine whether it preserves the K–Pg boundary. U–Pb ages of detrital zircons from the directly underlying Puntudo Chico Formation indicate that the base of the La Colonia Formation is younger than ∼71.7 Ma. The studied palynological assemblage includes Quadraplanus brossus, which is restricted to the Maastrichtian in other southern hemisphere records. Paleomagnetic results show that the ∼125 meter Cerro Buitre Norte section of the La Colonia Formation and the overlying Cerro Bororó Formation has dominantly normal polarity except for a ∼5 meter zone of reversed polarity near the base that is interpreted to correlate to Chron C30R of the Geomagnetic Polarity Timescale (GPTS), and a ∼13 meter zone of reversed polarity near the top of the La Colonia Formation that is interpreted to correlate to Chron C29R. The presence of a magnetozone correlated to C29R in the La Colonia Formation suggests that it preserves the K–Pg boundary, making it an important target for continued paleontological collection to better understand the K–Pg extinction and recovery dynamics in South America.
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.
Flowering plants (angiosperms) are the most diverse of all land plants, becoming abundant in the Cretaceous and achieving dominance in the Cenozoic. However, the exact timing of their origin remains a controversial topic, with molecular clocks generally placing their origin much further back in time than the oldest unequivocal fossils. To resolve this discrepancy, we developed a Bayesian method to estimate the ages of angiosperm families on the basis of the fossil record (a newly compiled dataset of ~15,000 occurrences in 198 families) and their living diversity. Our results indicate that several families originated in the Jurassic, strongly rejecting a Cretaceous origin for the group. We report a marked increase in lineage accumulation from 125 to 72 million years ago, supporting Darwin’s hypothesis of a rapid Cretaceous angiosperm diversification. Our results demonstrate that a pre-Cretaceous origin of angiosperms is supported not only by molecular clock approaches but also by analyses of the fossil record that explicitly correct for incomplete sampling.
Gondwanatheria is an enigmatic mammaliaform clade distributed in the Cretaceous and Paleogene of South America, Africa, Madagascar, India and Antarctica. The Mesozoic record in South America is restricted to the Latest Cretaceous of Río Negro and Chubut provinces, Argentina and Magallanes Region of southern Chile. The aim of the present contribution is to describe a new specimen of gondwanatherian mammaliaforms from beds belonging to the Maastrichtian Chorrillo Formation, cropping out 30 km SW of El Calafate, Santa Cruz Province, Argentina. It is represented by a single molariform referable to the species Magallanodon baikashkenke with which it shares a unique combination of characters. Analysis of the unique combination of characters exhibited by Magallanodon shed doubts on the monophyly of Ferugliotheriidae and suggest that South American taxa may be closely related to each other. The wide geographical distribution and occurrence of gondwanatherians on geological units of diverse origins suggest that they were capable of facing disparate environmental conditions.
The Late Cretaceous record offshore Argentina comprises an extensive hybrid/mixed (turbidite-contourite) depositional system over 280,000 km2. This offers a key site to investigate complex assemblages of morphosedimentary features and their depositional processes during one of the major tectonic events in the southern hemisphere: the breakup of Gondwana (125 Ma) and the opening of the South Atlantic Ocean. The hybrid depositional system was studied using a new 2D multichannel seismic reflection dataset and well data. This system developed along the continental slope and rise between 3500 and 6500 m SSL with nineteen 300–500 m thick, mounded drifts, separated by sixteen 2–5 km wide submarine channels. Seismic interpretation and correlations suggest four main evolutionary stages: a) the pre-drift stage (∼125–89.8 Ma) from the Aptian to the Coniacian marks thermal subsidence of the margin followed by initiation of a turbidite depositional system; b) the onset stage (∼89.8–81 Ma) from the Coniacian to the Campanian records the first exchange between SE turbidity flows and weak SW bottom currents; c) the growth stage (∼81–66 Ma) records maximum growth from the Campanian to the Maastrichtian, characterized by the SW progradation and expansion of the hybrid system due to more frequent interactions between turbidity and alongslope bottom currents; and d) the burial stage (∼66 Ma) marks the cessation of the hybrid system in the Paleocene due to bottom current intensification and transition to a pure contourite system, which persists until the present day. These four evolutionary phases register the Mesozoic to Cenozoic paleoceanographic fluctuations, associated with the northward opening of the South Atlantic Ocean and the establishment of a new deep-water circulation regime. Control factors for this hybrid depositional system link inherited morphological structures, regional tectonic events, a mutable oceanic circulation, and recurrent gravity-driven processes. The present results were also compared with other mixed systems of similar or different geological age to contextualize the paleoceanographic and conceptual implications for deep-marine sedimentary environments. This comparison allowed us to identify two end members of drift and channel migration (upstream versus downstream), differentiated by the dominance of alongslope bottom currents vs. downslope turbidity currents. The distribution of the morphological elements and their lateral migration reflects the imprint of the most influential process and the changing energy, velocity, frequency and timing between the two processes. A future challenge will be to determine the full spectrum of hybrid systems, from turbidite-dominated to contourite-dominated settings, and how they differ vertically and spatially in the sedimentary record.
We describe Magallanodon baikashkenke gen. et. sp. nov., a new gondwanatherian mammal from the Late Cretaceous of the Magallanes Region in southern Chile (Río de Las Chinas Valley, Estancia Cerro Guido, north of Puerto Natales city, Última Esperanza Province). The mammal-bearing layer is placed within the Late Campanian-Early Maastrichtian levels of the Dorotea Formation (Magallanes/Austral Basin). The new remains constitute the southernmost record of a Mesozoic gondwanatherian mammal, as well as the frst Mesozoic mammal from Chile. This taxon is comparable in size to the hypsodont-toothed Gondwanatherium (Late Cretaceous) and Sudamerica (Early Paleocene) but with noticeably brachyodont molariforms supported by four to fve roots. As in other gondwanatherians, it has at least one hypertrophied, rodent-like incisor in the upper jaw. The new taxon is here diagnosed and described, and is regarded as a possible ferugliotheriid (?Ferugliotheriidae). If confrmed, it would represent the largest known taxon for this family. Its molariform occlusal crown pattern, after wear, resembles that of other gondwanatherians, particularly ferugliotheriids and that of the sudamericid Gondwanatherium. This adds new evidence on the phylogenetic proximity of ferugliotheriid and sudamericid gondwanatherians. An analysis of the enamel microstructure of the upper incisor of Magallanodon was performed demonstrating several crucial similarities with the pattern shown by Gondwanatherium (Sudamericidae). We discuss the signifcance of Magallanodon for understanding the acquisition, within gondwanatherians, of a lophed molariform pattern. Finally, we discuss the signifcance of the new fnding in the context of southern biotas, including those of Patagonia and Antarctica.
Terrestrial record of recovery
The extinction that occurred at the end of the Cretaceous period is best known as the end of the nonavian dinosaurs. In theory, this paved the way for the expansion of mammals as well as other taxa, including plants. However, there are very few direct records of loss and recovery of biotic diversity across this event. Lyson et al. describe a new record from the Cretaceous-Paleogene in Colorado that includes unusually complete vertebrate and plant fossils that describe this event in detail, including the recovery and expansion of mammalian body size and increasing plant and animal biotic diversity within the first million years.
Science , this issue p. 977
In the last three decades, records of tribosphenidan mammalsfrom India, continental Africa, Madagascar and South Americahave challenged the notion of a strictly Laurasian distributionof the group during the Cretaceous. Here, we describe a lowerpremolar from the Late Cretaceous Adamantina Formation, SãoPaulo State, Brazil. It differs from all known fossil mammals,except for a putative eutherian from the same geologic unityandDeccanolestes hislopi, from the Maastrichtian of India. Theincompleteness of the material precludes narrowing downits taxonomic attribution further than Tribosphenida, butit is larger than most coeval mammals and shows a thinlayer of parallel crystallite enamel. The new taxon helpsfilling two major gaps in the fossil record: the paucity ofMesozoic mammals in more northern parts of South Americaand of tribosphenidans in the Cretaceous of that continent.In addition, high-precision U-Pb geochronology provided apost-Turonian maximal age (≤87.8 Ma) for the type stratum,which is overlain by the dinosaur-bearing Marília Formation, constraining the age of the Adamantina Formation at the site to late Coniacian–late Maastrichtian. This represents the first radioisotopic age for the Bauru Group, a key stratigraphic unit for the study of Cretaceous tetrapods in Gondwana.
The early evolution of mammals is associated with the linked evolutionary origin of diphyodont tooth replacement, rapid juvenile growth and determinate adult growth. However, specific relationships among these characters during non-mammalian cynodont evolution require further exploration. Here, polarized light microscopy revealed incremental lines, resembling daily laminations of extant mammals, in histological sections of enamel in eight non-mammalian cynodont species. In the more basal non-probainognathian group, enamel extends extremely rapidly from cusp to cervix. By contrast, the enamel of mammaliamorphs is gradually accreted, with slow rates of crown extension, more typical of the majority of non-hypsodont crown mammals. These results are consistent with the reduction in dental replacement rate across the non-mammalian cynodont lineage, with greater rates of crown extension required in most non-probainognathians, and slower crown extension rates permitted in mammaliamorphs, which have reduced patterns of dental replacement in comparison with many non-probainognathians. The evolution of mammal-like growth patterns, with faster juvenile growth and more abruptly terminating adult growth, is linked with this reduction in dental replacement rates and may provide an additional explanation for the observed pattern in enamel growth rates. It is possible that the reduction in enamel extension rates in mammaliamorphs reflects an underlying reduction in skeletal growth rates at the time of postcanine formation, due to a more abruptly terminating pattern of adult growth in these more mammal-like, crownward species.
New dental and dentary fossils collected in the Upper Cretaceous La Colonia Formation in central Patagonia provide new evidence on the morphology, feeding ecology, and relationships of the enigmatic mammal Reigitherium. The newly discovered specimens described here include elements of the upper dentition and several partial dentaries, elucidating fundamental questions of serial homology and postcanine dental formula (four premolars and three molars). This new evidence supports a nested position of Reigitherium within the advanced meridiolestidan clade Mesungulatoidea. Apomorphic features of the upper and lower molariform elements include intense enamel crenulation circumscribed within the primary trigon and trigonid, elevated cingulids, and the neomorphic appearance of cusps/cuspulids, all of which increase overall crown complexity. A Dental Topography Analysis comparing Reigitherium and its sister taxon Peligrotherium to Cretaceous and Cenozoic therians demonstrates functional similarity between the mesungulatoids and South American marsupial taxa that succeed them in the small-to medium-sized herbivore niche during the Paleocene. Previous taxonomic attributions of Reigitherium are discussed and comparisons with other meridiolestidans highlight the remarkable radiation of this group in the Cretaceous of South America.
Among the collections made during the 1896–1899 Princeton Expeditions to Patagonia are three specimens with partial crania and postcrania of Necrolestes patagonensis Ameghino, 1891, from Santa Cruz Province, Argentina. Necrolestes has anatomical specializations found in extant subterranean mammals, and other features identifying Necrolestes as a late Miocene member of the South American Meridiolestida, a non-therian clade that otherwise is limited to the Cretaceous and Paleocene. Anatomical aspects of the three Princeton specimens have been reported previously but they are more fully described here after additional preparation. The most novel discoveries concern specializations of the snout that help identify Necrolestes as a head-lift digger, as are the extant African golden moles and Australian marsupial moles. As in other meridiolestidans and most non-therian mammals, Necrolestes has a septomaxilla at the external nasal aperture. However, the septomaxilla of Necrolestes has structures that buttress an internarial bar and what we interpret as ossified external nasal cartilages, a remarkable adaptation for digging not known in any extant subterranean therians. The upturned snout of Necrolestes produces a shovel effect and also is known in two lineages of subterranean placentals, namely Oligocene palaeanodonts and Miocene proscalopid soricomorphs. The braincase, ear region, and basicranial vasculature are built on a pattern like that in other non-therian mammals, but additional spaces in the squamosal and exoccipital bones increase the volume of the middle ear, which in extant therians is associated with enhancement of low-frequency hearing, another subterranean adaptation. Our findings reinforce the meridiolestidan affinities for Necrolestes and reconstruction of its subterranean habit.
We report the discovery of Anebodon luoi, a new genus and species of zhangheotheriid symmetrodont mammal from the Lujiatun site of the Lower Cretaceous Yixian Formation, China. The fossil is represented by an associated partial skull and dentaries with a nearly complete dentition, and with a dental formula of I4/3 C1/1 P5/4 M3/4. This new taxon lacks the high molar count typical of derived symmetrodonts, differing from the well-represented zhangheotheriids Zhangheotherium and Maotherium in having a postcanine dental formula that resembles more primitive tinodontid symmetrodonts on the one hand, and sister taxa to therians such as Peramus on the other. Upper and lower distal premolars are strongly molariform and are captured undergoing replacement, clarifying positional homology among related taxa. We also describe the rostrum and, for the first time in a symmetrodont, much of the orbital mosaic. Importantly, our new taxon occupies a basal position within the Zhangheotheriidae and permits discussion of trechnotherian character evolution, ultimately shedding additional light on the evolution of therians.
The Neuquén Basin, located at the foothills of the Central Andes of Argentina, with superb and extended exposures is a natural laboratory. It is important not only for its continuous sedimentary record and superb palaeontological biota but also for its significant economic hydrocarbon resources. This synthesis is intended to provide an update only on the Cretaceous timespan in the basin development. It is characterized by a succession of sedimentary rocks that were deposited in different environments over time, interspersed with volcanic rocks associated with an active volcanic arc. High-precision geochronology dating on the tuffs establishes robust chronological ages to constrain the different stages. As this brief review only shows a fragment of the information presently available, an effort has been made to compile a comprehensive and up-to-date bibliography to compensate for the brevity of the text.
Abstract
There are few studies focused on spore and pollen clumps in paleopalynological samples, and these are only reports from the Northern Hemisphere. These aggregates may be of animal or foral origin. The goal of this contribution is to provide the frst detailed study of spore and pollen clumps from the Southern Hemisphere, and to discuss their possible origin, botanical afnities, and pollination modes, based on their morphological characteristics, preservation and comparison with putative liv-
ing representatives. Three fern spore clumps and 18 angiosperm pollen clumps were recognized in Maastrichtian-Danian La Colonia Formation sediments that outcrop at Chubut Province, Patagonia, Argentina. Most clumps are monospecifc and com-
posed of undamaged elements but some of them have two types of spore/pollen with corroded and/or fragmented exines. These fndings represent the most diverse and abundant record of fern and angiosperm clumps from the Late Cretaceous and early
Paleocene so far known from South America and the Southern Hemisphere. These results are indicative of the need for comprehensive large-scale studies on pollination of modern taxa and careful processing of palynological samples to lessen the
already large bias in paleopalynological interpretations. The paucity of information on clumps in the fossil record has impaired our comprehension of dispersion/pollination in deep time.
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.
Frogs (Anura) are nowadays common and abundant constituents of terrestrial and freshwater ecosystems near globally and their fossil record shows that they were already important during Cretaceous times. However, their fossils are often very incomplete, challenging their identification, which, coupled to historical reasons, has led to their marginalization in studies of Cretaceous tetrapod assemblages. We here report on the identities of frogs from three upper Campanian–lower Maastrichtian assemblages from Chilean (Dorotea Formation) and Argentinean (Allen and Los Alamitos formations) Patagonia, with focus on humeral morphology. Records from the Dorotea Formation represent the first described Mesozoic frogs from Chile and include the southernmost record of pipids worldwide. In the three assemblages we have identified humeri of the pipid Kuruleufenia and of calyptocephalellid frogs, proving humeral morphology valuable in diagnosing and identifying Cretaceous frogs from Patagonia. These frogs are diagnostic components of the South American Allenian tetrapod assemblage that was widespread across Patagonia near the end of the Cretaceous.
The deposits of the Chorrillo Formation (Maastrichtian) were accumulated during a ‘continental window’ that occurred during the Late Cretaceous in the Austral-Magallanes foreland basin, southern Patagonia, Argentina. The aim of the present contribution is to describe the depositional conditions as well as new vertebrate and plant fossils from this unit. The analysis of these deposits resulted in the definition of five architectural elements: Complex sandy narrow sheets channels (SS), Complex gravelly narrow sheets channels (GS), Sandstone lobes (SL), Thick fine-grained deposits (GF) and Thin dark fine-grained deposits (DF). These were separated into channelized and non-channelized units and represent the accumulation in a fine-grained dominated, fossil rich fluvial depositional system. Vertebrates fossil records include two species of frogs of the genus Calypteocephalella (representing the southernmost record of Pipoidea), snakes belonging to Madtsoiidae and Anilioidea (the latter ones being the first records for the basin), chelid turtles similar to Yaminuechelys-Hydromedusa, meiolaniiform turtles, titanosaur sauropods, megaraptoran theropods, new remains of the elasmarian Isasicursor santacrucensis (including the first cranial remains available for this species), hadrosaur ornithischians, enantiornithine birds. Sharks and elasmosaurs are also recorded and may possibly derive from the overlying marine Calafate Formation. These new taxa, together with previous findings from the Chorrillo Formation, are included into a stratigraphic column, thus providing valuable information that sheds new light on faunistic composition and paleobiogeography of high-latitude biotas of Gondwana.
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.
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.
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.
Abelisaurid theropods are well known in Cretaceous beds along South America, particularly Patagonia. However, the record of latest Cretaceous abelisauroids is still far from satisfactory. Until recently, few taxa were described from Maastrichtian beds: Carnotaurus sastrei and Quilmesaurus curriei, but also perhaps Abelisaurus comahuensis. In this contribution, we describe a new genus and species (Niebla antiqua gen. et sp. nov.), of medium-sized abelisaurid coming from Late Cretaceous (Maastrichtian) beds from Río Negro province, northern Patagonia, Argentina. The paleohistological analysis shows that this individual has reached the somatical maturity and that it represents an adult of a mid-sized abelisaurid. The specimen is represented by a nearly complete braincase, fragmentary jaw and teeth, relatively complete scapulocoracoid, dorsal ribs and incomplete vertebrae. The new taxon is relatively small, much smaller than other coeval abelisaurids such as Carnotaurus and Abelisaurus. The braincase shows autapomorphic features such a dorsoventrally tall basal tuber and postemporal foramen enclosed by parietal and exoccipitals. The scapulocoracoid is notably similar to that of Carnotaurus in having a posterodorsally oriented glenoid, a dorsoventrally expanded and wide coraco-scapular plate, and a very narrow and straight scapular blade. These features are very different from those of other abelisaurids, which may indicate a unique conformation of the pectoral girdle among these South American theropods.
The ichnogenus Brasilichnium Leonardi, 1981 is an early mammal track, presently recognized in several continents, but originally found in the aeolian deposits of Botucatu Formation in the Paraná Basin, Brazil. The sandstones of the Botucatu Formation are generally fine-grained and well-sorted, containing no pebbles; its color can be white, yellowish or reddish, but more commonly it is pinkish. Nearly always, it is silicified and therefore compact and hard, making it a very suitable building material. These sedimentary rocks are interpreted as a Lower Cretaceous desert regarded as the largest paleodesert in the Earth’s history. Although the aeolian environment is generally considered inadequate for track preservation, taphonomic events linked to moist dunes and microbial mats foster a good geological context for an early diagenization of the sediments. Brasilichnium is associated with a large ichnofauna of other mammal, theropod and ornithopod tracks, in addition to invertebrate traces. It is today a worldwide ichnogenus that allows an overview of the arid terrestrial ecosystems during Mesozoic. The presence of tritylodontids in the world after the end of the Triassic and up to the Late Cretaceous, and the possibility of attributing Brasilichnium and related ichnogenera to them, has been discussed at great length. It seems preferable, however, to suggest that these tracks can only belong to the crown-group Mammalia. Thus, these track-bearing deposits bear witness to small- and medium-size mammaliaform trackmakers; some of them would be remarkably larger than the coeval mammals represented by skeletal remains. This chronological discrepancy between ichnological and bone findings is not a surprising phenomenon: it is also known in other clades. A complete check of these tracks will have to be carried out. It would also be advisable to perform a review of all the tracks recently attributed to Brasilichnium world-wide. The Botucatu and Caiuá ichnofaunas and, globally, the ichnogenus Brasilichnium, should be stressed as an important window into the arid terrestrial ecosystems during the Gondwanic Cretaceous. This study is the result of forty-four years of research in the Botucatu Formation by the first author.
Madtsoiids constitute a successful group of extinct snakes widely distributed across Gondwana and the European archipelago during Late Cretaceous times, surviving in reduced numbers to the Pleistocene. They are renowned for including some of the largest snakes that have ever crawled on earth, yet diverse small madtsoiids are also known. Uncovering the evolutionary trends that led these snakes into disparate body sizes has been hampered mainly by the lack of phylogenetic consensus and the paucity of taxa with novel combinations of features. Here we describe a new large madtsoiid snake based on isolated vertebrae from the La Colonia Formation (Maastrichtian–Danian) of Patagonia, Argentina. A comprehensive phylogenetic analysis recovers Madtsoiidae as a basal ophidian lineage and the new snake as sister to a clade of mostly big-to-gigantic taxa, providing insights into early stages and evolutionary trends towards madtsoiid gigantism.
The Golfo San Jorge basin and other Mesozoic basins occupy extensive areas of the extra-Andean Argentinean Patagonia. These basins initiated and evolved from the Jurassic as rift basins on an igneous-metamorphic and sedimentary Paleozoic basement. Several studies have indicated that basement structures might have influenced their configuration in their initial stages.
Based on an updated synthesis of the structures and fabrics of the Paleozoic basement in the Patagonian foreland, and its comparison with fundamental structures of the Mesozoic basins, this work investigates the existence of relations among them, identifying common control types to most of these basins.
The comparison suggests that W-E to WNW-ESE structures controlled those basin sectors located near the Atlantic margin, with scarce evidence of basement control. In the Golfo San Jorge basin, basement control can be observed in the thinnest sedimentary sectors on basements, as part of the Río Chico paleo-high.
In general, it is possible to establish closer relations with the basement structures towards the continent interior, in the areas with structural reliefs that are currently striking NNW, NW, and WNW. Here, the main structures layout in some of these depocenters could be compared with the direction and vergence of the Paleozoic foliations and shear zones.
The lower Paleozoic structures of the Deseado orogen influenced those sectors further from the Atlantic margin of the Deseado basin and, possibly, the San Bernardo fold belt. Northward of the Golfo San Jorge basin, curved orogenic fabrics linked to the Gondwanic structures determine variations in the orientation of the main depocentres of the Cañadón Asfalto basin. Its influence reaches the northern boundary of Patagonia in the Neuquén basin depocenters, located in the Huincul ridge.
This book summarizes major aspects of the evolution of South American metatherians, including their epistemologic, phylogenetic, biogeographic, faunal, tectonic, paleoclimatic, and metabolic contexts. A brief overview of the evolution of each major South American lineage ("Ameridelphia", Sparassodonta, Didelphimorphia, Paucituberculata, Microbiotheria, and Polydolopimorphia) is provided. It is argued that due to physiological constraints, metatherian evolution closely followed the conditions imposed by global temperatures. In general terms, during the Paleocene and the early Eocene multiple radiations of metatherian lineages occurred, with many adaptive types exploiting insectivorous, frugivorous, and omnivorous adaptive zones. In turn, a mixture of generalized and specialized types, the latter mainly exploiting carnivorous and granivorous-folivorous adaptive zones, characterized the second half of the Cenozoic. In both periods, climate was the critical driver of their radiation and turnovers.
The dryolestoid mammal Dorsetodon haysomi gen. et sp. nov. is described from the Purbeck Limestone Group (Lower Cretaceous) of southern England, on the basis of lower molars. Dorsetodon is assigned to the Paurodontidae, a family of Theria previously known only from North America. The distinction between Paurodontidae and Henkelotheriidae (from the Upper Jurassic of Portugal), although maintained for lack of solid contrary data, is argued to have been based on variable or subjective characters. A further small mammal, Chunnelodon alopekodes gen. et sp. nov., representing an undetermined cladothenan family, is also described from lower molar teeth. The non-procumbent paraconid on the lower molar places Chunnelodon as a sister-taxon to the Laurasian Dryolestoidea.
Three new genera of dryolestoid mammals from the Late Cretaceous of Patagonia are based on isolated molars. Two of them plus Casamiquelia BONAPARTE are referred to the Casamiqueliidae n. fam. The current state of knowledge of the Los Alamitos Theria and the relationships of "Barberenia" with dryolestoids are discussed. The Los Alamitos fauna is the only assemblage of mammals recorded in the Late Cretaceous of Gondwana. There is no current evidence to consider that this fauna evolved in isolation.