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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.
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A large therian mammal
from the Late Cretaceous of South
America
Nicolás R. Chimento
1,3*, Federico L. Agnolín
1,2,3, Jordi García‑Marsà
1,3, Makoto Manabe
4,
Takanobu Tsuihiji
5 & Fernando E. Novas
1,3
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.
Mesozoic mammals are usually regarded as small-sized, insectivore, shrew-like creatures of nocturnal habits14.
is traditional view has recently changed thanks to discoveries that notably increased the ecological and mor-
phological disparity of Mesozoic mammals, including swimming, burrowing and gliding taxa510, as well as
dog-sized forms1113. Among them, the fossil record of Late Cretaceous therian mammals from the Laurasian
continents includes a wide array of small-bodied shrew-like insectivores of therian anities1,2. Because the fossil
record of modern-line mammals is notably rich in the Northern Hemisphere, it has been usually argued that
the early evolution and origin of therians occurred exclusively on Laurasian landmasses, with therian mammals
being rare—if not entirely absent—in Gondwana1418. However, some molecular analyses1926 in conjunction
with sparce paleontological data2735 are lending support to the view that some therian lineages, at least, evolved
and diversied in the Southern Hemisphere in Late Mesozoic times.
Here we report on a therian mammal from the Mesozoic of Patagonia. Present discovery is of high relevance,
because it supports the idea that southern landmasses constituted an important theatre for the early evolution
of modern mammals. It is also indicative of the persistence of wide gaps in the fossil record of the Southern
Hemisphere, up to now representing about 5% of the global fossil record of Mesozoic mammals33.
Class Mammalia Linnaeus 1758
Subclass eria Parker and Haswell, 1897
Patagomaia nov. gen.
Generic diagnosis
Patagomaia is a large mammal (~ 14kg) distinguished by the following unique combination of character states:
fused acetabulum with a complete rim lacking a dorsal emargination; femur with subspherical head having a
well-dened fovea capitis; femur head separated from the rest of the bone by a well-dened and medially tilted
neck; lesser trochanter of femur small and located on the posteromedial surface of the sha; distal end of femur
with nearly symmetrical distal condyles and reduced epicondyles. Patagomaia further diers from other Mesozoic
mammals in having the autapomorphic condition of a thick, well-dened, and obliquely oriented intercondylar
ridge delimiting a deep fossa at the distal end of the femur.
OPEN
1Laboratorio de Anatomía Comparada y Evolución de los Vertebrados (LACEV), Museo Argentino de Ciencias
Naturales “Bernardino Rivadavia” (MACN-CONICET), Av. Ángel Gallardo 470, C1405DJR Ciudad Autónoma de
Buenos Aires, Argentina. 2Fundación de Historia Natural “Félix de Azara”, Departamento de Ciencias Naturales
y Antropología, CEBBAD - Universidad Maimónides, Hidalgo 767, C1405BDB Buenos Aires, Argentina. 3Consejo
Nacional de Investigaciones Cientícas y Técnicas (CONICET), Buenos Aires, Argentina. 4National Museum of
Nature and Science, 4-1-1 Amakubo, Tsukuba 305-0005, Japan. 5Department of Earth and Planetary Science, The
University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 305-0005, Japan. *email: nicochimento@hotmail.com
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Type species
Patagomaia chainko sp. nov.
Etymology
Patago, from Patagonia; maia, mother in Greek.
Patagomaia chainko sp. nov.
Zoobank registration: urn:lsid:zoobank.org:pub:39766F43-6876-4460-9830-D80646C312CC
Holotype
MPM-PV-23365 (Museo Padre Molina, Río Gallegos, Santa Cruz, Argentina), associated postcranial remains
including the distal end of the le ulna, two fragments of the preacetabular wing of the le ilium, acetabular
region of the le hemipelvis, fragment of the ischial blade, proximal end of the right femur; distal end of the le
femur, proximal end of the le tibia, and other indeterminate bone fragments (Fig.1; Supplementary Figs.1,
3, 4, 6, 7).
Referred material
MPM-PV-23366, partial le acetabulum and ischium (see Supplementary Fig.2); incomplete right femoral sha
(see Supplementary Fig.5).
Etymology
e species name is derived from the Aonikenk language: chaink, large and ko, bone.
Diagnosis
e same as for genus by monotypy.
Type locality and age
La Anita Farm, Santa Cruz, Argentina. e remains were collected at a new site (S 50° 30 39.888, W 72° 33
18.035) geographically close to and at the same stratigraphic level as the Isasicursor 2 site (Chorrillo Forma-
tion, lower Maastrichtian, Upper Cretaceous)36,37. Both the holotype and referred specimens were found within
an area of about 20 × 30m. In close proximity, fossil wood and indeterminate hadrosaurid remains were found.
Description
e preacetabular wing of the ilium has a well-dened acetabular ridge that separates the dorsal gluteal fossa from
the ventral iliac fossa, as in other early diverging therians3840. ere is a thickened surface for the origin of the m.
rectus femoris anterior to the acetabulum, similar to some placental mammals but in contrast to other therians41,
non-therian mammals and some early diverging therians (e.g., multituberculates Akidolestes, Henkelotherium,
Vincelestes, Ukhaatherium)4143 in which there is a distinct preacetabular tubercle (see Supplementary Informa-
tion 1; Supplementary Fig.8).
e pubis, ischium, and ilium of Patagomaia are strongly fused in the acetabular region, in sharp contrast
with the unfused condition in many non-therian mammaliaforms (e.g., morganucodontids, dryolestoids, doco-
dontans, multituberculates, eutriconodonts)7,11,4247.
In Patagomaia, the acetabulum is deep and subspherical, and it is surrounded by a complete rim lacking the
dorsal emargination that characterizes non-therian mammaliaforms (e.g., docodontans, multituberculates, Vince-
lestes; see Supplementary Figs.1, 2, 11, 16)7,45. e acetabular rim shows a prominent anterior wall, corresponding
with a subspherical femoral head. e lunate surface of the acetabulum is wide, with a deep acetabular fossa
posteroventrally continuous with a narrow acetabular notch. Although the iliopubic eminence is represented
only by its base, it appears to have been robust, as in monotremes and some therians42, but dierent from many
other Mesozoic taxa (e.g., Adalatherium, Vincelestes; see Supplementary Fig.11).
e femoral head of Patagomaia is clearly oset from the rest of the bone, unlike non-therian mammals in
which it is close to the main axis of the femur (e.g., monotremes, Akidolestes, Vincelestes)42. e femoral neck
is constricted and medially oriented, forming an angle of about 55° with the main axis of femur, whereas non-
therian mammals lack a constricted femoral neck (e.g., monotremes, Akidolestes, Vincelestes)42. e trochanteric
fossa is well-dened, wide, and distally bounded by a crescent-shaped crest for the insertion of the m. quadratus
femoris39,40, in contrast to the shallow poorly dened fossa in non-therian mammals (e.g., Akidolestes, Henkeloth-
erium, Vincelestes)42,43. e greater trochanter is robust, anteroposteriorly broad, and projected posteriorly; it
reaches the same level as the femoral head proximally, resembling many therians39,48,49 but unlike non-therian
mammals (e.g., monotremes, multituberculates, Henkelotherium, Vincelestes)4245,50,51. e lesser trochanter of
Patagomaia is relatively small and posteroventrally displaced. A thick crest extends posterodistally from the base
of the greater trochanter. Such a crest is usually associated with the presence of a prominent third trochanter,
suggesting that this trochanter was also developed in Patagomaia but is broken o and missing from the holotype
(Supplementary Figs.3 and 11).
e proximal end of the femur of Patagomaia lacks diagnostic features of multituberculates, such as the pres-
ence of a post-trochanteric fossa, a prominent greater trochanter that extends proximally beyond the femoral
head and is separated from it by a deep incisure, a plate-like lesser trochanter at the conuence of the greater
trochanter and neck that strongly protrudes ventrally, or the presence of a subtrochanteric tubercle45,50,51.
e posterior position of the lesser trochanter gives the femur of Patagomaia a subquadrangular con-
tour in proximal view, as in therian mammals (e.g., Argyrolagus, Ukhaatherium, leptictids, Meniscotherium,
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Microgale)41,49,52. is condition diers from most non-therian mammaliaforms (e.g., morganucodontans, doco-
dontans, Henkelotherium, Vincelestes, Necrolestes)43,53,54 in which the greater trochanter, femoral head, and lesser
trochanter are almost aligned (see Supplementary Fig.9).
Figure1. Images of Patagomaia chainko holotype remains, MPM-PV-23365 References: (a) fragments of the
le pelvis; (b) silhouette and skeletal scheme with details of the preserved bones; (c) distal end of the le ulna;
(d) proximal end of the right femur and distal end of the le femur; e, proximal half of the le tibia. Scale bar:
20mm.
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e distal end of the femur of Patagomaia is transversely narrow, with the distal condyles proximo-distally low
and nearly symmetrical in size and shape, in contrast with most non-therian mammals (e.g., morganucodontans,
monotremes, Haldanodon; Docofossor, Necrolestes)44,5355 in which the distal condyles are markedly asymmetrical
(see Supplementary Fig.10). e femoral condyles of Patagomaia are separated posteriorly by a deep but narrow
intercondylar groove. ere is a thick and prominent intercondylar ridge that is obliquely oriented and is distally
delimited by a pit-like concavity (see Supplementary Fig.4). is condition is unknown in most other mam-
mals, being observed in only a few extant therians (e.g., Arctictis, Hystrix, Meles; see Supplementary Fig.12). In
Patagomaia, the patellar surface is represented by a shallow concavity delimited by low crests.
e tibia of Patagomaia shows symmetrical proximal facets for articulation with the femoral condyles, in
contrast to the asymmetrical articular facets of non-therian mammaliaforms (e.g., morganucodontans, doco-
dontans, Akidolestes, Vincelestes; see Supplementary Figs.6)7,42,54. A deep and well-dened fossa extends along
the posterior surface of the sha of the tibia, a trait only seen in some eutherians (e.g., leptictids56, periptychids57,
carnivores; see Supplementary Fig.13). e anterior tibial tubercle is low and extends distally as a crest.
Available elements show that they belong to a medium-sizedmammal, comparable in size to a the canidLyca-
lopex culpaeus. Following methodologies described in the SI, we estimated a body mass of ~ 14kg for the holotype
specimen of Patagomaia chainko (Supplementary Information 4; Supplementary Table1; Supplementary Fig.16).
e histology of the femur and tibia of Patagomaia reveals that the cortex is composed of parallel-bered and
lamellar bone tissue and sparse/moderatedensity of longitudinal canals, closely resembling some Mesozoic
eutherians (e.g., Barunlestes, Zalambdalestes)58. e presence of a well-dened External Fundamental System
indicates the somatic maturity of the holotype of Patagomaia chainko (Fig.2; Supplementary Information 2).
is suggests that the size and body mass estimated here for the holotype of Patagomaia chainko likely represents
the maximum ones that this animal could have reached.
Discussion
To analyse the phylogenetic anities of Patagomaia chainko, it was scored into three dierent comprehensive
mammalian data matrices13,59,60. e incomplete nature of the holotype specimen, plus the fact that most of these
datasets heavely rest on skull and teeth information, we were able to score just a small number of characters (less
than 2.5%) for Patagomaia. However, the analyses consistently recover Patagomaia within eria (Supplementary
Information 5; Supplementary Figs.17, 18 and 19). Synapomorphic features of eria7,44,47,52 present in Patago-
maia are: acetabulum completely fused and devoid of a dorsal emargination; femur with well-dierentiated and
medially-oriented head and neck; lesser trochanter small and posteroventrally displaced; distal end of femur
with symmetrical articular condyles and reduced epicondyles; proximal end of tibia with symmetrical articular
facets (in agreement with the condition of distal femoral condyles). is set of characters clearly distinguishes
Patagomaia from non-therian mammals of the Southern Hemisphere, such as meridiolestidans (i.e., Necrolestes),
multituberculates50, and gondwanatherians (i.e., Adalatherium)13.
It is worth noting that, although Patagomaia reveals therian anities, it diers from Paleogene South Ameri-
can representatives of this clade (e.g., notoungulates, litopterns, astrapotherians, xenarthrans, and sparassodonts)
in having a shallow patellar groove delimited by poorly dened crests that do not form a deep trochlea. In sum,
Patagomaia does not exhibit morphological features that may ally it with any of the mammalian clades (i.e.,
Gondwanatheria, Dryolestoidea, Marsupialia, Ungulata) frequently recorded in Cretaceous and early Paleogene
beds from South America. e prevailing view has been that the early evolutionary radiation of therian mammals
occurred in the Northern Hemisphere1517 although it has overlooked the occurrences of therians and stem-
therians in the Cretaceous of Africa, Madagascar, India and South America2733. In this context, Patagomaia
constitutes an important addition to the meagre record of Mesozoic therians from Gondwana and indicates that
some stages of early therian evolution occurred in the Southern Hemisphere1926.
We estimate that the body mass of the holotype individual of Patagomaia ranged between 2.6 and 26kg, with
an average estimate of approximately 14kg (a value obtained using the mean of 14 dierent regressions; see
Materials and Methods and Supplementary Information 4). Even the smallest estimates would place it among
the larger Mesozoic mammals, while the average and higher estimates exceed by far those of the largest Meso-
zoic mammals previously known: the Early Cretaceous Chinese eutriconodont Repenomamus (approximately
10kg), and the Late Cretaceous gondwanatherian Vintana (8.9kg). is would make Patagomaia the largest
known Mesozoic mammal.
Most mammalian taxa recorded in Campanian–Maastrichtian assemblages of the Northern Hemisphere have
an estimated body mass below 100g, and only 1% of them are estimated to have reached a body mass of 1 kg61.
In sharp contrast, at least 17 valid mammalian taxa (mainly belonging to Monotremata, Gondwanatheria and
Meridiolestida) are known from Patagonian faunal assemblages, eight of which surpass 1kg in body mass (see
Supplementary Information 4). Similarly, large-bodied (much larger than 1kg) mammals such as Vintana sertichi
and Adalatherium hui are known from the latest Cretaceous of Madagascar53. is suggests that the evolution of
larger body size among Gondwanan mammalian taxa began prior to the end-Cretaceous mass extinction event,
thus preceding their Laurasian relatives in reaching large body size by at least 5 million years.
is new discovery demonstrates that Late Cretaceous mammalian faunas from South America were taxo-
nomically diverse, not only including gondwanatherians, dryolestoids and monotremes, but also early therians.
Patagomaia also reveals that the evolution of large body size among Late Cretaceous mammals was more complex
than previously understood.
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Materials and methods
Description
We follow the terminology of postcranial anatomy and myology used by Muizon38, Argot39,40,62,63, Fostowicz-
Frelik64, Gambaryan etal.65 and Warburton etal.66. Paleohistological methods are explained in the Supplementary
Figure2. Simplied time-calibrated cladogram showing the phylogenetic anities of Patagomaia chainko,
geographic location and paleohistological images. e simplied cladogram shows our interpretation unifying
the three analyses conducted (see Supplementary Information 5). Map showing the fossil locality. e specimen
here reported was recovered at the new site located at S 50° 30 39.888 and W 72° 33 18.035, close to the
Isasicursor 2 site (marked with a red star) in the Maastrichtian Chorillo Formation. Transverse section of the
femur (le) showing the External Fundamental System (white arrowheads); and tibia (right) in polarized light
with lambda compensator. ER, Erosion room; PFB, parallel-bered bone tissue; TB, trabecular bone; VC,
vascular canal. Scale bar: 0.75mm.
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Information 3.
Body mass estimation
Estimates of the body mass of Patagomaia were made by taking measurements of the postcranial remains, using
regressions that have already been used in other fossil mammals. Measurements were taken with a digital caliper.
In addition, other regressions, based on cranial and dental measurements already published by other authors,
were used to calculate the body mass of other mammals from the Cretaceous of South America. e averages of
all regressions were calculated and compared with the results obtained in Patagomaia. e methodology used
for the body mass estimations is explained in the Supplementary Information 4.
Phylogenetic analyses
ree data matrices were used with the aim of testing the phylogenetic position of Patagomaia chainko among
mammals. We chose these data sets because they are very complete and updated matrices, and each one is focused
on dierent groups of Mesozoic mammals (see Supplementary Information 5).
Data availability
e datasets analyzed during the current study are included in this published article (and its Supplementary
Information le).
Received: 4 September 2023; Accepted: 29 January 2024
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Acknowledgements
e present paper is the result of the joint Argentine-Japanese explorations carried out in March 2020, March
2022, and February 2023. We thank the late Coleman Burke (New York) for his encouragement and nancial
assistance to carry out the rst eld explorations to La Anita farm. Dr. Yoshihiro Hayashi, former Director Gen-
eral, National Museum of Nature and Science, Japan, for his support of the project by funding a major part of the
expedition from the internal grant of the museum. Special thanks are due to Santiago Miner (LACEV-MACN),
who kindly helped with the reconstruction of 3D images of Patagomaia, Kai R.K. Jäger (Section Palaeontology,
Institute of Geosciences, Rheinische Friedrich-Wilhelms-Universität Bonn, Germany) for providing 3D models
of the Henkelotherium, and David Krause (Denver Museum of Nature and Science, Colorado) for sharing 3D
models of Adalatherium. We thank M. Martinelli and M. Ezcurra (MACN-PV), Pablo Teta and Sergio Lucero
(MACN-Ma), for allowingaccess to fossil specimensunder their care.e specimen of Patagomaia was collected
Content courtesy of Springer Nature, terms of use apply. Rights reserved
8
Vol:.(1234567890)
Scientic Reports | (2024) 14:2854 | https://doi.org/10.1038/s41598-024-53156-3
www.nature.com/scientificreports/
by G.E. Lo Coco (LACEV-MACN). e authors would like to thank Oscar Canto and Carla Almazán (Secretaría
de Cultura) for supporting our projects and explorations in Santa Cruz. Special thanks are also due to Federico
Braun for allowing access to his property and F. Echeverría, D. Fraser and A. Prieto for the hospitality and their
valuable geographic knowledge of the Anita territories. Authors would also like to thank the entire eld crew,
including C. Sakata, C. Miyamae, H. Kamei, M.P. Isasi,F. Brissón-Egli, A. Moreno Rodríguez, G. Lio, S. Miner,
G. Muñoz, J. De Pasqua, C. ompson, D. Piazza, G. Lo Coco, A. Misantone, G. Stoll, S. Rozadilla, A.M. Aran-
ciaga Rolando, M.J. Motta, G. Álvarez-Herrera, andR. Álvarez-Nogueirafor their assistance and logistics during
eldwork. We especially thank Mirta González and Dr Victoria Sánchez (MACN-Icn) for their assistance in the
use of the petrographic microscope.We also are grateful toRoberto Pujana (MACN,BA Pb)for thepreparation
of the thin-sections. We also thank the geologists D. Moyano-Paz, M. Coronel, D.G. Poiré, M.S. Raigemborn
and S. Lizzoli, and paleobotanist E. Vera for their valuable comments on the stratigraphy and regional geology
of the Chorrillo Formation. We would like to express our sincere thanks to Benjamin Breenden, for his reading
and contributions to the better writing of the manuscript. anks also to Elsa Panciroli, Robin Beck, Guillermo
Rougier and an anonymous reviewer, for their enlightening comments that greatly improved the quality of the
manuscript.
Author contributions
N.R.C., F.L.A. and F.E.N. conceptualized the study. N.R.C. phylogenetic analyses. J.G.M. paleohistological study.
F.E.N., M.M. and T.T. acquired fundings. N.R.C., F.L.A. and J.G.M. made the gures. All authors contributed to
the writing and general revision of the manuscript.
Competing interests
e authors declare no competing interests.
Additional information
Supplementary Information e online version contains supplementary material available at https:// doi. org/
10. 1038/ s41598- 024- 53156-3.
Correspondence and requests for materials should be addressed to N.R.C.
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... ). In fact, Chimento et al. 1 explicitly discuss the presence of this patellar groove in Patagomaia, and further highlighted it as "pg" in some of their figures (e.g., Fig. 1; the figures, unfortunately, lack a key for their abbreviations). We therefore corrected the score of the patellar groove as present. ...
... The possibility that these characters are general traits of Gondwanatheria cannot be ruled out. Chimento et al. 1 mentioned that the poorly preserved iliopubic eminence of Patagomaia (broken at its tip) "appears to have been robust" and thus different from Adalatherium. However, this is only a guess regarding the size of an unpreserved structure. ...
... The strict consensus tree of the equal weights parsimony analysis now recovers Patagomaia in a polytomy alongside gondwanatherians, as well as therians (Fig. 2a). Unlike the analysis in Chimento et al. 1 , we additionally used implied weights parsimony and Bayesian analyses, which are widely accepted approaches to further explore the tree space 9,10 . These did not recover any support for therian affinities, but instead, both approaches recovered Patagomaia as a gondwanatherian (Figs. ...
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Early suckler? One trait that is unique to mammals is milk suckling. Suckling requires the presence of stability and motion in the throat, both of which require a complex hyoid apparatus. Zhou et al. describe a mammaliform docodontan fossil from the Jurassic that was preserved with a nearly intact hyoid (see the Perspective by Hoffmann and Krause). The structure is complex and saddle shaped, like that seen in modern mammals, suggesting that a muscularized throat was present before the development of mammals. Science , this issue p. 276 ; see also p. 222
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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.
Book
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.
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Docodonta are one of the earliest diverging groups of mammaliaforms, and their morphology provides key information on the transition between non-mammalian cynodonts and Mammalia. We describe the partial skulls of two docodontans Borealestes serendipitus and Borealestes cuillinensis sp. nov. from the Kilmaluag Formation (Middle Jurassic: Bathonian), Isle of Skye, Scotland. We visualize their cranial anatomy using laboratory and synchrotron X-ray micro-CT. The skulls belong to two partial skeletons, currently comprising the most complete Mesozoic mammal fossils reported from the British Isles. The associated upper and lower dentitions show that the lower dentition of Borealestes is not diagnostic to species level. We establish, B. cuillinensis, based on upper molar characters, and re-identify upper molars previously assigned to ‘Borealestes’ mussettae as belonging to B. cuillinensis. ‘Borealestes’ mussettae, based on distinctive lower molars, is found to be morphologically and phylogenetically distinct from Borealestes, necessitating assignment to a new genus, Dobunnodon gen. nov. The skulls of Borealestes retain many plesiomorphic features seen in Morganucodon but absent in more crownward mammaliaforms. Our study highlights that generic and species taxonomy of docodontans are more reliable when based on both upper and lower teeth, while lower molar morphology may underrepresent the true diversity of Mesozoic mammaliaforms.