Large mammal-like tracks from Lower Cretaceous of Brazil + monotreme dental morphology origin + marsupial origins + Prochelidella turtle shell histology
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Ben Creisler
Aug 3, 2026, 2:15:38 AM (3 days ago) Aug 3
to DinosaurMa...@googlegroups.com
Ben Creisler
====
Recent papers:
The Botucatu Formation of the Paraná Basin, southern Brazil, preserves some of the largest mammal-like tracks attributed to Mammaliaformes from the Early Cretaceous. Here, we describe two new large tetradactyl pes trackways (specimens UERJ IC-170 and UERJ IC-171) which exceed in size all the mammaliaform footprints previously described from the same formation. These trackways exhibit distinct morphological features, including short, rounded digit prints and narrow trackways consistent with a quadrupedal gait. Based on their morphology and after comparison with known ichnotaxa, we assign these tracks to the ichnofamily Chelichnopodidae, expanding the ichnodiversity of the Botucatu paleoerg. The large size of these tracks challenges the prevailing view of predominantly small body size in Mesozoic mammaliaforms and suggests that larger body sizes evolved earlier and more frequently than previously recognized. These findings impact our understanding of the macroevolutionary patterns of mammaliaform size and ecology during the Early Cretaceous in Gondwana.
Nicolás R. Chimento (2026)
The Bonaparte hypothesis and the evolution of dental homologies in monotremes
Australian Zoologist 45(2): AZ26025.
https://doi.org/10.1071/AZ26025
https://connectsci.au/az/article-abstract/45/2/AZ26025/275020/The-Bonaparte-hypothesis-and-the-evolution-of
Monotremes occupy a pivotal position in mammalian evolution, yet the origin and homology of their dentition remain highly debated. This study revisits and updates the ‘Bonaparte hypothesis’, which proposes that monotreme dental patterns originated from Gondwanan dryolestoid ancestors, particularly meridiolestidans. Drawing on recent fossil discoveries from South America and Australia, including new monotreme and meridiolestidan material, a comprehensive comparative analysis of dental and mandibular morphology is presented. Multiple lines of evidence reveal notable similarities in cusp arrangement, crown organisation, root structure, and mandibular features between monotremes, meridiolestidans, and Northern Hemisphere dryolestoids. These include the structure of triakididrepanons, patterns of root fusion, development of cingulids, and the presence of a hypertrophied mandibular canal. The results support the plausibility of complex dental transformations, potentially involving the fusion of adjacent tooth units, as originally proposed by Bonaparte. Although the hypothesis remains difficult to test due to the fragmentary fossil record and the lack of developmental evidence, it is consistent with current data and compatible with models proposing a Gondwanan origin of tribosphenic dentition. These findings highlight the importance of Gondwanan taxa in early mammalian evolution and underscore the need for new fossil discoveries to further resolve the origins of monotreme dental morphology.
Pedro Victor Buck, Aline Marcele Ghilardi, Tito Aureliano, Marcus Vinicius Berao Ade, Rodolfo Dino, Luzia Antonioli, Márcio Luiz Geremias, Marcelo Adorna Fernandes & Hermínio Ismael de Araújo-Júnior (2026)
Remarkably large mammal-like tracks from the Botucatu Formation (Lower Cretaceous), Paraná Basin, Brazil: implications for the evolution of large size in Mammaliaformes
Journal of South American Earth Sciences 106223
doi: https://doi.org/10.1016/j.jsames.2026.106223
https://www.sciencedirect.com/science/article/abs/pii/S0895981126002750
Highlights
New large mammaliaform trackways from Botucatu Formation described
Largest mammal-like tracks from Lower Cretaceous Gondwana
Ichnology reveals greater size diversity in mammaliaforms
ABSTRACT
Remarkably large mammal-like tracks from the Botucatu Formation (Lower Cretaceous), Paraná Basin, Brazil: implications for the evolution of large size in Mammaliaformes
Journal of South American Earth Sciences 106223
doi: https://doi.org/10.1016/j.jsames.2026.106223
https://www.sciencedirect.com/science/article/abs/pii/S0895981126002750
Highlights
New large mammaliaform trackways from Botucatu Formation described
Largest mammal-like tracks from Lower Cretaceous Gondwana
Ichnology reveals greater size diversity in mammaliaforms
ABSTRACT
The Botucatu Formation of the Paraná Basin, southern Brazil, preserves some of the largest mammal-like tracks attributed to Mammaliaformes from the Early Cretaceous. Here, we describe two new large tetradactyl pes trackways (specimens UERJ IC-170 and UERJ IC-171) which exceed in size all the mammaliaform footprints previously described from the same formation. These trackways exhibit distinct morphological features, including short, rounded digit prints and narrow trackways consistent with a quadrupedal gait. Based on their morphology and after comparison with known ichnotaxa, we assign these tracks to the ichnofamily Chelichnopodidae, expanding the ichnodiversity of the Botucatu paleoerg. The large size of these tracks challenges the prevailing view of predominantly small body size in Mesozoic mammaliaforms and suggests that larger body sizes evolved earlier and more frequently than previously recognized. These findings impact our understanding of the macroevolutionary patterns of mammaliaform size and ecology during the Early Cretaceous in Gondwana.
====
Nicolás R. Chimento (2026)
The Bonaparte hypothesis and the evolution of dental homologies in monotremes
Australian Zoologist 45(2): AZ26025.
https://doi.org/10.1071/AZ26025
https://connectsci.au/az/article-abstract/45/2/AZ26025/275020/The-Bonaparte-hypothesis-and-the-evolution-of
Monotremes occupy a pivotal position in mammalian evolution, yet the origin and homology of their dentition remain highly debated. This study revisits and updates the ‘Bonaparte hypothesis’, which proposes that monotreme dental patterns originated from Gondwanan dryolestoid ancestors, particularly meridiolestidans. Drawing on recent fossil discoveries from South America and Australia, including new monotreme and meridiolestidan material, a comprehensive comparative analysis of dental and mandibular morphology is presented. Multiple lines of evidence reveal notable similarities in cusp arrangement, crown organisation, root structure, and mandibular features between monotremes, meridiolestidans, and Northern Hemisphere dryolestoids. These include the structure of triakididrepanons, patterns of root fusion, development of cingulids, and the presence of a hypertrophied mandibular canal. The results support the plausibility of complex dental transformations, potentially involving the fusion of adjacent tooth units, as originally proposed by Bonaparte. Although the hypothesis remains difficult to test due to the fragmentary fossil record and the lack of developmental evidence, it is consistent with current data and compatible with models proposing a Gondwanan origin of tribosphenic dentition. These findings highlight the importance of Gondwanan taxa in early mammalian evolution and underscore the need for new fossil discoveries to further resolve the origins of monotreme dental morphology.
====
Free pdf:
Thomas H. Rich, Timothy F. Flannery & Patricia Vickers-Rich (2026)
The diphyletic origin of marsupials
Australian Zoologist 45(2): AZ26012.
doi: https://doi.org/10.1071/az26012
https://connectsci.au/az/article/45/2/AZ26012/275008/The-diphyletic-origin-of-marsupials
There is ample evidence that one group of marsupials, the ameridelphians, arose during the late Early Cretaceous in the Northern Hemisphere and subsequently dispersed to South America. It has long been accepted that marsupials subsequently crossed Antarctica to reach Australia, becoming a distinct group, the australidelphians. The Australian fossil evidence pertaining to this view was notoriously poor until recently. In 1997, the first known Mesozoic Australian tribosphenic mammal was discovered in late Early Cretaceous deposits. Subsequently two more tribosphenic mammalian genera and species were collected at the same locality. After appearing, did these tribosphenic mammals simply go extinct or did they give rise to the marsupials subsequently found there, the australidelphians? Morphological analysis of these tribosphenic mammals suggests that one of the three species in particular might be annectant to marsupials. This coupled with the geographic distribution of the australidelphians and ameridelphians, which are distinguished from one another by a consistent, distinctly different calcanea is suggestive of a separate origin of the two groups.
====
Thomas H. Rich, Timothy F. Flannery & Patricia Vickers-Rich (2026)
The diphyletic origin of marsupials
Australian Zoologist 45(2): AZ26012.
doi: https://doi.org/10.1071/az26012
https://connectsci.au/az/article/45/2/AZ26012/275008/The-diphyletic-origin-of-marsupials
There is ample evidence that one group of marsupials, the ameridelphians, arose during the late Early Cretaceous in the Northern Hemisphere and subsequently dispersed to South America. It has long been accepted that marsupials subsequently crossed Antarctica to reach Australia, becoming a distinct group, the australidelphians. The Australian fossil evidence pertaining to this view was notoriously poor until recently. In 1997, the first known Mesozoic Australian tribosphenic mammal was discovered in late Early Cretaceous deposits. Subsequently two more tribosphenic mammalian genera and species were collected at the same locality. After appearing, did these tribosphenic mammals simply go extinct or did they give rise to the marsupials subsequently found there, the australidelphians? Morphological analysis of these tribosphenic mammals suggests that one of the three species in particular might be annectant to marsupials. This coupled with the geographic distribution of the australidelphians and ameridelphians, which are distinguished from one another by a consistent, distinctly different calcanea is suggestive of a separate origin of the two groups.
====
====
Juan Marcos Jannello, Ignacio Jorge Maniel, Marcelo Saúl de la Fuente & Leonardo Sebastián Filippi (2026)
Microstructure bone diversity on the Oldest Pan-Chelidae Turtles (Testudines: Pleurodira): Understanding shell paleohistological variation and life history of Prochelidella spp.
Cretaceous Research 106464
doi: https://doi.org/10.1016/j.cretres.2026.106464
https://www.sciencedirect.com/science/article/abs/pii/S0195667126001539
Highlights
Unusual compactness shell in freshwater extinct turtle taxa.
Prochelidella portezuelae osteosclerotic state.
Abstract
The fossil record of reptiles plays crucial role in reconstructing the faunal history of Patagonia. Among pleurodiran turtles, Prochelidella spp. represent the oldest known genera within the crown Chelidae or a basal panchelid clade according to the chosen phylogenetic hypothesis. These turtles are widely distributed across several Patagonian basins in Argentina, including Austral-Magallanes, Golfo San Jorge, Somuncurá-Cañadón Asfalto, and Neuquén. Currently, five species of Prochelidella are recognized. This is the first paleohistological analysis of shell bones from four Cretaceous Prochelidella species from Patagonia. We aim to integrate paleoecological, systematic, and morphological data, with emphasis on the taxonomic insights provided by microanatomy and osteohistology of shell elements. We describe histological patterns of shell bones from Prochelidella portezuelae, Prochelidella cerrobarcinae, Prochelidella palomoi, and Prochelidella buitreraensis, all from the Cretaceous of Patagonia. These data are compared with those from closely related taxa, such as Mendozachelys wichmanni, Rionegrochelys caldieroi, and Linderochelys rinconensis. We focuses on postcranial elements, primarily costal plate materials. Our findings reveal significant paleohistological variation within Prochelidella, including lines of arrested growth, varying degrees of compactness, ridges/valley surfaces (minor vermiculation ornamentation), and a notably thick internal cortex. Some outstanding osteohistologies, like osteosclerotic state in Prochelidella portezuelae or structural fibres restricted to the deepest of the external cortex, with layer tissue of periosteal origin in Prochelidella buitreraensis, also reported in other groups such as Solemys vermiculata, suggest that the dermal fibre recruitment through metaplasia plays an essential role during the early stages of outer cortex development and provide insights into its life history.
Microstructure bone diversity on the Oldest Pan-Chelidae Turtles (Testudines: Pleurodira): Understanding shell paleohistological variation and life history of Prochelidella spp.
Cretaceous Research 106464
doi: https://doi.org/10.1016/j.cretres.2026.106464
https://www.sciencedirect.com/science/article/abs/pii/S0195667126001539
Highlights
Unusual compactness shell in freshwater extinct turtle taxa.
Prochelidella portezuelae osteosclerotic state.
Abstract
The fossil record of reptiles plays crucial role in reconstructing the faunal history of Patagonia. Among pleurodiran turtles, Prochelidella spp. represent the oldest known genera within the crown Chelidae or a basal panchelid clade according to the chosen phylogenetic hypothesis. These turtles are widely distributed across several Patagonian basins in Argentina, including Austral-Magallanes, Golfo San Jorge, Somuncurá-Cañadón Asfalto, and Neuquén. Currently, five species of Prochelidella are recognized. This is the first paleohistological analysis of shell bones from four Cretaceous Prochelidella species from Patagonia. We aim to integrate paleoecological, systematic, and morphological data, with emphasis on the taxonomic insights provided by microanatomy and osteohistology of shell elements. We describe histological patterns of shell bones from Prochelidella portezuelae, Prochelidella cerrobarcinae, Prochelidella palomoi, and Prochelidella buitreraensis, all from the Cretaceous of Patagonia. These data are compared with those from closely related taxa, such as Mendozachelys wichmanni, Rionegrochelys caldieroi, and Linderochelys rinconensis. We focuses on postcranial elements, primarily costal plate materials. Our findings reveal significant paleohistological variation within Prochelidella, including lines of arrested growth, varying degrees of compactness, ridges/valley surfaces (minor vermiculation ornamentation), and a notably thick internal cortex. Some outstanding osteohistologies, like osteosclerotic state in Prochelidella portezuelae or structural fibres restricted to the deepest of the external cortex, with layer tissue of periosteal origin in Prochelidella buitreraensis, also reported in other groups such as Solemys vermiculata, suggest that the dermal fibre recruitment through metaplasia plays an essential role during the early stages of outer cortex development and provide insights into its life history.
====
Mickey Mortimer
Aug 3, 2026, 5:36:10 PM (3 days ago) Aug 3
to Dinosaur Mailing Group
"Pedro Victor Buck, Aline Marcele Ghilardi, Tito Aureliano, Marcus
Vinicius Berao Ade, Rodolfo Dino, Luzia Antonioli, Márcio Luiz
Geremias, Marcelo Adorna Fernandes & Hermínio Ismael de
Araújo-Júnior (2026)
Remarkably large mammal-like tracks from the
Botucatu Formation (Lower Cretaceous), Paraná Basin, Brazil:
implications for the evolution of large size in Mammaliaformes
Journal of South American Earth Sciences 106223
doi: https://doi.org/10.1016/j.jsames.2026.106223
https://www.sciencedirect.com/science/article/abs/pii/S0895981126002750
Highlights
New large mammaliaform trackways from Botucatu Formation described
Largest mammal-like tracks from Lower Cretaceous Gondwana
Ichnology reveals greater size diversity in mammaliaforms
ABSTRACT
Journal of South American Earth Sciences 106223
doi: https://doi.org/10.1016/j.jsames.2026.106223
https://www.sciencedirect.com/science/article/abs/pii/S0895981126002750
Highlights
New large mammaliaform trackways from Botucatu Formation described
Largest mammal-like tracks from Lower Cretaceous Gondwana
Ichnology reveals greater size diversity in mammaliaforms
ABSTRACT
The Botucatu Formation of the Paraná Basin, southern Brazil, preserves some of the largest mammal-like tracks attributed to Mammaliaformes from the Early Cretaceous. Here, we describe two new large tetradactyl pes trackways (specimens UERJ IC-170 and UERJ IC-171) which exceed in size all the mammaliaform footprints previously described from the same formation. These trackways exhibit distinct morphological features, including short, rounded digit prints and narrow trackways consistent with a quadrupedal gait. Based on their morphology and after comparison with known ichnotaxa, we assign these tracks to the ichnofamily Chelichnopodidae, expanding the ichnodiversity of the Botucatu paleoerg. The large size of these tracks challenges the prevailing view of predominantly small body size in Mesozoic mammaliaforms and suggests that larger body sizes evolved earlier and more frequently than previously recognized. These findings impact our understanding of the macroevolutionary patterns of mammaliaform size and ecology during the Early Cretaceous in Gondwana."
So... how big are they?! The entire point of the article is the size of the trackmaker, yet nowhere in the abstract or highlights is there even the most approximate mass or living mammal comparison. Might as well be an SVP abstract...
Mickey Mortimer
Gregory Paul
Aug 3, 2026, 6:08:47 PM (2 days ago) Aug 3
to dinosaurma...@googlegroups.com
Exactly. Most SVP abstracts have more data in them.
GSPaul
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Ben Creisler
Aug 3, 2026, 6:35:09 PM (2 days ago) Aug 3
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Ben Creisler
I don't have access to the paper but did some online checks using the specimen numbers.
A Google query ' "UERJ-IC-170" Formação Botucatu' brings an image of the tracks with a scale bar.
Research Gate has an abstract that gives some additional details about the prints:
Translated:
The specimen, designated UERJ-IC-170 and described in this study, consists of a sequence of four footprints preserved in concave epirelief, all attributed to the feet of the trackmaker moving quadrupedally. The prints exhibit a tetradactyl, mesaxonic condition, with digits that are short and rounded at their distal ends. Morphological and locomotor parameters were measured using ImageJ software; the mean width and length were 9.83 ± 1.22 cm and 13.41 ± 0.49 cm, respectively, with a mean length/width ratio of 0.73 ± 0.07. The footprints display an oval morphology, with the greatest dimension along the transverse axis. Mean values for stride length, double-step length, and total trackway width are 20.95 ± 2.81 cm, 40.81 ± 2.57 cm, and 17.66 ± 3.59 cm, respectively. The mean stride angle is 147.35 ± 4.07°, suggesting a possible upright limb posture for the trackmaker.
However, a query on "UERJ-IC-171" Formação Botucatu brings up sources that use the specimen designation for a bird fossil!
To view this discussion visit https://groups.google.com/d/msgid/DinosaurMailingGroup/1253165149.3297242.1785794916298%40mail.yahoo.com.
Gregory Paul
Aug 3, 2026, 7:47:31 PM (2 days ago) Aug 3
to dinosaurma...@googlegroups.com
Looks like big dog size.
GSPaul
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