Mosasaur tail fin convergent evolution + post-hatchling rhynchosaur from the Triassic of Brazil
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Ben Creisler
Dec 11, 2025, 1:21:31 PM (7 days ago) Dec 11
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Ben Creisler
Yang Song and Johan Lindgren (2025)
Convergence in aquatic locomotion: reconstructing mosasaurian (Squamata: Mosasauria) tail fins from osteological correlates and covariation with extant sharks
Paleobiology (advance online publication)
DOI: https://doi.org/10.1017/pab.2025.10080
https://www.cambridge.org/core/journals/paleobiology/article/convergence-in-aquatic-locomotion-reconstructing-mosasaurian-squamata-mosasauria-tail-fins-from-osteological-correlates-and-covariation-with-extant-sharks/7636CD449F28DECD96F99536D5FFACAB
Abstract
The rarity of preserved soft tissues in the fossil record has limited our understanding of the life appearance of extinct vertebrates. However, through examination of various osteological features and comparisons with extant ecomorphological analogues, we can predict some of the external characteristics of ancient taxa. Specifically, for the Cretaceous mosasaurians (a group of seagoing squamates), extant sharks serve as suitable modern analogues because of their comparable caudal vertebral morphologies and ecologies. In this study, we build a novel framework by combining two-block partial least squares (2B-PLS) regression analysis and geometric morphometrics to investigate the relationship between tail fin shape and the underlying musculoskeletal morphology in sharks and mosasaurians. Our analysis reveals a strong correlation between the “tail fin expanded soft tissue” (TFEST) and shape of the associated caudal skeleton/musculature. Moreover, the covariation pattern between these two structures is remarkably similar in sharks and mosasaurians. Based on these findings, we then develop a predictive model that reconstructs the tail fin in mosasaurian taxa without known soft tissues. Our model indicates that all hydropedal forms possessed a bilobed, hypocercal (downturned) fluke, while plesiopedal species lacked a well-developed fleshy dorsal lobe. The variation in fluke morphologies between the four different evolutionary lineages of derived mosasaurians (mosasaurines, halisaurines, plioplatecarpines, and tylosaurines) suggests multiple independent origins of a bilobed tail fin. This study thus highlights the complexity of aquatic adaptations in mosasaurians and demonstrates the utility of predictive models when reconstructing the life appearance of extinct animals.
Non-technical Summary
Our knowledge about the life appearance of extinct marine reptiles is limited by the rarity of preserved soft tissues in the fossil record. Therefore, we developed a new framework to predict the tail fin shape of mosasaurians—a major group of aquatic lizards that thrived in the world’s oceans some 98 to 66 million years ago—based on skeletal data and comparisons with living sharks. By analyzing how the backbone and associated muscles of shark tails relate to the fleshy fin outline, and by verifying that this relationship is also applicable to mosasaurians with preserved body outlines, we built a predictive model capable of reconstructing the tail fins of mosasaurian species lacking direct soft-tissue evidence. Our novel approach combines two-block partial least squares (2B-PLS) regression and geometric analysis and reveals that mosasaurians evolved a variety of tail fin shapes that likely reflect different lifestyles. Coastal species had long and almost eel-like tails ideal for maneuvering in shallow waters, while species living in open-ocean habitats evolved semi-lunate flukes similar to those of pelagic sharks. We also found that a bilobed, downward bent (hypocercal) tail evolved independently at least four times in Mosasauria, which highlights the evolutionary complexity of their secondary adaptation to aquatic life during the Late Cretaceous. This study demonstrates the potential of combining geometric analysis with data from modern analogues when reconstructing the life appearance of extinct animals.
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Jossano Rosso Morais, Rodrigo Temp Müller, Leonardo Kerber & Flávio Augusto Pretto (2025)
A post-hatchling rhynchosaur from the Brazilian Triassic and the ontogenetic development of key characters within Hyperodapedontinae
Journal of Systematic Palaeontology 23(1): 2581267
doi: https://doi.org/10.1080/14772019.2025.2581267
https://www.tandfonline.com/doi/full/10.1080/14772019.2025.2581267
Hyperodapedontinae is the most abundant and diverse clade of Rhynchosauridae, an iconic group of Triassic archosauromorphs, easily recognizable by their maxillomandibular apparatus, formed by a grooved maxillary tooth plate and a dentary blade. As the number of teeth (and possibly sulci and blades) can vary both ontogenetically and phylogenetically, understanding the development of such morphological traits is important to confidently score phylogenetic datasets. Here we present the ontogenetically most immature rhynchosaur ever recorded for the Brazilian Triassic. Measuring only 2.5 cm in skull length, CAPPA/UFSM 0295 most probably represents a perinate, as indicated by its unworn dentition. Though in a very early stage of development, the animal already possessed several key characters of phylogenetic importance, such as the well-developed anguli oris crest, a ventrally closed infratemporal fenestra, and a mixture of conical and pyramidal teeth in the maxilla. Yet some features, such as the number of maxillary tooth rows, are, as expected, strongly influenced by its ontogenetic status. The specimen shows only single labial and single lingual rows, highlighting the need for caution when scoring these characters in phylogenetic studies. Among Brazilian rhynchosaurs, the presence of a single maxillary sulcus, along with two rows of dentary teeth and an open infraorbital foramen, is observed only in Macrocephalosaurus mariensis, to which the specimen is referred. The diminutive size and virtually unworn dentition strongly suggest CAPPA/UFSM 0295 represents a perinate individual. This constitutes the first record of a perinate Hyperodapedontinae and one of the oldest archosauromorph hatchlings ever recorded in continental settings.
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New papers:
Free pdf:
Yang Song and Johan Lindgren (2025)
Convergence in aquatic locomotion: reconstructing mosasaurian (Squamata: Mosasauria) tail fins from osteological correlates and covariation with extant sharks
Paleobiology (advance online publication)
DOI: https://doi.org/10.1017/pab.2025.10080
https://www.cambridge.org/core/journals/paleobiology/article/convergence-in-aquatic-locomotion-reconstructing-mosasaurian-squamata-mosasauria-tail-fins-from-osteological-correlates-and-covariation-with-extant-sharks/7636CD449F28DECD96F99536D5FFACAB
Abstract
The rarity of preserved soft tissues in the fossil record has limited our understanding of the life appearance of extinct vertebrates. However, through examination of various osteological features and comparisons with extant ecomorphological analogues, we can predict some of the external characteristics of ancient taxa. Specifically, for the Cretaceous mosasaurians (a group of seagoing squamates), extant sharks serve as suitable modern analogues because of their comparable caudal vertebral morphologies and ecologies. In this study, we build a novel framework by combining two-block partial least squares (2B-PLS) regression analysis and geometric morphometrics to investigate the relationship between tail fin shape and the underlying musculoskeletal morphology in sharks and mosasaurians. Our analysis reveals a strong correlation between the “tail fin expanded soft tissue” (TFEST) and shape of the associated caudal skeleton/musculature. Moreover, the covariation pattern between these two structures is remarkably similar in sharks and mosasaurians. Based on these findings, we then develop a predictive model that reconstructs the tail fin in mosasaurian taxa without known soft tissues. Our model indicates that all hydropedal forms possessed a bilobed, hypocercal (downturned) fluke, while plesiopedal species lacked a well-developed fleshy dorsal lobe. The variation in fluke morphologies between the four different evolutionary lineages of derived mosasaurians (mosasaurines, halisaurines, plioplatecarpines, and tylosaurines) suggests multiple independent origins of a bilobed tail fin. This study thus highlights the complexity of aquatic adaptations in mosasaurians and demonstrates the utility of predictive models when reconstructing the life appearance of extinct animals.
Non-technical Summary
Our knowledge about the life appearance of extinct marine reptiles is limited by the rarity of preserved soft tissues in the fossil record. Therefore, we developed a new framework to predict the tail fin shape of mosasaurians—a major group of aquatic lizards that thrived in the world’s oceans some 98 to 66 million years ago—based on skeletal data and comparisons with living sharks. By analyzing how the backbone and associated muscles of shark tails relate to the fleshy fin outline, and by verifying that this relationship is also applicable to mosasaurians with preserved body outlines, we built a predictive model capable of reconstructing the tail fins of mosasaurian species lacking direct soft-tissue evidence. Our novel approach combines two-block partial least squares (2B-PLS) regression and geometric analysis and reveals that mosasaurians evolved a variety of tail fin shapes that likely reflect different lifestyles. Coastal species had long and almost eel-like tails ideal for maneuvering in shallow waters, while species living in open-ocean habitats evolved semi-lunate flukes similar to those of pelagic sharks. We also found that a bilobed, downward bent (hypocercal) tail evolved independently at least four times in Mosasauria, which highlights the evolutionary complexity of their secondary adaptation to aquatic life during the Late Cretaceous. This study demonstrates the potential of combining geometric analysis with data from modern analogues when reconstructing the life appearance of extinct animals.
=====
Jossano Rosso Morais, Rodrigo Temp Müller, Leonardo Kerber & Flávio Augusto Pretto (2025)
A post-hatchling rhynchosaur from the Brazilian Triassic and the ontogenetic development of key characters within Hyperodapedontinae
Journal of Systematic Palaeontology 23(1): 2581267
doi: https://doi.org/10.1080/14772019.2025.2581267
https://www.tandfonline.com/doi/full/10.1080/14772019.2025.2581267
Hyperodapedontinae is the most abundant and diverse clade of Rhynchosauridae, an iconic group of Triassic archosauromorphs, easily recognizable by their maxillomandibular apparatus, formed by a grooved maxillary tooth plate and a dentary blade. As the number of teeth (and possibly sulci and blades) can vary both ontogenetically and phylogenetically, understanding the development of such morphological traits is important to confidently score phylogenetic datasets. Here we present the ontogenetically most immature rhynchosaur ever recorded for the Brazilian Triassic. Measuring only 2.5 cm in skull length, CAPPA/UFSM 0295 most probably represents a perinate, as indicated by its unworn dentition. Though in a very early stage of development, the animal already possessed several key characters of phylogenetic importance, such as the well-developed anguli oris crest, a ventrally closed infratemporal fenestra, and a mixture of conical and pyramidal teeth in the maxilla. Yet some features, such as the number of maxillary tooth rows, are, as expected, strongly influenced by its ontogenetic status. The specimen shows only single labial and single lingual rows, highlighting the need for caution when scoring these characters in phylogenetic studies. Among Brazilian rhynchosaurs, the presence of a single maxillary sulcus, along with two rows of dentary teeth and an open infraorbital foramen, is observed only in Macrocephalosaurus mariensis, to which the specimen is referred. The diminutive size and virtually unworn dentition strongly suggest CAPPA/UFSM 0295 represents a perinate individual. This constitutes the first record of a perinate Hyperodapedontinae and one of the oldest archosauromorph hatchlings ever recorded in continental settings.
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