Marine predator endothermy evidence in tooth enamel from Cretaceous Western Interior Seaway + mammaliaform tracks from Middle Jurassic of China
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Ben Creisler
1:29 PM (7 hours ago) 1:29 PM
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Ben Creisler
Lida Xing, Jens N. Lallensack, Hang Yin, Hendrik Klein, Anthony Romilio, Junmin Gao & Qi Qi (2026)
New Middle Jurassic mammaliaform tracks from northern Shaanxi Province, China.
Fossil Record 29(1): 37-56.
doi: https://doi.org/10.3897/fr.29.169194
https://fr.pensoft.net/article/169194/
The Middle Jurassic Yan’an Formation in Zizhou County, Shaanxi Province, China, preserves abundant vertebrate ichnofossils. In 2019, several morphologically distinctive small tetradactyl and pentadactyl tracks, as well as possible pterosaur tracks, were discovered at the Wangzhuang and Longweimao tracksites. These tracks were documented using 3D photogrammetry and morphological comparison suggests that these tracks share morphological similarities with Ameghinichnus. Their distinctive features indicate that they may represent an undetermined ichnospecies within this genus; therefore, they are provisionally referred to Ameghinichnus isp., which represents the first record of Ameghinichnus in Asia. The pterosaur tracks preserved alongside the mammaliaform tracks may represent one of the earliest known pterosaur ichnofossils in China. Palaeoecological evidence suggests that Ameghinichnus isp. was produced by small, agile mammaliaforms inhabiting low-energy fluvial–lacustrine environments. This discovery extends the spatiotemporal range of Ameghinichnus, enriches the Jurassic mammaliaform ichnological record of East Asia and provides new insights into their palaeoecology and early evolutionary history.
New papers:
Chelsea M. Comans, Thomas S. Tobin & Rebecca L. Totten (2026)
Evidence for endothermy from tooth enamel(oid) oxygen isotopes in marine predators of the Late Cretaceous Western Interior Seaway, USA
Palaeogeography, Palaeoclimatology, Palaeoecology 113578
doi: https://doi.org/10.1016/j.palaeo.2026.113578
https://www.sciencedirect.com/science/article/abs/pii/S0031018226000416
Highlights
Geochemical comparison of fossil marine predators yields information about thermoregulation.
Isotope results suggest mosasaurs Platecarpus and Tylosaurus were endothermic.
Regional endothermy is likely for the large bony fish Xiphactinus and sharks Ptychodus, Cretoxyrhina.
Ectothermy is likely for the shark Cretalamna and other shark taxa.
Abstract
We analyze stable oxygen isotopes (δ18Op) of tooth enamel(oid) from fossil marine predators from the Smoky Hill Chalk Member of the Niobrara Formation (Kansas, USA) to investigate the thermoregulatory modes of shark, bony fish, and mosasaur taxa that lived in the Western Interior Seaway (WIS) of North America during the Late Cretaceous. Through comparing these taxa with the co-occurring ectothermic bony fish †Enchodus, which serves as a proxy for ambient conditions, we assess the relative δ18Op values of 21 individual organisms across nine genera, using data measured from 47 fossil teeth. The mosasaur taxa (†Platecarpus and †Tylosaurus) exhibit significantly lower δ18Op values than †Enchodus that are consistent in the direction and magnitude expected for endothermy in this group. Most shark taxa, by contrast, are likely ectothermic because their δ18Op values overlap with those of †Enchodus. The durophagous shark †Ptychodus and large predatory bony fish †Xiphactinus, however, record significantly lower δ18Op values than Enchodus and other sharks, which we interpret as indicative of elevated body temperatures consistent with endothermy and migration. Comparison with the Gulf Coastal Plain (GCP) of North America reveals that †Ptychodus, †Cretoxyrhina, †Platecarpus, and †Tylosaurus have similarly low δ18Op values across both regions. The consistency of δ18Op value offsets for †Ptychodus across species and regions suggests that endothermy was a trait inherent to this genus. Differences in δ18Op value offsets (from local †Enchodus values) between WIS and GCP for †Tylosaurus and †Cretoxyrhina may reflect differences in respective study sample size, methodology, and ecological factors, namely migration, and regional ecological differences and ontogenetic habitat partitioning within †Cretoxyrhina. Further, these new data support previous findings that †Cretalamna was ectothermic and †Cretoxyrhina was endothermic. The prevalence of endothermy in Late Cretaceous sharks is likely higher than previously thought, challenging hypotheses of endothermy evolution in Late Cretaceous sharks.
Chelsea M. Comans, Thomas S. Tobin & Rebecca L. Totten (2026)
Evidence for endothermy from tooth enamel(oid) oxygen isotopes in marine predators of the Late Cretaceous Western Interior Seaway, USA
Palaeogeography, Palaeoclimatology, Palaeoecology 113578
doi: https://doi.org/10.1016/j.palaeo.2026.113578
https://www.sciencedirect.com/science/article/abs/pii/S0031018226000416
Highlights
Geochemical comparison of fossil marine predators yields information about thermoregulation.
Isotope results suggest mosasaurs Platecarpus and Tylosaurus were endothermic.
Regional endothermy is likely for the large bony fish Xiphactinus and sharks Ptychodus, Cretoxyrhina.
Ectothermy is likely for the shark Cretalamna and other shark taxa.
Abstract
We analyze stable oxygen isotopes (δ18Op) of tooth enamel(oid) from fossil marine predators from the Smoky Hill Chalk Member of the Niobrara Formation (Kansas, USA) to investigate the thermoregulatory modes of shark, bony fish, and mosasaur taxa that lived in the Western Interior Seaway (WIS) of North America during the Late Cretaceous. Through comparing these taxa with the co-occurring ectothermic bony fish †Enchodus, which serves as a proxy for ambient conditions, we assess the relative δ18Op values of 21 individual organisms across nine genera, using data measured from 47 fossil teeth. The mosasaur taxa (†Platecarpus and †Tylosaurus) exhibit significantly lower δ18Op values than †Enchodus that are consistent in the direction and magnitude expected for endothermy in this group. Most shark taxa, by contrast, are likely ectothermic because their δ18Op values overlap with those of †Enchodus. The durophagous shark †Ptychodus and large predatory bony fish †Xiphactinus, however, record significantly lower δ18Op values than Enchodus and other sharks, which we interpret as indicative of elevated body temperatures consistent with endothermy and migration. Comparison with the Gulf Coastal Plain (GCP) of North America reveals that †Ptychodus, †Cretoxyrhina, †Platecarpus, and †Tylosaurus have similarly low δ18Op values across both regions. The consistency of δ18Op value offsets for †Ptychodus across species and regions suggests that endothermy was a trait inherent to this genus. Differences in δ18Op value offsets (from local †Enchodus values) between WIS and GCP for †Tylosaurus and †Cretoxyrhina may reflect differences in respective study sample size, methodology, and ecological factors, namely migration, and regional ecological differences and ontogenetic habitat partitioning within †Cretoxyrhina. Further, these new data support previous findings that †Cretalamna was ectothermic and †Cretoxyrhina was endothermic. The prevalence of endothermy in Late Cretaceous sharks is likely higher than previously thought, challenging hypotheses of endothermy evolution in Late Cretaceous sharks.
====
Free pdf:
Lida Xing, Jens N. Lallensack, Hang Yin, Hendrik Klein, Anthony Romilio, Junmin Gao & Qi Qi (2026)
New Middle Jurassic mammaliaform tracks from northern Shaanxi Province, China.
Fossil Record 29(1): 37-56.
doi: https://doi.org/10.3897/fr.29.169194
https://fr.pensoft.net/article/169194/
The Middle Jurassic Yan’an Formation in Zizhou County, Shaanxi Province, China, preserves abundant vertebrate ichnofossils. In 2019, several morphologically distinctive small tetradactyl and pentadactyl tracks, as well as possible pterosaur tracks, were discovered at the Wangzhuang and Longweimao tracksites. These tracks were documented using 3D photogrammetry and morphological comparison suggests that these tracks share morphological similarities with Ameghinichnus. Their distinctive features indicate that they may represent an undetermined ichnospecies within this genus; therefore, they are provisionally referred to Ameghinichnus isp., which represents the first record of Ameghinichnus in Asia. The pterosaur tracks preserved alongside the mammaliaform tracks may represent one of the earliest known pterosaur ichnofossils in China. Palaeoecological evidence suggests that Ameghinichnus isp. was produced by small, agile mammaliaforms inhabiting low-energy fluvial–lacustrine environments. This discovery extends the spatiotemporal range of Ameghinichnus, enriches the Jurassic mammaliaform ichnological record of East Asia and provides new insights into their palaeoecology and early evolutionary history.
====
Gregory Paul
4:10 PM (4 hours ago) 4:10 PM
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Why I remember back in them there olden times, when the notion that they were all scaly dinosaurs, much less sea reptiles and fish, were tachymetabolic was considered radical nonsense by many. Endothermy was a rare specialization of birds and mammals of course.
GSPaul
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Thomas Yazbek
4:18 PM (4 hours ago) 4:18 PM
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Mosasaurs would appear to be the only known endothermic squamates. It makes you wonder what the thermoregulatory mechanisms they employed were...
Thomas
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Jura
4:49 PM (3 hours ago) 4:49 PM
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Alternatively, it makes you wonder if the proxies being used here actually have a causal link to obligate endothermy. My suspicion is that they don't.
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