Paleoteius, new lizard from Late Cretaceous of Argentina + lizard trackway from Jurassic of Spain + Alligator intramandibular joint biomechanics

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

Not yet mentioned reptile papers:

Paleoteius lakui gen. et sp. nov.

Federico L. Agnolín, Mauro Aranciaga-Rolando, Gerardo Álvarez-Herrera, Martín D. Ezcurra, Ana Moreno Rodríguez, Pablo Chafrat, Nahuel Vega, Agustín Scanferla, Krister T. Smith & Fernando E. Novas (2026)
A new late Cretaceous squamate from Patagonia sheds light on Gondwanan diversity
Scientific Reports (advance online publication)
doi: https://doi.org/10.1038/s41598-026-40914-8
https://www.nature.com/articles/s41598-026-40914-8

[We are providing an unedited version of this manuscript to give early access to its findings. Before final publication, the manuscript will undergo further editing. Please note there may be errors present which affect the content, and all legal disclaimers apply.]


The fossil record of Mesozoic terrestrial squamates is very poor in the Southern Hemisphere. Currently recognised species represent less than 6% of the number described for Laurasian landmasses. Here, we describe the most complete Late Cretaceous terrestrial squamate known from South America. The specimen was collected in the Maastrichtian Allen Formation of northern Patagonia (Río Negro Province, Argentina) and represents a single individual that preserves a partial skull, vertebrae, and appendicular bones. The skeleton shows a unique combination of character states that allow erecting the new genus and species Paleoteius lakui. Morphological phylogenetic analyses recovered Paleoteius lakui as a Scincomorpha more closely related to Xantusiidae than to other species. When a molecular backbone is applied to those analyses, Paleoteius lakui is found outside Scincomorpha, but within a clade composed of Jurassic to Palaeogene species found as non-crown scincomorphans in previous studies. The affinities of Paleoteius lakui either as a stem-xantusiid or as a more early-diverging clade indicate the presence of a yet unrecognised lineage of squamates in South America. Paleoteius lakui is related to Laurasian forms with a probable cosmopolitan ancestry during the Jurassic–earliest Cretaceous. The new finding helps fill the still poorly known record of Mesozoic Gondwanan squamates.

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Laura Piñuela, Lida Xing, Ángel García-Pérez & José Carlos García-Ramos (2026)
First report of lizard trackways from the Jurassic of Europe
Ichnos (advance online publication)
doi: https://doi.org/10.1080/10420940.2026.2634103
https://www.tandfonline.com/doi/full/10.1080/10420940.2026.2634103


Two well-preserved lizard trackways from the Kimmeridgian Lastres Formation of Asturias (northern Spain) are described. Trackway 1 (T1) consists of seven tracks (four manus and three pes), whereas Trackway 2 (T2) includes six tracks (three manus and three pes). The estimated body lengths of the T1 and T2 trackmakers are slightly below 50 cm and 30 cm, respectively. Both trackways, preserved as natural casts, show asymmetric and ectaxonic manus and pes prints, ranging from tetradactyl to pentadactyl. Digit lengths follow the pattern IV > III > II, with digit V, when present, similar to or slightly longer than digit I. Distinct claw marks are preserved on most digit impressions, particularly in T1. Lizard footprints are uncommon in the Jurassic, and the Upper Jurassic records are especially rare. These new trackways represent the only well-documented lizard trackways from the Upper Jurassic. They are attributed to Rhynchosauroides and constitute the latest occurrence of this ichnogenus in the global record. Experimental comparisons with modern lizards suggest that the tail placement and footprints arrangement in T1 may provide clues to gait and movement patterns in these fossil trackmakers.

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John David Fortner, Kaleb Craig Sellers, Kevin Mallory Middleton & Casey Monahan Holliday (2026)
Biomechanics of the intramandibular joint in Alligator mississippiensis
Journal of Anatomy (advance online publication)
doi: https://doi.org/10.1111/joa.70102
https://onlinelibrary.wiley.com/doi/10.1111/joa.70102


Tetrapod vertebrates possess skulls composed of variably articulating bones which they use to apprehend, process, and ingest food. Natural selection must therefore optimize craniomandibular sutures for load resistance, but sutural patency is required for normal craniofacial development to occur. While mammals seemingly escaped this constraint in their mandible by simplifying it into a single bony element (i.e., the dentary), sauropsids retain a composite mandible with a prominent, and occasionally flexible, intramandibular joint (IMJ) separating the rostral, dentigerous elements from the caudal elements onto which the jaw muscles insert. How sauropsids simultaneously construct a mandible robust enough for feeding that nevertheless maintains sutural patency for proper growth is a biomechanical paradox of keen interest to functional morphologists. Sauropsids may either passively reduce IMJ strain by expanding IMJ complexity or actively by using isometric contraction of specialized jaw muscles to resist excursion. American alligator (Alligator mississippiensis) mandibles possess a rather complex IMJ that must accommodate extreme magnitude and highly dynamic loads during feeding. Importantly, they also possess large m. intramandibularis (mIM) and m. pterygoideus ventralis (mPTv) muscles that may reduce IMJ strain during feeding, making them an ideal taxon to investigate the effect of joint morphology and muscle activity on IMJ and mandibular strain. We therefore constructed several 3D finite element models of Alligator mandibles with varyingly shaped IMJs to test the effect of IMJ orientation, complexity, and differential muscle activity on mandibular bending deformation and joint strain. Simple planar IMJs, regardless of orientation, reduce positive sagittal bending and medial wishboning deformation, and increase inversion of each hemimandible's dorsal margin. Changes in bending deformation during bilateral bites as the joint surface was reoriented from rostrally sloped to Vertical to caudally sloped are partially attributable to changes in joint surface area, though bending deformation is sensitive to both bite point location and joint orientation during unilateral bites. Increasing IMJ surface area reduces IMJ strain magnitudes, with a highly complex IMJ experiencing the most uniform and lowest magnitude joint ligament strains. Differential activation of mIM and mPTv do not significantly reduce IMJ strains but do affect mandibular bending deformation, suggesting that available joint surface area, and not isometric muscle contraction, is the greatest variable controlling IMJ strains in adult Alligator. Instead, mIM may significantly control bite point reaction forces due to its very long moment arm, whereas mPTv indirectly reduces medial wishboning by pulling the caudal elements against the pterygoid buttress, inducing a powerful, laterally directed reaction force on the caudal elements. However, while sauropsids appear susceptible to medial wishboning owing to the prominent medially directed pull of their jaw muscles, overall relationships between IMJ form and flexibility are unclear, as groups with complex intramandibular sutures may be akinetic (e.g., crocodylians) or kinetic (e.g., varanids). Further research will clarify IMJ morphological diversity and disparity among reptiles and divulge form-function relationships of this critical, but underappreciated, aspect of their feeding apparatus.

[Andreas Johansson]

Do Agnolin et al. intend any difference between ”non-crown” and ”stem”?

—
Andreas Johansson

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