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Laosuchus is an extinct genus of chroniosuchian that lived around the Permian-Triassic boundary^[1]^[2]. It is known from the skull of a single species, Laosuchus naga.

History and Etymology

L. naga was found in the Luang Prabang Basin of Northern Laos, part of the Indochina block. The site was first discovered by J. B. H. Counillon in 1896 as part of the Pavie's third Mission. Counillon was tasked with mapping mineral resources for the French colonial empire. L. naga was discovered during a 2005 expedition to the area, along with remains of dicynodonts^[3]^[4]. It was later described by Arbez, Sidor, and Steyer in 2018.^[1] Its name comes from the Nāga, a snake-like deity that appears in multiple east Asian religions.

Description and Phylogeny

Laosuchus naga is represented by a single skull and articulated left hemimandible designated as specimen MDS-LPQ 2005-09, stored at the Musée des Dinosaures in Savannakhet. The skull, roughly 26 centimeters in length, is similar in shape to that of crocodiles. Its long snout bore marginal labyrinthodont teeth with an average height of 9 millimeters. Its nares are similar in shape to Madygenerpeton pustulatus. Like M. pustulatus, it also has oval-shaped orbits that are raised above the skull roof, but the orbits are proportionally smaller. Its choanae are relatively long compared to other chroniosuchians^[1].

L. naga has a number of traits that make it unique among chroniosuchia. It lacks palatal tusks, bearing only small denticles on the palate. Its pineal foramen is significantly reduced with a 1mm diameter(M. pustulatus, C. dongusensis, and B. schumanni have diameters of 2mm, 2.5mm, and 3.5mm respectively^[5]^[6]). A transverse flange extends from the pterygoid, contacting the maxilla. CT-scanning revealed an autapomorphic internal crest on the dorsal palate that follows the internal margin of the choanae. The parasphenoid bears a thin ventro-medial ridge, a condition also seen in Discosauriscus austriacus^[7] Its tabular horn and posterior squamosal contact, closing the otic notch^[1].

There are a number of traits that L. naga share with other chroniosuchians. It bears a subtriangular fontanelle on its premaxilla, a poorly ossified braincase, a well developed pterygoid flange, and a relatively narrow parasphenoid bearing a crest between the basicranial joint^[1].

CT-scanning also revealed the presence of canals within the mandible, snout, and bones in front of the orbit. The canals in the skull roof are interpreted as being related to a lateral line system. Canals in the tip of the snout and mandible are more complex and could be neurovascular canals, which modern animals use for thermoreception, electroreception, or mechanoreception. The paleontologists who described L. naga suggest that the lateral line system was used to detect prey beneath the water surface while the neurovascular system could let it detect movement at the water surface similar to modern crocodiles^[1].

Phylogenetic relationships

Geological context and implications

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^ ^a ^b ^c ^d ^e ^f Arbez, Thomas; Sidor, Christian A.; Steyer, J.-Sébastien (2019-07-18). "Laosuchus naga gen. et sp. nov., a new chroniosuchian from South-East Asia (Laos) with internal structures revealed by micro-CT scan and discussion of its palaeobiology". Journal of Systematic Palaeontology. 17 (14): 1165–1182. doi:10.1080/14772019.2018.1504827. ISSN 1477-2019.
^ Rossignol, Camille; Bourquin, Sylvie; Poujol, Marc; Hallot, Erwan; Dabard, Marie-Pierre; Nalpas, Thierry (2016-04-15). "The volcaniclastic series from the Luang Prabang Basin, Laos: A witness of a triassic magmatic arc?". Journal of Asian Earth Sciences. 120: 159–183. doi:10.1016/j.jseaes.2016.02.001. ISSN 1367-9120.
^ Steyer, J. Sebastien (2009). "The geological and palaeontological exploration of Laos; following in the footsteps of J. B. H. Counillon and A. Pavie". Geological Society, London, Special Publications. 315 (1): 25–32. doi:10.1144/SP315.3. ISSN 0305-8719.
^ Bercovici, Antoine; Bourquin, Sylvie; Broutin, Jean; Steyer, Jean-Sébastien; Battail, Bernard; Véran, Monette; Vacant, Renaud; Khenthavong, Bounxou; Vongphamany, Sotsy (2012-10-22). "Permian continental paleoenvironments in Southeastern Asia: New insights from the Luang Prabang Basin (Laos)". Journal of Asian Earth Sciences. 60: 197–211. doi:10.1016/j.jseaes.2012.08.019. ISSN 1367-9120.
^ Clack, J., & Klembara, J. (2009). AN ARTICULATED SPECIMEN OF CHRONIOSAURUS DONGUSENSIS AND THE MORPHOLOGY AND RELATIONSHIPS OF THE CHRONIOSUCHIDS. Patterns And Processes In Early Vertebrate Evolution, 81(81), 15-42.
^ Schoch, Rainer R.; Voigt, Sebastian; Buchwitz, Michael (2010-11-01). "A chroniosuchid from the Triassic of Kyrgyzstan and analysis of chroniosuchian relationships". Zoological Journal of the Linnean Society. 160 (3): 515–530. doi:10.1111/j.1096-3642.2009.00613.x. ISSN 0024-4082.
^ Klembara, Jozef (1997-03-29). "The cranial anatomy of Discosauricsus Kuhn, a seymouriamorph tetrapod from the Lower Permian of the Boskovice Furrow (Czech Republic)". Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences. 352 (1351): 257–302. doi:10.1098/rstb.1997.0021. PMC 1691931.{{cite journal}}: CS1 maint: PMC format (link)

[:03-1] ^ ^a ^b ^c ^d ^e ^f Arbez, Thomas; Sidor, Christian A.; Steyer, J.-Sébastien (2019-07-18). "Laosuchus naga gen. et sp. nov., a new chroniosuchian from South-East Asia (Laos) with internal structures revealed by micro-CT scan and discussion of its palaeobiology". Journal of Systematic Palaeontology. 17 (14): 1165–1182. doi:10.1080/14772019.2018.1504827. ISSN 1477-2019.

[2] Rossignol, Camille; Bourquin, Sylvie; Poujol, Marc; Hallot, Erwan; Dabard, Marie-Pierre; Nalpas, Thierry (2016-04-15). "The volcaniclastic series from the Luang Prabang Basin, Laos: A witness of a triassic magmatic arc?". Journal of Asian Earth Sciences. 120: 159–183. doi:10.1016/j.jseaes.2016.02.001. ISSN 1367-9120.

[3] Steyer, J. Sebastien (2009). "The geological and palaeontological exploration of Laos; following in the footsteps of J. B. H. Counillon and A. Pavie". Geological Society, London, Special Publications. 315 (1): 25–32. doi:10.1144/SP315.3. ISSN 0305-8719.

[4] Bercovici, Antoine; Bourquin, Sylvie; Broutin, Jean; Steyer, Jean-Sébastien; Battail, Bernard; Véran, Monette; Vacant, Renaud; Khenthavong, Bounxou; Vongphamany, Sotsy (2012-10-22). "Permian continental paleoenvironments in Southeastern Asia: New insights from the Luang Prabang Basin (Laos)". Journal of Asian Earth Sciences. 60: 197–211. doi:10.1016/j.jseaes.2012.08.019. ISSN 1367-9120.

[5] Clack, J., & Klembara, J. (2009). AN ARTICULATED SPECIMEN OF CHRONIOSAURUS DONGUSENSIS AND THE MORPHOLOGY AND RELATIONSHIPS OF THE CHRONIOSUCHIDS. Patterns And Processes In Early Vertebrate Evolution, 81(81), 15-42.

[6] Schoch, Rainer R.; Voigt, Sebastian; Buchwitz, Michael (2010-11-01). "A chroniosuchid from the Triassic of Kyrgyzstan and analysis of chroniosuchian relationships". Zoological Journal of the Linnean Society. 160 (3): 515–530. doi:10.1111/j.1096-3642.2009.00613.x. ISSN 0024-4082.

[7] Klembara, Jozef (1997-03-29). "The cranial anatomy of Discosauricsus Kuhn, a seymouriamorph tetrapod from the Lower Permian of the Boskovice Furrow (Czech Republic)". Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences. 352 (1351): 257–302. doi:10.1098/rstb.1997.0021. PMC 1691931.{{cite journal}}: CS1 maint: PMC format (link)

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