Estudios Geológicos 78 (1)
enero-junio 2022, e145
ISSN-L: 0367-0449
https://doi.org/10.3989/egeol.44639.613

First record of fossil sauropterygians from the Upper Triassic of Southwestern Spain (Ayamonte, Huelva province)

Primer registro de sauropterigios fósiles del Triásico Superior del suroeste de España (Ayamonte, provincia de Huelva)

Matías Reolid

Departamento de Geología, Universidad de Jaén, Jaén, Spain.

https://orcid.org/0000-0003-4211-3946

Fernando Muñiz

Departamento de Cristalografía, Mineralogía y Química Agrícola, Universidad de Sevilla, Sevilla, Spain.

https://orcid.org/0000-0002-5727-3646

Antonio Toscano

Departamento de Ciencias de la Tierra, Universidad de Huelva, Huelva, Spain.

https://orcid.org/0000-0003-2144-5714

Zain Belaústegui

Departament de Dinàmica de la Terra i de l’Oceà, Facultat de Ciències de la Terra, Universitat de Barcelona, Barcelona, Spain
Institut de Recerca de la Biodiversitat (IRBio), Universitat de Barcelona, Barcelona, Spain

https://orcid.org/0000-0002-1707-9670

ABSTRACT

This work reports the first record of a sauropterygian reptile remain from the uppermost Triassic of the westernmost part of the South Iberian Palaeomargin. The fossil bone, found in the Upper Triassic carbonate succession of Ayamonte (Huelva, Spain), corresponds to a neural arch of a sauropterygian. The carbonate succession was deposited in very shallow marine environment simultaneous with igneous activity during the Rhaetian (latest Triassic). The studied remain is isolated, disarticulated and presents fractures that evidence transport but also potential activity of scavengers. The neural arch is preserved as calcium phosphate enriched in some elements (e.g. Sr) relative to the surrounding carbonate sediment. The anatomic features do not allow a detailed taxonomic identification. This remain extends the record of sauropterygians to the westernmost end of the Tethys following the South Iberian Palaeomargin and evidences the colonization of the Algarve Basin during the extensional phase related with the progress of the rifting of Pangaea and the opening of the Tethys to the west.

Keywords: 
Fossil vertebrate; Germanic facies; Upper Triassic; Westernmost Tethys
RESUMEN

Este trabajo se centra en el estudio del primer registro de un resto fósil de sauropterigio procedente del Triásico superior del sector oriental del Paleomargen Suribérico. El resto, encontrado en la sucesión sedimentaria carbonatada del Triásico que aflora en Ayamonte (Huelva, España), corresponde a un arco neural de un sauropterigio. La sucesión carbonatada se depositó en un ambiente marino muy somero simultáneamente a cierta actividad ígnea durante el Rhaetiense (Triásico superior). El resto fósil aparece aislado, desarticulado y presenta algunas fracturas que evidencian cierto transporte por corrientes, sin descartar la posible interacción de organismos carroñeros. El arco neural está conservado como fosfato cálcico con enriquecimiento en Sr, de acuerdo con los mapeos composicionales realizados sobre el resto fósil y el sedimento circundante. Los rasgos anatómicos no han permitido su identificación taxonómica detallada. El resto estudiado extiende el registro de reptiles sauropterigios hacia el occidente colonizando el Paleomargen Suribérico, en este caso la Cuenca del Algarve, conforme se producía la rotura (rifting) de Pangea y el Tethys se abría paso hacia el oeste con la progresiva inundación de nuevas cuencas.

Palabras clave: 
Vertebrado fósil; Facies Germánicas; Triásico superior; Tethys Occidental

Recibido el 19 de marzo de 2022; Aceptado el 9 de junio de 2022; Publicado online el 21 de julio de 2022

Citation/Cómo citar este artículo: Reolid, M. et al. (2022). First record of fossil sauropterygians from the Upper Triassic of Southwestern Spain (Ayamonte, Huelva province). Estudios Geológicos 78(1): e145. https://doi.org/10.3989/egeol.44639.613.

CONTENT

Introduction

 

Marine reptiles were diverse and abundant in the Triassic marine ecosystems of Western Tethys, both in the Germanic and in the Alpine basins, forming part of the Germanic Bioprovince and the Tethyan Bioprovince, as compiled in Rieppel (2000)Rieppel, O. (2000). Sauropterygia I. In: Encyclopedia of Paleoherpetology (Wellnhofer, P., Ed.), 12A, Dr. Friedrich Pfeil Verlag, München, 134 pp. and Motani (2009)Motani, R. (2009). The evolution of marine reptiles. Evolution: Education and Outreach, 2: 224-235. https://doi.org/10.1007/s12052-009-0139-y . In the Alpine Triassic, in addition to Sauropterygia, Ichthyosauria and Thalattosauria have also been recorded (e.g. Müller, 2005Müller, J. (2005). The anatomy of Askeptosaurus italicus from the Middle Triassic of Monte San Giorgio, and the interrelationships of thalattosaurs (Reptilia, Diapsida). Canadian Journal of Earth Sciences, 42: 1347-1367. https://doi.org/10.1139/e05-030 ; Dalla Vecchia, 2006Dalla Vecchia, F.M. (2006). A new sauropterygian reptile with plesiosaurian affinity from the Later Triassic of Italy. Rivista Italiana di Paleontologia e Stratigrafia, 112: 207-225., 2008Dalla Vecchia, F.M. (2008). First record of Simosaurus (Sauropterygia, Nothosauroidea) from the Carnian (Late Triassic) of Italy. Rivista Italiana di Paleontologia e Stratigrafia, 114: 273-285.; Kolb et al., 2011Kolb, C.; Sánchez-Villagra, M.R. & Scheyer, T.M. (2011). The palaeohistology of the basal ichthyosaur Mixosaurus Baur, 1887 (Ichthyopterygia, Mixosauridae) from the Middle Triassic: Palaeobiological implications. Comptes Rendus Palevol, 10: 403-411. https://doi.org/10.1016/j.crpv.2010.10.008 ). The Besano Formation in the Southern Alps from Italy and Switzerland, is especially rich in marine reptiles (e.g. Tschanz, 1989Tschanz, K. (1989). Lariosaurus buzzii n. sp. from the Middle Triassic of Monte San Giorgio (Switzerland) with comments on the classification of nothosaurs. Palaeontographica Abteilung A 208: 137-161.; Beardmore & Furrer, 2016Beardmore, S.R. & Furrer, H. (2016). Preservation of Pachypleurosauridae (Reptilia; Sauropterygia) from the Middle Triassic of Monte San Giorgio, Switzerland. Neues Jahrbuch fur Geologie und Palaontologie-Abhandlungen, 280: 221-240. https://doi.org/10.1127/njgpa/2016/0578 ; Rieppel, 2019Rieppel, O. (2019). Mesozoic sea dragons: Triassic marine life from the Ancient tropical lagoon of Monte San Giorgio. Indiana University Press, 256 pp. https://doi.org/10.2307/j.ctvd58t86 ). At the Germanic Basin, from the Muschelkalk facies, the most abundant reptiles in shallow marine sediments were Sauropterygia (e.g. Rieppel, 2000Rieppel, O. (2000). Sauropterygia I. In: Encyclopedia of Paleoherpetology (Wellnhofer, P., Ed.), 12A, Dr. Friedrich Pfeil Verlag, München, 134 pp.; Klein et al., 2015Klein, N.; Voeten, D.F.A.E.; Lankamp, J.; Bleeker, R.; Sichelschmidt, O.J.; Liebrand, M.; Nieweg, D.C. & Sander, P.M. (2015). Postcranial material of Nothosaurus marchicus from the Lower Muschelkalk (Anisian) of Winterswijk, The Netherlands, with remarks on swimming styles and taphonomy. Paläontologische Zeitzfrich, 89: 961-981. https://doi.org/10.1007/s12542-015-0273-5 ).

The Germanic facies also developed along the Iberian Palaeomargin, today outcropping in the Algarve Basin, the Betic External Zones, the Iberian Range, the Catalonian Coastal Range and the Pyrenean Cordillera. The record of marine reptiles in these areas is also relatively rich but most of the reported remains consist of isolated elements (e.g. Sanz, 1976Sanz, J.L. (1976). Lariosaurus balsami (Sauropterygia, Reptilia) de Estaeda (Huesca). Estudios Geológicos, 32: 547-567., 1983aSanz, J.L. (1983a). Los Nothosaurios (Reptilia, Sauropterygia) Españoles. Estudios Geológicos, 39: 193-215., bSanz, J.L. (1983b). Consideraciones sobre el género Pistosaurus. El suborden Pistosauria (Reptilia, Sauropterygia). Estudios Geológicos, 39: 451-458.; Alafont, 1992Alafont, L.S. (1992). Notosaurios y Placodontos (Reptilia) del Triásico Medio de Bienservida-Villarrodrigo. Instituto de Estudios Albacetenses, Diputación de Albacete, Serie 1, 60, Albacete, Spain, 131 pp.; Sanz et al., 1993Sanz, J.L.; Alafont, L.S. & Moratalla, J.J. (1993). Triassic reptile faunas from Spain. In: Evolution, Ecology and Biogeography of the Triassic Reptiles, (Mazin, J.M. & Pinna, G., Eds.), Paleontologia Lombarda NS, 2: 153-164.; Niemeyer, 2002Niemeyer, J. (2002). Invertebraten und Vertebraten aus dem Muschelkalk von Siles (Jaén), Spanien. Münstersche Forschungen zur Geologie und Paläontogie, 94: 1-99.; Quesada & Aguera González, 2005Quesada, J.M. & Aguera González S. (2005). Descripción del primer ejemplar de Ceresiosaurus (Reptilia: Sauropterygia), hallado en la Península Ibérica en el Ladiniense (Triásico Medio) de Mont-Ral-Alcover (Tarragona). Estudios Geológicos, 61: 247-269. https://doi.org/10.3989/egeol.05613-667 ; Fortuny et al., 2011Fortuny, J.; Bolet, A.; Sellés, A.G.; Cartanyà, J. & Galobart, A. (2011). New insights on the Permian and Triassic vertebrates from the Iberian Peninsula with emphasis on the Pyrenean and Catalonian basins. Journal of Iberian Geology, 37: 65-86. https://doi.org/10.5209/rev_JIGE.2011.v37.n1.5 ; Reolid et al., 2014Reolid, M.; Pérez-Valera, F.; Benton, M.J. & Reolid, J. (2014). Marine flooding event in continental Triassic facies identified by a nothosaur and placodont bone bed (South Iberian Paleomargin). Facies, 60: 277-293. https://doi.org/10.1007/s10347-013-0360-6 ; de Miguel Chaves et al., 2015De Miguel Chaves, C.A.; Pérez-García, A.; Cobos, R.; Royo-Torres, F.; Ortega, F. & Alcalá, L. (2015). A diverse Late Triassic tetrapod fauna from Manzanera (Teruel, Spain). Geobios, 48: 479-490. https://doi.org/10.1016/j.geobios.2015.09.002 , 2016Miguel Chaves, C. de; García-Gil, S.; Ortega, F.; Sanz, J.L. & Pérez-García, A. (2016). First Triassic tetrapod (Sauropterygia, Nothosauridae) from Castilla y León: evidence of an unknown taxon for the Spanish record. Journal of Iberian Geology, 42: 29-38. https://doi.org/10.5209/rev_JIGE.2016.v42.n1.51210 , 2017Miguel Chaves, C. de; Ortega, F. & Pérez-García, A. (2017). The eosauropterygian fossils from the Middle Triassic of Canales de Molina (Guadalajara, Spain). Journal of Iberian Geology, 43: 129-138. https://doi.org/10.1007/s41513-017-0011-9 , 2018Miguel Chaves, C. de; Ortega, F. & Pérez-García, A. (2018). New highly pachyostotic nothosauroid interpreted as a filter-feeding Triassic marine reptile. Biology Letters, 14: 20180130. https://doi.org/10.1098/rsbl.2018.0130 , 2020Miguel Chaves, C. de; Ortega, F. & Pérez-García, A., 2020. The Iberian Triassic fossil record of Sauropterygia: an update. Journal of Iberian Geology, 46: 445-464. https://doi.org/10.1007/s41513-020-00137-w ; Campos & Mateus, 2018Miguel Chaves, C. de; Ortega, F. & Pérez-García, A., 2020. The Iberian Triassic fossil record of Sauropterygia: an update. Journal of Iberian Geology, 46: 445-464. https://doi.org/10.1007/s41513-020-00137-w ; Márquez-Aliaga et al., 2019Márquez-Aliaga, A.; Klein, N.; Reolid, M.; Plasencia, P.; Villena, J.A. & Martínez-Pérez, C. (2019). An enigmatic marine reptile, Hispaniasaurus cranioelongatus (gen. et sp. nov.) with nothosauroid affinities from the Ladinian of the Iberian Range (Spain). Historical Biology, 31: 223-233. https://doi.org/10.1080/08912963.2017.1359264 ; Pérez-Valera et al., 2020Pérez-Valera, J.A.; Berrocal-Casero, M. & Pérez-Valera, F. (2020). First Triassic tetrapod (Eusauropterygia) in the Triassic of the Subbetic domain of the Betic Cordillera (Southeastern Spain). Paläontologische Zeitzfrich, 94: 343-352. https://doi.org/10.1007/s12542-019-00500-y ; Ruciński, 2020Ruciński, M.R. (2020). Novel placodont material and paleoenvironment analysis of Triassic deposits of Rocha da Pena (Algarve, southern Portugal). MSc Universidade Nova de Lisboa, 104 pp.). Only one record of sauropterygians in the Iberian Peninsula comes from Alpine facies, being an isolated rib from the Ladinian of the Alpujarride Complex (Betic Internal Zones; Reolid & Reolid, 2020Reolid, M. & Reolid, J. (2020). First recod of Triassic marine reptiles (Nothosauria, Sauropterygia) from the Alpujarride Complex (Internal Zones of the Betic Cordillera, Spain). Estudios Geológicos, 76: e126. https://doi.org/10.3989/egeol.43592.535 ).

In the southwestern part of the Iberian Palaeomargin the record of marine reptiles is comparatively scarce. Only at the Algarve Basin (South Portugal), in the Carnian Grés de Silves Group, fossil vertebrates have been reported from the Rocha da Pena bonebed with numerous temnospondyl remains of Metoposaurus algarvensis (Brusatte et al., 2015Brusatte, S.L.; Butler, R.J.; Mateus, O. & Steyer, J.S. (2015). A new species of Metoposaurus from the Late Triassic of Portugal and comments on the systematics and biogeography of metoposaurid temnospondyls. Journal of Vertebrate Paleontology, 35: e912988. https://doi.org/10.1080/02724634.2014.912988 ). In the Grés de Silves Group have been also reported a probably basal representative of phytosaurs (Mateus et al., 2014Mateus, O.; Butler, R.J.; Brusatte, S.L.; Whiteside, J.H. & Steyer, J.S. (2014). The first phytosaur (Diapsida, Archosauriformes) from the Late Triassic of the Iberian Peninsula. Journal of Vertebrate Paleontology, 34: 970-975. https://doi.org/10.1080/02724634.2014.840310 ) and the placodont Henodus; the latter represented by isolated remains including osteoderms and cranial remains (Campos et al., 2017Campos, H.; Mateus, O. & Moreno-Azanza, M. (2017). Preliminary results on the stratigraphy and taphonomy of multiple bonebeds in the Triassic of Algarve. XV Encuentro de Jóvenes Investigadores en Paleontología, Pombal, Portugal, 83-87.; Campos & Mateus, 2018Campos, H. & Mateus, O. (2018). The first record of placodonts in Portugal and its chronological and paleoecological implications. XVI annual meeting of the European Association of Vertebrate Palaeontologists, Caparica, Portugal, 38.; Ruciński, 2020Ruciński, M.R. (2020). Novel placodont material and paleoenvironment analysis of Triassic deposits of Rocha da Pena (Algarve, southern Portugal). MSc Universidade Nova de Lisboa, 104 pp.).

The objective of this short note is reporting the first record of an isolated vertebra of sauropterygians from the Upper Triassic of the eastern part of the Algarve Basin in Ayamonte (Huelva province, Southwestern Spain).

Geological setting

 

The studied remain was recorded in the Triassic deposits that outcrops in the east side of the Guadiana River, close to Ayamonte town, and more exactly in the hill of the Parador Nacional (coord. 37º13´32´´N, 7º24´26´´W) (Fig. 1A and B). The Triassic of Ayamonte constitute the eastern end of the Algarve Basin. This basin is extended in E-W direction from Ayamonte (Spain) to Cape San Vicente (Portugal) and constitutes the westernmost part of the South Iberian Palaeomargin.

medium/medium-EGEOL-78-01-e145-gf1.png
Figure 1.  Geological setting. A. Geological map of southwestern Iberia with indication of Ayamonte Town. B. Geological map of the surroundings of the Ayamonte (Huelva) with location of the outcrop with fossil remain (modified from Alonso-Chaves et al., 2020Alonso-Chaves, F.M.; García-Navarro, E.; Fernández, C. & Mayoral, E. (2020). Tectónica extensional durante el Triásico Superior en el extremo oriental de la cuenca del Algarve (Ayamonte, España) y la reactivación de fallas durante el Plioceno-Cuaternario. Geogaceta, 67: 19-22.). C. Lithological columns on the eastern margin of the Guadiana River (modified from Alonso-Chaves et al., 2020Alonso-Chaves, F.M.; García-Navarro, E.; Fernández, C. & Mayoral, E. (2020). Tectónica extensional durante el Triásico Superior en el extremo oriental de la cuenca del Algarve (Ayamonte, España) y la reactivación de fallas durante el Plioceno-Cuaternario. Geogaceta, 67: 19-22.) and location of the fossil bone.

In Ayamonte, the Triassic is unconformably overlying the shales of the Lower Carboniferous. The lower part of the Triassic succession (around 15 m thick) is constituted by red siltstones and sandstones with gypsum and some microconglomerate intervals. These deposits were interpreted as related to continental or coastal deposits (Alonso-Chaves et al., 2020Alonso-Chaves, F.M.; García-Navarro, E.; Fernández, C. & Mayoral, E. (2020). Tectónica extensional durante el Triásico Superior en el extremo oriental de la cuenca del Algarve (Ayamonte, España) y la reactivación de fallas durante el Plioceno-Cuaternario. Geogaceta, 67: 19-22.; Santos et al., 2022Santos, A.; Popovic, N. & Mayoral, E. (2022). Palaeoecology of Late Triassic marine assemblages from the proto-Atlantic Basin (Ayamonte, SW Spain). Proceedings of the Geologists Association, 133: 47-66. https://doi.org/10.1016/j.pgeola.2021.11.002 ). The trace fossils (Taenidium isp. Labyrintichnus terrerensis, Planolites isp.) are indicative of the Scoyenia ichnofacies typical of swamp-like to alluvial plain environments (Santos et al., 2022Santos, A.; Popovic, N. & Mayoral, E. (2022). Palaeoecology of Late Triassic marine assemblages from the proto-Atlantic Basin (Ayamonte, SW Spain). Proceedings of the Geologists Association, 133: 47-66. https://doi.org/10.1016/j.pgeola.2021.11.002 ). These deposits are the continuation (in the eastern side of the Guadiana River) of the Grés de Silves Group, which was described in the Portuguese part of the Algarve Basin and assigned to the Carnian by Palain (1976)Palain, C., 1976. Une série détrique terrigène. Les «Grès de SiIves»: Trias et Lias inférieur du Portugal. Memória dos Serviços Geológicos de Portugal, Nova Série, 25: 377 pp.. The beginning of the sedimentation in this basin occurred during the early Carnian according to the palynological analysis of Vilas-Boas et al. (2022)Vilas-Boas, M.; Paterson, N.W.; Pereira, Z.; Fernandes, P. & Cirilli, S. (2022). The age of the first pulse of continental rifting associated with the breakup of Pangea in Southwest Iberia; new palynological evidence. Journal of Iberian Geology, 48: 181-190. https://doi.org/10.1007/s41513-022-00189-0 .

The upper part of the Triassic sedimentary succession (around 26 m thick) is constituted by a carbonate interval composed by marlstones, marly-limestones, limestones and dolostones interbedded with basic volcanic rocks (dolerites) and volcano-sedimentary deposits to the top (see detailed description in Alonso-Chaves et al., 2020Alonso-Chaves, F.M.; García-Navarro, E.; Fernández, C. & Mayoral, E. (2020). Tectónica extensional durante el Triásico Superior en el extremo oriental de la cuenca del Algarve (Ayamonte, España) y la reactivación de fallas durante el Plioceno-Cuaternario. Geogaceta, 67: 19-22.; Fig. 1C). The faunal assemblage of the succession is dominated by bivalves (mainly Trigonodus, Isocyprina, Pleuromya and Isognonom) and gastropods (mainly Coelostylina, Spirostylus and Mathilda). Santos et al. (2022)Santos, A.; Popovic, N. & Mayoral, E. (2022). Palaeoecology of Late Triassic marine assemblages from the proto-Atlantic Basin (Ayamonte, SW Spain). Proceedings of the Geologists Association, 133: 47-66. https://doi.org/10.1016/j.pgeola.2021.11.002 recorded Palaeonucula subovalis, Modiolus cf. minimus, and Isocyprina concentrica; bivalves indicative of a Rhaetian age (Vörös, 1981Vörös, A. (1981). A survey of the Rhaetian (Upper Triassic) Bivalvia from Borzavár (Bakony Mts., Hungary). Annales Musei Nationalis Hungaricï, 73: 33-54. ; Márquez-Aliaga et al., 2010Márquez-Aliaga, A.; Damborenea, S.; Gómez, J.J. & Goy, A. (2010). Bivalves from the Triassic-Jurassic transition in northern Spain (Asturias and western Basque-Cantabrian Basin). Ameghiniana, 47: 185-205. https://doi.org/10.5710/AMGH.v47i2.3 ). The stratigraphic record of other genera of mollusks reported from this section support the assignation to the uppermost Triassic. According to Ros (2009)Ros, S. (2009). Dinámica de la paleodiversidad de los bivalvos del Triásico y Jurásico Inferior. PhD Thesis Universidad de Valencia, 564 pp. and Márquez-Aliaga et al. (2010)Márquez-Aliaga, A.; Damborenea, S.; Gómez, J.J. & Goy, A. (2010). Bivalves from the Triassic-Jurassic transition in northern Spain (Asturias and western Basque-Cantabrian Basin). Ameghiniana, 47: 185-205. https://doi.org/10.5710/AMGH.v47i2.3 , the first occurrence of bivalves Pteromya cf. tatei and the genus Isocyprina is in Rhaetian rocks. The first occurrence of the gastropod Cylindrobullina cf. avenoides in Europe and South America is also recorded in the Rhaetian (Ferrari, 2015Ferrari, S.M. (2015). Systematic revision of Late Triassic marine gastropods from Central Perú: considerations on the Late Triassic/Early Jurassic faunal turnover. Andean Geology, 42: 71-96.).

According to the presence of trace fossils (Thalassinoides isp., Treptichnus pollardi and Helminthoidichnites tenuis), fossil macroinvertebrates and sedimentary structures, Santos et al. (2022)Santos, A.; Popovic, N. & Mayoral, E. (2022). Palaeoecology of Late Triassic marine assemblages from the proto-Atlantic Basin (Ayamonte, SW Spain). Proceedings of the Geologists Association, 133: 47-66. https://doi.org/10.1016/j.pgeola.2021.11.002 have interpreted a shallow marginal environment with brackish and low energy conditions.

Materials and methods

 

The studied remain appeared in the broken surface of a red limestone block and it is constituted by two halves identified as NOT/AY/001a and NOT/AY/001b. Polished slabs and thin sections of the red limestone block have been prepared and the microfacies being analysed with an Olympus SZ60 microscope at the Universidad de Jaén.

The fossil bone and hosting rock were scanned at the Universidad de Jaén using a Bruker XR-microfluorescence M4 Tornado equipped with a rhodium target X-ray tube with a high voltage of 50 kV, a current of 600 μA and pressure of 20 mbar. The spotsize of the X-ray optics was 25 μm. The maximum penetration depth from which fluorescence X-rays can still reach the detector is less than 20 μm. This low penetration allowed analyses of the sediment surface showing lateral compositional changes, especially the contrast between the fossil bone and the surrounding sedimentary rock. The geochemical compositional maps obtained for each element are represented by a range of colour intensity that indicates the relative concentration of each element.

Results

 

The fossil bone is recorded in red to purple limestone which is roughly laminated with calcisiltite to calcarenite thin layers, locally rich in organic matter and native sulfur. The sediment surrounding the fossil bone is constituted by a bioclastic packstone of peloids, lumps and thin-shelled bivalves. Bivalves are commonly disarticulated and concave-up in the bed, most of them < 1 cm (Fig. 2A). Microgastropods and coal fragments are also common in the bed with the fossil bone.

medium/medium-EGEOL-78-01-e145-gf2.png
Figure 2.  A. Polished slab showing the presence of abundant thin-shelled disarticulated bivalves. B. View of the neural arch of sauropterygian. C. Sketch of the neural arch with indication of anatomic parts. D. Detail of the spongy tissue preserved in the fossil bone with infilling of iron oxides and growth of pyrolusite in the bone surface.

The fossil bone is an isolated vertebral arch without centrum (Figs. 2b and 2C). The maximum width of the neural arch is 46 mm and the maximum height is 44 mm. The neural arch presents a high and thin neural spine (31 mm high and 4 mm width). The square-sided transverse processes are thick (10 mm high and 21 mm width) and the distal margins are slightly rounded. The transverse processes present areas with spongy bone tissue. The neural canal or vertebral foramen is elevated and relatively circular with 8 mm in diameter. The development of the transverse apophysis sensu Alafont (1992)Alafont, L.S. (1992). Notosaurios y Placodontos (Reptilia) del Triásico Medio de Bienservida-Villarrodrigo. Instituto de Estudios Albacetenses, Diputación de Albacete, Serie 1, 60, Albacete, Spain, 131 pp. (maximum width of neural arch / height of neural canal) is 5.76, whereas the development of the neural canal (neural canal height*100 / neural arch height) is 18.26. Since the studied specimen is a section in a fracture surface, it is not possible to study the morphology of the zygosphene-zygantrum articulation.

The upper part of the left transverse process presents a semicircular fracture with around 1 cm in diameter (Fig. 2B and C). Other minor fractures are evidenced in other places of the transverse processes and the neural spine. The spongy bone tissue present infilling by iron oxides whereas pyrolusite is locally observable at the surface (Fig. 2D).

According to the geochemical compositional maps (Fig. 3), the bone is preserved as calcium phosphate. The Sr content is especially high in the bone and is absent in the surrounding sediment which is enriched in Si, Fe, Ba and Cu in respect to the bone.

medium/medium-EGEOL-78-01-e145-gf3.png
Figure 3.  Compositional maps of XR-microfluorescence of the fossil neural arch highlighting the distribution of Ca, P, Sr, Fe, Ba and Cu. Note the intensity of colour indicate more relative concentration.

Discussion

 

Taxonomic attribution

 

The isolated neural arch is attributed to a sauropterygian as evidenced by the morphology of the thin and elongated neural spine with relatively reduced development of the square-sided transverse processes. These features allow to exclude the assignation to phytosaurs, because this group exhibit more robust vertebrae with thicker neural spines for supporting dorsal armor plates and thick and large transverse processes (Case, 1932Case, E.C. (1932). A perfectly preserved segment of the armor of a phytosaur, with associated vertebrae. Contributions from the Museum of Paleontology Michigan University, 4: 57-80.; Lucas et al., 2002Lucas, S.G.; Heckert, A.B. & Kahle, R. (2002). Postcranial anatomy of Angistorhinus, a late Triassic phytosaur from West Texas. New Mexico Museum of natural History and Science Bulletin, 21: 157-164.; Witzmann et al., 2014Witzmann, F.; Schwarz-Wings, D.; Hampe, O.; Fritscg, G. & Asbach, P. (2014). Evidence of spondyloarthropathy in the spine of a Phytosaur (Reptilia: Archosauriformes) from the Late Triassic of Halberstadt, Germany. PLos ONE 9: e85511. https://doi.org/10.1371/journal.pone.0085511 ). Other typical aquatic diapsid reptiles of Late Triassic (Nicholls, 1999Nicholls, E.L. (1999) A reexamination of Thalattosaurus and Nectosaurus and the relationships of the Thalattosauria (Reptilia, Diapsida). PaleoBios, 19: 1-29.; Rieppel et al., 2000Rieppel, O., Liu, J. & Bucher, H. (2000). The first record of a thalaltosaur reptile from the late Triassic of Southern China (Guizhou Province, PR China). Journal of Vertebrate Paleontology, 20: 507-514. https://doi.org/10.1671/0272-4634(2000)020[0507:TFROAT]2.0.CO;2 ), the thalattosaurians, are also excluded due to the characteristic small neural canal of vertebrae in spite of the neural archs of thalattosaurs are of moderate height, almost in the Superfamily Askeptosauroidea (Müller, 2005Müller, J. (2005). The anatomy of Askeptosaurus italicus from the Middle Triassic of Monte San Giorgio, and the interrelationships of thalattosaurs (Reptilia, Diapsida). Canadian Journal of Earth Sciences, 42: 1347-1367. https://doi.org/10.1139/e05-030 ).

The development of the transverse apophysis shows a ratio of 5.76 whereas this value is around 7.3 in the Family Pachypleurosauridae and ranges between 3.7 to more than 8.5 in Placodontia (Alafont, 1992Alafont, L.S. (1992). Notosaurios y Placodontos (Reptilia) del Triásico Medio de Bienservida-Villarrodrigo. Instituto de Estudios Albacetenses, Diputación de Albacete, Serie 1, 60, Albacete, Spain, 131 pp.). Transverse processes of the dorsal vertebrae are specially elongated in placodonts such as Placodus gigas (Rieppel, 2000Rieppel, O., Liu, J. & Bucher, H. (2000). The first record of a thalaltosaur reptile from the late Triassic of Southern China (Guizhou Province, PR China). Journal of Vertebrate Paleontology, 20: 507-514. https://doi.org/10.1671/0272-4634(2000)020[0507:TFROAT]2.0.CO;2 ). This ratio in the dorsal vertebrae documented by Segesdi & Osi (2021)Segesdi, M. & Ösi, A. (2021). Sauropterygian remains from the Middle Triassic of Villány, Nungary - new information on the aquatic reptile fauna of Tisza Magaunit (Triassic southern Eurasian shelf region). Palaeobiodiversity and Palaeoenvironments, 101: 985-1011. https://doi.org/10.1007/s12549-020-00480-x ranges from 5.2 to 6.4 for Nothosaurus and is around 5 for simosaurids.

The development of the neural canal in the studied vertebra is 18.26, whereas this ratio is 27.8 in pachypleurosaurids and from 25 to 31 in placodontids (Alafont, 1992Alafont, L.S. (1992). Notosaurios y Placodontos (Reptilia) del Triásico Medio de Bienservida-Villarrodrigo. Instituto de Estudios Albacetenses, Diputación de Albacete, Serie 1, 60, Albacete, Spain, 131 pp.). According to the vertebrae reported by Segesdi & Osi (2021)Segesdi, M. & Ösi, A. (2021). Sauropterygian remains from the Middle Triassic of Villány, Nungary - new information on the aquatic reptile fauna of Tisza Magaunit (Triassic southern Eurasian shelf region). Palaeobiodiversity and Palaeoenvironments, 101: 985-1011. https://doi.org/10.1007/s12549-020-00480-x , the development of the neural canal is lower in Nothosaurus (10.9 to 12.9) and in simosaurids (8.6). However, the values proposed by Alafont (1992)Alafont, L.S. (1992). Notosaurios y Placodontos (Reptilia) del Triásico Medio de Bienservida-Villarrodrigo. Instituto de Estudios Albacetenses, Diputación de Albacete, Serie 1, 60, Albacete, Spain, 131 pp. comparing the development of transverse processes and the development of the neural canal, are variable along the postcranial skeleton from cervical to caudal vertebrae. Therefore, the taxonomic assignation of the vertebra within Sauropterygia is complicated. Pachypleurosaurids can be excluded because their size was much smaller than the studied specimen. In fact, this specimen was relatively large due to the size of the neural arch, probably reaching more than 2 m in length. But an adult specimen of Nothosaurus presents larger dorsal vertebrae than the studied specimen, reaching more than 8 cm height for the neural spine and 6 cm width for the neural arch (Segesdi & Osi, 2021Segesdi, M. & Ösi, A. (2021). Sauropterygian remains from the Middle Triassic of Villány, Nungary - new information on the aquatic reptile fauna of Tisza Magaunit (Triassic southern Eurasian shelf region). Palaeobiodiversity and Palaeoenvironments, 101: 985-1011. https://doi.org/10.1007/s12549-020-00480-x ). Therefore, the studied neural arch could correspond to a caudal vertebrae of an adult Nothosaurus.

Nevertheless, considering the age of the studied fossil remain (Upper Triassic, probably Rhaetian), the fossil record of sauropterygians (Rieppel, 1999Rieppel, O. (1999). Phylogeny and paleobiogeography of Triassic Sauropterygia: problems solved and unresolved. Palaeogeography, Palaeoclimatology, Palaeoecology, 153: 1-15. https://doi.org/10.1016/S0031-0182(99)00067-X ), and the features of the neural arch, the studied remain would also correspond to a cyamodontoid placodont such as Henodus or Psephoderma. Most of the sauropterygians, included other placodonts, nothosauroids and pistosauroids, are mainly restricted to Middle Triassic (see Rieppel, 1999Rieppel, O. (1999). Phylogeny and paleobiogeography of Triassic Sauropterygia: problems solved and unresolved. Palaeogeography, Palaeoclimatology, Palaeoecology, 153: 1-15. https://doi.org/10.1016/S0031-0182(99)00067-X ). Henodus was recorded from the Algarve as Carnian (Campos & Mateus, 2018Campos, H. & Mateus, O. (2018). The first record of placodonts in Portugal and its chronological and paleoecological implications. XVI annual meeting of the European Association of Vertebrate Palaeontologists, Caparica, Portugal, 38.; Ruciński, 2020Ruciński, M.R. (2020). Novel placodont material and paleoenvironment analysis of Triassic deposits of Rocha da Pena (Algarve, southern Portugal). MSc Universidade Nova de Lisboa, 104 pp.) and only Psephoderma has been reported from Rhaetian (Rieppel, 1999Rieppel, O. (1999). Phylogeny and paleobiogeography of Triassic Sauropterygia: problems solved and unresolved. Palaeogeography, Palaeoclimatology, Palaeoecology, 153: 1-15. https://doi.org/10.1016/S0031-0182(99)00067-X ). Recently, a new placodont, Parahenodus, has been described from the Upper Triassic (Carnian-Norian) of Spain (de Miguel Chaves et al., 2018Miguel Chaves, C. de; Ortega, F. & Pérez-García, A. (2018). New highly pachyostotic nothosauroid interpreted as a filter-feeding Triassic marine reptile. Biology Letters, 14: 20180130. https://doi.org/10.1098/rsbl.2018.0130 ). Although this genus has been described from a unique partial skull (vertebrae have not been recorded), it could also be considered as a potential candidate for the taxonomic assignation of the studied remain. If this assignation to Superfamily Cyamodontoidea is accepted, this would be a neural arch from a caudal vertebra and not from a dorsal vertebra of cyamodontoids. The dorsal vertebrae of cyamondontoids present characteristically elongated, broaded and curved transverse processes, and these features are not observed in the studied neural arch of Ayamonte. However, the neural canal of cyamodontoids is relatively high and narrow (Rieppel, 2000Rieppel, O. (2000). Sauropterygia I. In: Encyclopedia of Paleoherpetology (Wellnhofer, P., Ed.), 12A, Dr. Friedrich Pfeil Verlag, München, 134 pp.) distinct to the studied specimen and the taxonomic uncertainty persists.

The characteristic compact vertebrae of sauropterygians are usually well preserved but the studied specimen present numerous fractures. The typically high neural spines of the sauropterygians are commonly well preserved but commonly disarticulated respect to the centrum, as occurs in the studied vertebra. The disarticulation of centra and neural arches is typical in bonebeds (Reolid et al., 2014Reolid, M.; Pérez-Valera, F.; Benton, M.J. & Reolid, J. (2014). Marine flooding event in continental Triassic facies identified by a nothosaur and placodont bone bed (South Iberian Paleomargin). Facies, 60: 277-293. https://doi.org/10.1007/s10347-013-0360-6 ). The main distortions in the vertebrae are fractures in neural spines and transverse processes as here reported. The fossil vertebra is isolated and other fossil bones have not been recorded, which is an evidence of transport and dispersion of bone remains by currents and potentially scavengers. Fragmentation of the bone points to a more or less extensive bioestratinomic exposure of the bone previous to the definitive burial.

Geochemical composition

 

The main composition of specimen (calcium phosphate) does not seem to have been modified during diagenesis. However, the observed enrichment in Sr could be interpreted as related to the replacement of Ca in bioapatite (hydroxylapatite) via co-precipitation or adsorption. Sr is commonly incorporated into bones as a biogenic trace element (e.g. Bocherens et al., 1994Bocherens, H.; Brinkman, D.B.; Dauphin, Y. & Mariotti, A. (1994). Microstructural and geochemical investigations on Late Cretaceous archosaur teeth from Alberta, Canada. Canadian Journal of Earth Sciences, 31: 783-792. https://doi.org/10.1139/e94-071 ; Gilbert et al., 1994Gilbert, C.; Sealy, J. & Sillen, A. (1994). An investigation of barium, calcium and strontium as palaeodietary indicators in the Southwestern Cape, South Africa. Journal of Archaeological Science 21, 173-184. https://doi.org/10.1006/jasc.1994.1020 ; Silen & Sealy, 1995Sillen, A. & Sealy, J.C. (1995). Diagenesis od strontium in fossil bone: a reconsideration of Nelson et al. (1986). Journal of Archaeological Science, 22: 313-320. https://doi.org/10.1006/jasc.1995.0033 ; Keenan et al., 2016Keenan, S.W. (2016). From bone to fossil: A review of the diagenesis of bioapatite. American Mineralogist, 101: 1943-1951. https://doi.org/10.2138/am-2016-5737 ; Rey et al., 2022Rey, L.; Tacail, T.; Santos, F.; Rottier, S.; Goude, G. & Balter, V. (2022). Disentangling diagenetic and biogenic trace elements and Sr radiogenic isotopes in fossil dental enamel using laser ablation analysis. Chemical Geology, 587: 120608. https://doi.org/10.1016/j.chemgeo.2021.120608 ). However, since Ba and Sr are incorporated by living organisms in the same way, and the Ba content in the studied bone is not remarkable (Fig. 3), probably part of the Sr has a diagenetic origin (Tuken et al., 2008Tusken, T.; Vennemann, T.W. & Pfretzschener, H.U. (2008). Early diagenesis of bone and tooth apatite in fluvial and marine settings: constraints from combined oxygen isotope, nitrogen and REE analysis. Palaeogeography, Palaeoclimatology, Palaeoecology, 266: 254-268. https://doi.org/10.1016/j.palaeo.2008.03.037 ). Sr-rich fluids may also have leached from sulfate-rich Triassic deposits or from doleritic volcanic rocks (Wey et al., 2022Wei, D.T.; Zhou, T.F.; Xia, Y.; Chen, J.; Xie, Z.J.; Liu, X.J.; Pang, B.C.; Tan, Q.P. & Bai, L.A. (2022). Ore fluid originrecorded by apatite chemistry: A case study on altered dolerite from the badu Carlin-type gold deposit, Youjiang basin, SW China. Ore Geology Reviews, 143: 104745. https://doi.org/10.1016/j.oregeorev.2022.104745 ). Therefore, Sr was very likely incorporated via adsorption in bioapatite during diagenesis.

On the other hand, the presence of iron and manganese oxides must be related to diagenetic processes. Iron oxides are infilling small voids of the spongy bone tissue and manganese oxides (pyrolusite) are located on the bone surface but not within the bone. The parallel distribution of Fe and Cu in the spongy bone tissue could be related to the presence of original sulphides (Tusken et al., 2008Tusken, T.; Vennemann, T.W. & Pfretzschener, H.U. (2008). Early diagenesis of bone and tooth apatite in fluvial and marine settings: constraints from combined oxygen isotope, nitrogen and REE analysis. Palaeogeography, Palaeoclimatology, Palaeoecology, 266: 254-268. https://doi.org/10.1016/j.palaeo.2008.03.037 ) related to microbial activity associated with decay of the soft tissues (e.g. Vietti et al., 2015Vietti, L.A., Bailey, J.V.; Fox, D.L. & Rogers, R.R. (2015). Rapid formation of framboidal sulfides on bone surfaces from a simulated marine carcass fall. Palaios, 30: 327-334. https://doi.org/10.2110/palo.2014.027 ; Pesquero et al., 2015Pesquero, M.D., Alcalá, L.; Bell, L.S. & Fernández-Jalvo, Y. (2015). Bacterial origin of iron-rich microspheres in Miocene mammalian fossils. Palaeogeography, Palaeoclimatology, Palaeoecology, 420: 27-34. https://doi.org/10.1016/j.palaeo.2014.12.006 ; Domenech-Carbo et al., 2016Domenech-Carbo, M.T.; Buendia-Ortuño, M.; pasies-Oviedo, T. & Osete-Cortina, L. (2016). Analytical study of waterlogged ivory from the Bajo de la Campana site (Murica, Spain). Microchemical Journal, 126: 381-405. https://doi.org/10.1016/j.microc.2015.12.022 ).

Sedimentary environment

 

Most of the specimens of sauropterygians recorded in Iberia are Middle Triassic in age (see de Miguel Chaves et al., 2020Miguel Chaves, C. de; Ortega, F. & Pérez-García, A., 2020. The Iberian Triassic fossil record of Sauropterygia: an update. Journal of Iberian Geology, 46: 445-464. https://doi.org/10.1007/s41513-020-00137-w ), but this specimen was recovered from the less common Upper Triassic record. In addition, this specimen, together with the placodont remains of the Algarve Basin (Campos & Mateus, 2018Campos, H. & Mateus, O. (2018). The first record of placodonts in Portugal and its chronological and paleoecological implications. XVI annual meeting of the European Association of Vertebrate Palaeontologists, Caparica, Portugal, 38.; Ruciński, 2020Ruciński, M.R. (2020). Novel placodont material and paleoenvironment analysis of Triassic deposits of Rocha da Pena (Algarve, southern Portugal). MSc Universidade Nova de Lisboa, 104 pp.), constitute the westernmost record of Triassic sauropterygians in the Tethys. The detrital red beds (silts and sands) of the Silves Group represent the earliest phase of sedimentation related to the initial rifting of Pangaea, dated as early Carnian (Vilas-Boas et al., 2022Vilas-Boas, M.; Paterson, N.W.; Pereira, Z.; Fernandes, P. & Cirilli, S. (2022). The age of the first pulse of continental rifting associated with the breakup of Pangea in Southwest Iberia; new palynological evidence. Journal of Iberian Geology, 48: 181-190. https://doi.org/10.1007/s41513-022-00189-0 ). The carbonate succession, dated as Rhaetian (Santos et al., 2022Santos, A.; Popovic, N. & Mayoral, E. (2022). Palaeoecology of Late Triassic marine assemblages from the proto-Atlantic Basin (Ayamonte, SW Spain). Proceedings of the Geologists Association, 133: 47-66. https://doi.org/10.1016/j.pgeola.2021.11.002 ), represents the flooding of the Algarve Basin and the colonization by mollusks (bivalves and gastropods) and marine vertebrates such as phytosaurs (Mateus et al., 2014Mateus, O.; Butler, R.J.; Brusatte, S.L.; Whiteside, J.H. & Steyer, J.S. (2014). The first phytosaur (Diapsida, Archosauriformes) from the Late Triassic of the Iberian Peninsula. Journal of Vertebrate Paleontology, 34: 970-975. https://doi.org/10.1080/02724634.2014.840310 ), the placodont Henodus (Campos & Mateus, 2018Campos, H. & Mateus, O. (2018). The first record of placodonts in Portugal and its chronological and paleoecological implications. XVI annual meeting of the European Association of Vertebrate Palaeontologists, Caparica, Portugal, 38.; Ruciński, 2020Ruciński, M.R. (2020). Novel placodont material and paleoenvironment analysis of Triassic deposits of Rocha da Pena (Algarve, southern Portugal). MSc Universidade Nova de Lisboa, 104 pp.) and the here studied sauropterygian of Ayamonte. The presence of dolerite rocks in the succession is congruent with the extensional phase of the rifting.

Conclusions

 

The studied remain corresponds to a neural arch of an Upper Triassic sauropterygian deposited in a very shallow marine environment. The studied remain is isolated, disarticulated and presents fractures, that evidence transport but also potential activity of scavengers. The neural arch is preserved as calcium phosphate and some elements such as Sr, are especially rich compared with the surrounding carbonate sediment; fact that could probably be related to substitution of Ca by Sr during the diagenesis. Although the anatomic features do not allow a more accurate taxonomic identification, the studied remain probably corresponds to the neural arch of a caudal cyamodontoid vertebra. This remain extends the record of sauropterygians to the westernmost end of the Tethys and evidences the colonization of the Algarve Basin during the extensional phase related with the progress of the rifting of Pangaea and the opening of the Tethys to the west.

ACKNOWLEDGEMENTS

 

Authors would like to thank Ramón Martín, Miguel Ángel Bernal and Benjamín Cabaco for informing us of the discovery of the fossil bone. Financial support through the projects PY20_00111 and RNM-200 Research Group (Junta de Andalucía, Spain) and PID2019-104625RB-100 (Spanish Govern) is gratefully acknowledged. Technical and human support provided by Centro de Instrumentación Científico-Técnica (CICT) of the University of Jaén is gratefully acknowledged. The authors thank the constructive comments of the reviewers Carlos de Miguel Chaves and Torsten Scheyer.

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