Barremian charophytes from the Maestrat Basin: taxonomy, palaeoecology, palaeobiogeography and biostratigraphy

Abstract

This thesis is focused on the taxonomical, biogeographical, palaeoecological and biostratigraphical characterization of the Barremian charophytes of the Maestrat Basin (Eastern Iberian Chain, Spain). Thirty-four charophyte taxa, including fructifications and thalli, which belong to the three families that co-existed during the Early Cretaceous, i.e., the Porocharaceae, the Characeae and the Clavatoraceae, were identified in two Barremian sections (Fredes and Herbers-Mas de Petxí). Twenty-six of them correspond to clavatoracean utricles, including Clavator grovesii var. jiuquanensis, which is herein firstly reported in the Iberian Chain. Similar clavatoracean richness is not found elsewhere, showing the importance of the Maestrat Basin as a hotspot for the clavatoracean species-richness during Barremian. New characters found in the clavatoracean utricles and thalli improved the knowledge on this important, mainly Cretaceous, family. The Echinochara lazarii utricle was reinterpreted as bilaterally symmetrical, rather than triradiated. This type of symmetry was previously unknown within genus Echinochara and in the whole subfamily Atopocharoidae, leading to a taxonomic emendation of this genus. This species was found attached to the thallus Charaxis spicatus allowing the reconstruction of the Echinochara whole-plant. The cortication of this thallus was reviewed and a new type of cortication, called double triplostichous cortication, was described. A second important result was the characterization of a gradualistic evolutionary lineage in Clavator calcitrapus. This lineage includes two anagenetic varieties, C. calcitrapus var. jiangluoensis and C. calcitrapus var. calcitrapus, linked by all possible intermediary morphotypes. This lineage has important biogeographic implications since the oldest variety had a subcosmopolitan range, while the second was endemic to Iberia. Finally, a previously unknown centripetal calcification pattern was found in the internodal cell of genus Munieria. This type of calcification is reminiscent of the one observed in the clavatoroid gyrogonites, providing further support to the hypothesis that this thallus belonged to the Clavatoraceae. The expanded sedimentary record of the Herbers-Mas de Petxí section and the abundance and diversity of charophyte remains found, provided a unique context, at least at European scale, to study the palaeoecology of charophyte species and the evolution of charophyte floras through the Barremian. Three sedimentary units, named from base to top Cantaperdius, Artoles and Morella formations, were analysed. Within these lithostratigraphic units, five charophyte associations were distinguished. 1) A charophyte association developed in freshwater lakes with low clastic input was found in the lower part of the Cantaperdius Formation and was mainly composed of Atopochara trivolvis var. triquetra, Clavator harrisii, Ascidiella stellata, and A. triquetra. Besides, Hemiclavator-rich populations were locally dominant. 2) An association characteristic of freshwater lakes with high clastic input mainly composed of Echinochara lazarii, Globator maillardii var. trochiliscoides, A. trivolvis var. triquetra, C. harrisii, Hemiclavator neimongolensis var. neimongolensis, and occasionally also Clavator calcitrapus. This association is mostly found in the upper part of the Cantaperdius Formation. 3) Porochara maestratica populations mainly thrived in low-clastic-influenced brackish settings; while 4) E. lazarii preferably thrived in clastic-influenced brackish settings. Both these brackish settings where abundant in the Artoles Formation. 5) The charophyte association developed in lakes formed in coastal mudflat to floodplain settings was mainly constituted by E. lazarii, A. trivolvis var. triquetra and var. trivolvis, and C. harrisii var. harrisii and var. reyi. This association is found in the Morella Formation. The environmental distribution of the associations recognized depended on the salinity, the clastic influence, and the depth. A new Barremian–early Aptian charophyte biostratigraphy is proposed. Two biozonations, European and Eurasian, were distinguished and correlated. Furthermore, these biozones were calibrated to the Geological Time Scale by means of strontium-isotope stratigraphy, enabling their correlation with the coeval marine realm. The European biostratigraphy is formed by two partial range biozones (characterized by the First Appearance Data, FAD), of index species that were endemic from the Western Tethyan Archipelago (extant Europe and North Africa): (1) Globator maillardii var. trochiliscoides (early Barremian) and (2) Ascidiella cruciata-Pseudoglobator paucibracteatus. The base of this latter biozone was calibrated with an oyster shell sample with an 87Sr/86Sr ratio of 0.707482, translating to a late early Barremian age, giving a total timespan of late early Barremian–early Aptian for this biozone. The Eurasian biozone is composed of three partial range biozones. (1) Atopochara trivolvis var. triquetra biozone is almost equivalent to the G. maillardii var. trochiliscoides biozone. (2) Hemiclavator neimongolensis var. neimongolensis biozone, which was calibrated with an oyster shell whose 87Sr/86Sr ratio is 0.707481 (late early Barremian). (3) The next biozone is defined by the FAD of Clavator grovesii var. jiuquanensis. Its base was dated with an oyster shell gathered 24.5 m below the FAD of the index species and its 87Sr/86Sr ratio is 0.707489 (late early Barremian). The FAD of C. grovesii var. corrugatus defines the next biozone. The biozonation described herein is intended to facilitate the correlation between different basins of the western Tethys, and between non-marine basins in whole Eurasia.

Aquesta tesi es focalitza en la caracterització taxonòmica, biogeogràfica, paleoecològica i biostratigràfica dels caròfits barremians de la conca del Maestrat (Cadena Ibèrica). Dels 34 taxons identificats, pertanyents a tres famílies que coexistiren durant el Cretaci Inferior (Porocharaceae, Characeae i Clavatoraceae) 26 pertanyen a utricles de clavatoràcies. Aquesta diversitat és única a escala mundial i mostra que el Maestrat fou un punt calent de biodiversitat d’aquesta família. Des del punt de vista morfològic s’ha mostrat que l’utricle d’Echinochara lazarii tenia una simetria bilateral, en comptes de presentar la simetrial triradial descrita originalment pel gènere. A més s’ha pogut reconstruir de la planta sencera d’ i Echinochara gràcies a haver descobert la connexió anatòmica entre Echinochara lazarii i el tal·lus descrit prèviament com Charaxis spicatus. Aquesta planta presentava una corticació desconeguda fins ara, que s’ha anomenat triplòstica doble. S’ha descrit també un nou llinatge evolutiu dins les clavatoràcies, format per dues varietats anagenètiques dins l’espècie de Clavator calcitrapus, que és de gran interès en paleobiogeografia i biostratigrafia. Finalment, la descripció d’una calcificació centrípeta als entrenusos del tal·lus de Munieria (un fòssil que ha estat motiu de controvèrsia respecte a la seva afinitat taxonòmica), dóna suport a la hipòtesi de que pertanyia a una clavatoràcia. L’estudi paleoecològic ha permès distingir diferents associacions de caròfits: 1) L’associació característica de llacs d’aigua dolça amb poca influència clàstica on predominava Atopochara trivolvis var. triquetra, Clavator harrisii, Ascidiella stellata i A. triquetra. 2) Una associació de llacs d’aigua dolça amb molta influència clàstica, dominada per Echinochara lazarii, Globator maillardii var. trochiliscoides, A. trivolvis var. triquetra, C. harrisii, Hemiclavator neimongolensis var. neimongolensis. 3) Porochara maestratica dominava als ambients salabrosos amb poca influència clàstica, mentre que 4) E. lazarii dominava ambients salabrosos amb més influència clàstica. 5) L’associació característica de basses i aiguamolls de fangars i planes d’inundació incloïa E. lazarii, A. trivolvis var. triquetra i C. harrisii. Es revisa la biostratigrafia dels caròfits del Barremià–Aptià inferior amb dues biozonacions, una per l’àmbit de l’Arxipèlag Cretaci de la Tetis (Europa, nord d’Àfrica i Orient mitjà) i una segona d’àmbit eurasiàtic. La primera biozonació està composta per dues biozones, les espècies índex de les quals eren endèmiques de l'arxipèlag: (1) Globator maillardii var. trochiliscoides (Barremià inferior) i (2) Ascidiella cruciata-Pseudoglobator paucibracteatus. La base d’aquesta última s’ha calibrat amb l’escala del temps geològic mitjançant uns valor d’ 87Sr/86Sr de 0,707482 (Barremià inferior terminal) obtinguts en una valva d’ostreid. Aquesta biozona inclou l’interval Barremià inferior terminal–Aptià inferior. La biozonació euroasiàtica conté tres biozones, les espècies índex de les quals estan presents a tota Euràsia. (1) La biozona Atopochara trivolvis var. triquetra té una edat de Barremià inferior. (2) La base de la biozona H. neimongolensis var. neimongolensis fou calibrada amb l’escala del temps geològic mitjançant una mostra d’ostreid que va proporcionar un valor de l’ 87Sr/86Sr de 0,7074781 (Barremià inferior superior). (3) La biozona Clavator grovesii var. jiuquanensis s’ha calibrat calibrat amb l’escala del temps geològic mitjançant una mostra d’ostreid que va proporcionar uns valors d’ 87Sr/86Sr de 0,707489 (Barremià superior inferior). Aquesta nova proposta biostratigràfica es preveu que faciliti la correlació entre les conques del Tetis i entre les conques continentals d’Euràsia.