In modern oceans, sunken carcasses of large pelagic vertebrates (e.g. whale-falls) play an essential role in the flux of nutrients in deep oligotrophic settings. Due to such a transfer of energy, carcasses generate peculiar and ephemeral ecosystems around them (deadfall communities). It has been speculated that Mesozoic marine reptiles could have played a role similar to modern cetaceans after death, possibly triggering the evolution of whale-fall specialist precursors. Most of previous taphonomic studies on marine reptiles from Europe were carried out on material from shallow-water deposits. Deeper settings, like mesopelagic and abyssopelagic deposits, were often neglected from taphonomical surveys on marine reptiles. This PhD project focused on skeletal preservation, scavenging traces, and microbial activity in marine reptiles from deep-water settings, with the Italian record as a case study. Ichthyosaurs, plesiosaurs, thalattosuchians, protostegids and mosasaurs were surveyed in order to correlate biostratinomic and diagenetic patterns in marine sauropsids from mesopelagic settings (from Veneto region) and abyssal plain deposits (from the Northern Apennines), and to identify stages and ecological successions similar to the ones observed in modern cetaceans. The mesopelagic specimens share a common bad state of preservation, consistent with the prolonged exposure of carcasses on a well-oxygenated seafloor. The preserved associated fossil fauna revealed an overrepresented presence of cephalopods such as nautiloids, belemnites and ammonoids. Hexanchiform shark teeth are also found associated to the carcasses, supporting a distinctive deep-water mobile scavenging community. Echinoids, sponges and other bioeroders are identified as representative of the enrichment-opportunist stage. Crinoids are unexpectedly recognized as part of the reef stage by colonization of the bare eroded bones as a viable hard substrate. Bromalites and stomach contents were also surveyed, which in most cases exhibit their specific taphonomic pattern. The abyssal plain marine reptiles are instead histologically and anatomically well preserved, though the intense tectonic history of the Northern Apennines is probably responsible for their strong fragmentation. SEM-EDS and TXRF analysis of the skeletal tissues revealed the widespread abundance of Mn and Fe, consistent with polymetallic nodules observed in modern abyssal plains. Fe and Mn oxides encrust and permeate skeletal material, most likely strengthening the bone tissue and preventing deformation. The description of this metallic coating represents a completely new fossilization mechanism for vertebrates in deep-sea settings. Modern cetacean bones from Recent abyssal plains were also investigated for comparison, to better frame the timing and origin of such polymetallic alteration of the bone tissue. As for the Cretaceous material, Mn permeates the bone tissue through nutrient foramina, osteocyte lacunae, Haversian canals and microbial/fungal bioerosions. The sulfophillic stage is the most well-represented deadfall stage recognized in this setting thanks to pyrite framboids found in histological sections and fragments of bone. The pyrite is localized only in the spongy bone, most likely representing S-reducing microbial activity during lipid decay. The two surveyed paleoenvironments complementary bring useful information to the understudied subject of Mesozoic pelagic vertebrate taphonomy in deep-water settings. Preservation patterns are confirmed to be environmentally-dependent, with distinct tapho-facies from each deposit. New taxa are also recognized as part of the deadfall communities, improving our understanding of these ecological successions. The thesis is structured upon a compilation of papers revolving around this survey.
Negli oceani attuali, le carcasse di grandi vertebrati pelagici (es. whale-falls) svolgono un ruolo essenziale nel flusso di nutrienti in ambienti oligotrofici profondi. Grazie a tale trasferimento di energia, queste carcasse generano attorno a loro ecosistemi temporanei e molto specializzati (comunità di deadfall). A lungo si è speculato sul fatto che anche i rettili marini del Mesozoico avrebbero potuto svolgere un ruolo simile a quello dei cetacei, promuovendo l'evoluzione dei precursori degli specialisti di whale-falls. I precedenti studi tafonomici sui rettili marini europei sono stati condotti solo su materiale proveniente da depositi di acque basse, spesso trascurando ambienti più profondi. Questo progetto di dottorato si è concentrato sulle modalità di conservazione dei resti scheletrici, sull'identificazione di associazioni fossili e di attività microbica nei rettili marini provenienti da ambienti di acque profonde attraverso lo studio del record italiano. Ittiosauri, plesiosauri, talattosuchi, protostegidi e mosasauri sono stati analizzati al fine di correlare tra loro i diversi modelli biostratinomici e diagenetici in ambienti mesopelagici (dal Veneto) e depositi abissali (dall'Appennino Settentrionale) e di identificare successioni ecologiche simili a quelle osservate nei cetacei attuali. Il record mesopelagico evidenzia un cattivo stato di conservazione compatibile con l'esposizione prolungata delle carcasse su un fondale ossigenato. La fauna fossile associata alle carcasse ha rivelato un grande contributo di cefalopodi tra nautiloidi, belemniti e ammonoidi. Denti di squali hexanchiformi sono stati rinvenuti in stretta associazione, fattore che supporta una comunità di mobile scavengers tipica di acque profonde. Echinoidi, spugne e altri produttori di bioerosioni sono identificati come rappresentativi dell’enrichment opportunistic stage. I crinoidi vengono inaspettatamente identificati come parte del reef stage, incrostando l’osso spugnoso degli esemplari studiati. Anche alcune bromaliti sono state analizzate, le quali hanno rivelato un modello tafonomico esclusivo. I rettili marini delle piane abissali si sono rivelati istologicamente ben conservati, anche se estremamente frammentati dalla storia tettonica dell’Appennino Settentrionale. L'analisi SEM-EDS e TXRF dei tessuti scheletrici ha rivelato la presenza diffusa di Mn e Fe, similmente ai noduli polimetallici che si formano nelle moderne piane abissali. Gli ossidi di Fe e Mn incrostano e permeano il materiale scheletrico, apparentemente rafforzandolo e avvantaggiandone la fossilizzazione. Queste incrostazioni polimetalliche rappresentano un processo di fossilizzazione nuovo nei vertebrati. Ossa di cetacei provenienti da piane abissali recenti sono state analizzate per confronto e per meglio inquadrare l'origine di questa alterazione metallica dei tessuti. Così come nel record cretaceo, il Mn permea il tessuto osseo attraverso forami nutritivi, lacune osteocitarie, canali Haversiani e bioerosioni microbiche/fungine. Il sulfophillic stage è la stadio di deadfall più comunemente rinvenuto in questi esemplari grazie a framboidi di pirite rinvenuti frequentemente in sezioni istologiche. La pirite è localizzata solo nell'osso spugnoso, come probabile proxy della mobilitazione dello zolfo da attività microbica. I due paleoambienti esaminati forniscono nuove informazioni sul tema poco studiato della tafonomia dei vertebrati mesozoici in ambienti di acque profonde. Vengono riconosciute distinte tafo-facies dipendenti dall’ambiente deposizionale, e nuovi taxa sono stati inclusi negli stadi di deadfalI, aumentando la nostra conoscenza su queste successioni ecologiche. La tesi è strutturata su una compilazione di articoli inerenti allo studio.
"Tafonomia e successioni ecologiche di deadfall nei rettili pelagici mesozoici da ambienti profondi"
SERAFINI, GIOVANNI
2024
Abstract
In modern oceans, sunken carcasses of large pelagic vertebrates (e.g. whale-falls) play an essential role in the flux of nutrients in deep oligotrophic settings. Due to such a transfer of energy, carcasses generate peculiar and ephemeral ecosystems around them (deadfall communities). It has been speculated that Mesozoic marine reptiles could have played a role similar to modern cetaceans after death, possibly triggering the evolution of whale-fall specialist precursors. Most of previous taphonomic studies on marine reptiles from Europe were carried out on material from shallow-water deposits. Deeper settings, like mesopelagic and abyssopelagic deposits, were often neglected from taphonomical surveys on marine reptiles. This PhD project focused on skeletal preservation, scavenging traces, and microbial activity in marine reptiles from deep-water settings, with the Italian record as a case study. Ichthyosaurs, plesiosaurs, thalattosuchians, protostegids and mosasaurs were surveyed in order to correlate biostratinomic and diagenetic patterns in marine sauropsids from mesopelagic settings (from Veneto region) and abyssal plain deposits (from the Northern Apennines), and to identify stages and ecological successions similar to the ones observed in modern cetaceans. The mesopelagic specimens share a common bad state of preservation, consistent with the prolonged exposure of carcasses on a well-oxygenated seafloor. The preserved associated fossil fauna revealed an overrepresented presence of cephalopods such as nautiloids, belemnites and ammonoids. Hexanchiform shark teeth are also found associated to the carcasses, supporting a distinctive deep-water mobile scavenging community. Echinoids, sponges and other bioeroders are identified as representative of the enrichment-opportunist stage. Crinoids are unexpectedly recognized as part of the reef stage by colonization of the bare eroded bones as a viable hard substrate. Bromalites and stomach contents were also surveyed, which in most cases exhibit their specific taphonomic pattern. The abyssal plain marine reptiles are instead histologically and anatomically well preserved, though the intense tectonic history of the Northern Apennines is probably responsible for their strong fragmentation. SEM-EDS and TXRF analysis of the skeletal tissues revealed the widespread abundance of Mn and Fe, consistent with polymetallic nodules observed in modern abyssal plains. Fe and Mn oxides encrust and permeate skeletal material, most likely strengthening the bone tissue and preventing deformation. The description of this metallic coating represents a completely new fossilization mechanism for vertebrates in deep-sea settings. Modern cetacean bones from Recent abyssal plains were also investigated for comparison, to better frame the timing and origin of such polymetallic alteration of the bone tissue. As for the Cretaceous material, Mn permeates the bone tissue through nutrient foramina, osteocyte lacunae, Haversian canals and microbial/fungal bioerosions. The sulfophillic stage is the most well-represented deadfall stage recognized in this setting thanks to pyrite framboids found in histological sections and fragments of bone. The pyrite is localized only in the spongy bone, most likely representing S-reducing microbial activity during lipid decay. The two surveyed paleoenvironments complementary bring useful information to the understudied subject of Mesozoic pelagic vertebrate taphonomy in deep-water settings. Preservation patterns are confirmed to be environmentally-dependent, with distinct tapho-facies from each deposit. New taxa are also recognized as part of the deadfall communities, improving our understanding of these ecological successions. The thesis is structured upon a compilation of papers revolving around this survey.File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.14242/80567
URN:NBN:IT:UNIMORE-80567