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“No est potestas super terram que comparetum ei” The giant ichthyosaur of the Severn Channel (SW England)

Authors:
  • Museum of Jurassic Marine Life

Abstract

Giant Late Triassic ichthyosaurs have been shrouded in mystery for more than two centuries. Due at large to their fragmentary fossil record, giant ichthyosaur palaeoecology, morphology and extinction are still a matter of important debate, making every new data crucial for the understanding of these past giants. We present the results of the latest studies on these marine reptiles, focusing on the morphology and bone histology of the newly described Ichthyotitan severnensis from the Rhaetian Westbury Formation of SW England (Lomax et al., 2024). Although the taxonomic assignment remains uncertain due to the incompleteness of the material, Ichthyotitan severnensis represents the first formally named giant ichthyosaur from the Rhaetian. The remains attributed to I. severnensis help us to reassess the size of ichthyosaurs in the latest Triassic, of which appears comparable to modern-day blue whales, indicating that extreme gigantism (length >25 m) evolved at least once outside of and long before Cetacea. The bone histology of I. severnensis confirms previous studies (Perillo & Sander, 2024) on the occurrence of an atypical histotype characterizing giant Late Triassic ichthyosaur lower jaws, probably linked to gigantism or to selective pressures in feeding. The case of I. severnensis highlights the crucial importance of collaboration between citizen scientists and palaeontologists for recovering new specimens, otherwise doomed to be lost. Finally, we address the need for further comparative studies with other ichthyopterigians and cetaceans to answer the many still open questions.
* marcelloperillo.96@gmail.com
¹ Paleontology division, Institute of Geosciences, University of Bonn, Bonn, Germany
² Palaeobiology Research Group, School of Earth Sciences, University of Bristol, Bristol, United Kingdom
³ Department of Earth and Environmental Sciences, The University of Manchester, Manchester, United Kingdom
4 The Etches Collection – Museum of Jurassic Marine Life, Dorset, United Kingdom
5 Braunton, Devon, United Kingdom
6 Dinosaurs Will Always Be Awesome, DWABA Museum, Orlando, Florida, United States of America
Marcello Perillo*¹, Dean R. Lomax²,³, Paul de la Salle4, Justin Reynolds5,
Ruby Reynolds5, James F. Waldron6
© Gabriel Ugueto
Modified from Allemand et al. 2023
Quick refresh on ichthyosaurs
No named large ichthyosaur in the Rhaetian!
Triassic
Upper
Early
Jurassic
Hettangian
201.3
Rhaetian
208.5
Tr-Jr ME
Modified from Allemand et al. 2023
Triassic
Upper
Early
Jurassic
Hettangian
201.3
Rhaetian
208.5
Tr-Jr ME
the traces of the past basin and of its inhabitants
Modified from Perillo & Sander 2024
Noi
siamo
qui
Aust Cliff
LilstockBlue Anchor
© Marcello Perillo
© Marcello Perillo © Jon Crossley
Aust Cliff
LilstockBlue Anchor
BRSMG Cb3669
BRSMG Cb4063 BRSMG Cb3870
19th century
Aust Cliff
LilstockBlue Anchor
BRSMG Cg3178
BRSMG Cb3669
BRSMG Cb4063 BRSMG Cb3870
BRSMG Cg2488
19th century
20182024
©
Sergey
Krasovskiy
And a really large one With permission
© Rhys Meyerkort 2024
Modified from Lomax et al. 2024
Blue Anchor
Lilstock
The first Rhaetian species
Yes, but how large?
The estimates are based on the morphology
and distance of the features of the proximal
part (M.A.M.E. and coronoid process)
With permission
© Rhys Meyerkort 2024
from Lomax et al. 2024
The largest one!
LilstockBlue Anchor
Cross-polarized lightCircular-polarized light
IFM
IFM
Modified from Lomax et al 2024 Modified from Perillo & Sander 2024
Intrinsic Fiber Matrix (IFM)
Occurrence of a typical bone tissue similar to metaplastic bone typical for giant ichthyosaurs
Periosteal Intrinsic Collagen Fibers build up a specific Matrix (IFM)
Don’t stop me (growing) nooow
Histology does not show signs of cessation of growth
Open periosteal canals
High vascularization
No EFS
Blue Anchor Lilstock
Aust Cliff
© Alessio Ciaffi
Don’t stop me (growing) nooow
We also have the way larger specimens from Aust Cliff
One of them can be homologized to the BA and Lil specs (the Aust Colossus)
It is 30% larger….
© Alessio Ciaffi
Geben Sie hier eine Formel ein.
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?
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Ichthyotitan severnensis
Nektonic shelled
invertebrates
Nektonic soft-bodied
invertebrates
Giant coeloids
Durophagous
e.g. Sargodon
Gyrolepis
albertii
Pachystropheus rhaeticus
Small fish
Tropifer laevis Shelled benthic invertebrates
Ceratodus sp.
Lissodus minimus
Placodus sp.
Severnichthys acuminatus
Modified from Cueille et al. 2020
plesiosaurs
ichthyosaurs
Rhomphayodon
Yet to be understood trophic role
What hit them through the T-J ME?
?
?
No histological comparable material is available! (cetaceans?)
©Vecteezy
Could we find similar
structures to IFM in
potential feeding
analogues?
Without a “tiny” help there would be no giant
Both discoveries were possible only thanks to amateur fossil collectors
Spreading knowledge
on fossil heritage
Self-regulating and
establishing contacts
with institutions
Increasing the
likelihood of
further discoveries
Communities of fossil
collectors
Positive
reinforcement
through media
High cultural
relevance of
fossils
Accessible
fossil heritage
Ichthyotitan severnensis
is the first named ichthyosaur of the
Rhaetian
Ichthyotitan severnensis
is estimated to be the largest marine
reptile recorded
Histology shows a unique bone histology (IFM), apparently
specific for Triassic giant ichthyosaurs lower jaws
~25 meters was not the maximum size reachable by
ichthyosaurs in the Late Triassic
The ecological role of these giants is yet to be understood,
comparative analysis on cetaceans may help finding answers
The nourishment of communities of citizen scientists and
collectors is fundamental to increase future discoveries
Conclusions
Rhaetian + 1 species :D
Thank you for your attention!
©
Sergey
Krasovskiy
Thank you for your attention!
marcelloperillo.96@gmail.com perillo_like_the_tree
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