Cretaceous shark evolution + Triassic temnospondyls in India and South-East Asia + Megateuthis, largest belemnite

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Ben Creisler

Jun 3, 2024, 5:45:53 PM (11 days ago) Jun 3
Ben Creisler

Some recent Mesozoic papers:

Free pdf:

Phillip C. Sternes, Lars Schmitz & Timothy E. Higham (2024)
The rise of pelagic sharks and adaptive evolution of pectoral fin morphology during the Cretaceous
Current Biology (advance online publication)

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Modern sharks were likely benthic or benthopelagic in origin
Sharks have independently expanded to the pelagic zone several times
Pectoral fin shape shows signatures of adaptive evolution
Sea surface temperature impacts shark evolution


The emergence and subsequent evolution of pectoral fins is a key point in vertebrate evolution, as pectoral fins are dominant control surfaces for locomotion in extant fishes. However, major gaps remain in our understanding of the diversity and evolution of pectoral fins among cartilaginous fishes (Chondrichthyes), a group with an evolutionary history spanning over 400 million years with current selachians (modern sharks) appearing about 200 million years ago. Modern sharks are a charismatic group of vertebrates often thought to be predators roaming the open ocean and coastal areas, but most extant species occupy the seafloor. Here we use an integrative approach to understand what facilitated the expansion to the pelagic realm and what morphological changes accompanied this shift. On the basis of comparative analyses in the framework of a time-calibrated molecular phylogeny, we show that modern sharks expanded to the pelagic realm no later than the Early Cretaceous (Barremian). The pattern of pectoral fin aspect ratios across selachians is congruent with adaptive evolution, and we identify an increase of the subclade disparity of aspect ratio at a time when sea surface temperatures were at their highest. The expansion to open ocean habitats likely involved extended bouts of sustained fast swimming, which led to the selection for efficient movement via higher aspect ratio pectoral fins. Swimming performance was likely enhanced in pelagic sharks during this time due to the elevated temperatures in the sea, highlighting that shark evolution has been greatly impacted by climate change.



How sharks survived a major spike in Earth’s temperature


Free pdf:

Carla San Román, Hugo Martín-Abad & Jesús Marugán-Lobón (2024)
A geometric morphometric protocol to correct postmortem body arching in fossil fishes
PeerJ 12:e17436

Postmortem body curvature introduces error in fish morphometric data. Compared to living fish, the causes of such body curvature in fossils may be due to additive taphonomic processes that have been widely studied. However, a protocol that helps to correct its effect upon morphometric data remains unexplored. Here, we test two different mathematical approaches (multivariate regression and the so-called ‘unbending functions’) available to tackle fish geometric morphometric data in two exceptionally preserved gonorynchiformes fossil fishes, Rubiesichthys gregalis and Gordichthys conquensis, from the Las Hoyas deposits (Early Cretaceous, Spain). Although both methods successfully correct body curvature (i.e., removing misleading geometric variation), our results show that traditional approaches applied in living fishes might not be appropriate to fossil ones, because of the additional anatomical alterations. Namely, the best result for 2D fossil fishes is achieved by correcting the arching of the specimens (mathematically “unbending” them). Ultimately, the effect of body curvature on morphometric data is largely taxon independent and morphological diversity mitigates its effect, but size is an important factor to take into account (because larger individuals tend to be less curved).  


Not yet mentioned:

Sanjukta Chakravorti, Aindrila Roy & Dhurjati Prasad Sengupta (2024)
Patterns of diversity of temnospondyl amphibians in India and South-East Asia
Patrons de diversité des amphibiens temnospondyles d’Inde et d’Asie du Sud-Est

While temnospondyl fossils have been relatively well documented in the Western world and Africa; research on temnospondyl fossils in South-East Asia and India has received less attention. Nonetheless, they add a substantial amount to the record of palaeobiogeographic distribution of the Triassic temnospondyl amphibians. During the Triassic, almost every temnospondyl family (aside from dissorophoids) was present in India. According to the findings, the assemblage and species diversity of Triassic temnospondyl amphibians peaked in the Early Triassic. With climatic stability and ecological saturation in the Middle and Late Triassic, diversification slowed down or gradually decreased. The Jaccard Similarity coefficient of the Triassic temnospondyl contents in South-East Asia and India shows an agreement in palaeobiogeography; the coefficients reflect the geography of Pangea. According to the Euler diagram, the junction of the Triassic temnospondyls in South-East Asia and India indicates that these regions of India did not have endemic Triassic temnospondyl populations.


Free pdf:

Christian Klug, Günter Schweigert, René Hoffmann, Dirk Fuchs, Alexander Pohle, Robert Weis & Kenneth De Baets (2024)
Anatomy and size of Megateuthis, the largest belemnite
Swiss Journal of Palaeontology 143: 23

Belemnite rostra are very abundant in Mesozoic marine deposits in many regions. Despite this abundance, soft-tissue specimens of belemnites informing about anatomy and proportions of these coleoid cephalopods are extremely rare and limited to a few moderately large genera like Passaloteuthis and Hibolithes. For all other genera, we can make inferences on their body proportions and body as well as mantle length by extrapolating from complete material. We collected data of the proportions of the hard parts of some Jurassic belemnites in order to learn about shared characteristics in their gross anatomy. This knowledge is then applied to the Bajocian genus Megateuthis, which is the largest known belemnite genus worldwide. Our results provide simple ratios that can be used to estimate belemnite body size, where only the rostrum is known.


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