Rise and fall of †Pycnodontiformes: Diversity, competition and extinction of a successful fish clade

Abstract

†Pycnodontiformes was a successful lineage of primarily marine fishes that broadly diversified during the Mesozoic. They possessed a wide variety of body shapes and were adapted to a broad range of food sources. Two other neopterygian clades possessing similar ecological adaptations in both body morphology (†Dapediiformes) and dentition (Ginglymodi) also occurred in Mesozoic seas. Although these groups occupied the same marine ecosystems, the role that competitive exclusion and niche partitioning played in their ability to survive alongside each other remains unknown. Using geometric morphometrics on both the lower jaw (as constraint for feeding adaptation) and body shape (as constraint for habitat adaptation), we show that while dapediiforms and ginglymodians occupy similar lower jaw morphospace, pycnodontiforms are completely separate. Separation also occurs between the clades in body shape so that competition reduction between pycnodontiforms and the other two clades would have resulted in niche partitioning. Competition within pycnodontiforms seemingly was reduced further by evolving different feeding strategies as shown by disparate jaw shapes that also indicate high levels of plasticity. Acanthomorpha was a teleostean clade that evolved later in the Mesozoic and which has been regarded as implicated in driving the pycnodontiforms to extinction. Although they share similar body shapes, no coeval acanthomorphs had similar jaw shapes or dentitions for dealing with hard prey like pycnodontiforms do and so their success being a factor in pycnodontiform extinction is unlikely. Sea surface temperature and eustatic variations also had no impact on pycnodontiform diversity patterns according to our results. Conversely, the occurrence and number of available reefs and hardgrounds as habitats through time seems to be the main factor in pycnodontiform success. Decline in such habitats during the Late Cretaceous and Palaeogene might have had deleterious consequences for pycnodontiform diversity. Acanthomorphs occupied the niches of pycnodontiforms during the terminal phase of their existence.

Fossilized cell structures identify an ancient origin for the teleost whole-genome duplication

“…The early Late Cretaceous coincided with a major burst of cladogenesis in marine fishes, the diversity of which is strongly and positively correlated with sea surface temperature during the Jurassic and Cretaceous (44,49). Although teleosts dominate this mid-Cretaceous radiation, other ray-finned fish groups show similar patterns [e.g., pycnodonts (50)], arguing for a general-rather than clade-specific-mechanism. The Cretaceous concluded 66 million y ago with a major mass extinction event.…”

A new hybodontiform shark (StrophodusAgassiz 1838) from the Lower Cretaceous (Valanginian-Hauterivian) of Colombia

“…The trophic structure of the Central Atlantic Ocean at the northernmost portion of South America during the Valanginian and Hauterivian is still poorly known. Durophagous hybodontiform sharks, as well as other shell-crushing fishes like pycnodontiforms, must have played an important ecological role as predators in these ancient tropical coastal ecosystems, due to the abundant and diverse invertebrate fauna present at the Carrizal and El Sapo members, similar to proposals for other Cretaceous and Jurassic faunas ( Kriwet, 2001 ; Cawley et al, 2021 ; Cooper & Martill, 2020 ).…”

Alternating regimes of shallow and deep-sea diversification explain a species-richness paradox in marine fishes

“…The fossil record shows that crown teleosts were not the dominant clade on reefs until ∼90 Ma ( 53 , 57 , 61 ), around the time when we inferred a dramatic rise in shallow marine diversity. Instead, the stem teleost Pycnodontiformes occupied niches during the Jurassic similar to percomorphs today ( 32 , 99 ). Morphometric and diversity-through-time analyses using fossils suggest pycnodontiforms displaced teleosts to marginal habitats until their decline in the Late Cretaceous ( 99 ).…”

“…Instead, the stem teleost Pycnodontiformes occupied niches during the Jurassic similar to percomorphs today ( 32 , 99 ). Morphometric and diversity-through-time analyses using fossils suggest pycnodontiforms displaced teleosts to marginal habitats until their decline in the Late Cretaceous ( 99 ). The extinction of incumbents and the subsequent expansion of teleosts on reefs together may be the inciting events forming the modern shallow–deep richness disparity.…”