Turning Points in Sarcopterygian Morphological Disparity from the Devonian to the Carboniferous

The transition from the Devonian to the Carboniferous marks a period of profound upheaval in vertebrate history. During this interval, sarcopterygians underwent major environmental disruptions and ecological reorganization, accompanied by far-reaching morphological changes that ultimately set the stage for the emergence of vertebrates on land.

Sarcopterygians include modern coelacanths and lungfishes, as well as all tetrapods, and their evolutionary history extends back to the Late Silurian. By the Devonian, they formed a major component of vertebrate diversity alongside placoderms. Beneath this apparent success, however, a deeper transformation was unfolding.

During the Middle Devonian, Earth's environments began to shift dramatically with the emergence of the first forests. Plants established stable and increasingly complex terrestrial ecosystems, reshaping aquatic systems through riverine transport and sedimentation. Root systems enhanced weathering and soil formation, delivering large amounts of terrestrial material into aquatic environments. This influx included not only organic matter, but also minerals and dissolved nutrients. Compared with organic material produced within aquatic systems by algae and microorganisms, these terrestrial inputs were structurally more complex, decomposed more slowly, and entered ecosystems in more pulsed and concentrated episodes. This transformation fundamentally altered nutrient structure and energy flow in aquatic habitats.

At the same time, the expansion of forests contributed to rising atmospheric oxygen levels, a shift associated with global cooling. These atmospheric changes interacted with ongoing geological processes, intensifying sea-level fluctuations and promoting the formation of extensive shallow seas and epicontinental basins. Together, these environmental transformations created an unprecedented diversity of habitats, enabling rapid diversification among sarcopterygians and driving their taxonomic richness to a peak.

Sarcopterygian species diversity through time（Image source：Vanhaesebroucke O et al. (2025)， CC BY 4.0 ）

This expansion of morphological disparity can be observed across multiple anatomical dimensions. Some species evolved elongated, fusiform bodies suited for sustained swimming, such as Gooloogongia loomesi. Others developed shorter, deeper body plans, likely adapted to specific ecological niches or behaviors, as seen in Allenypterus montanus. Caudal fin morphology also diversified, ranging from heterocercal to diphycercal configurations, reflecting distinct locomotor strategies.

Reconstruction of Gooloogongia loomesi（Image source：ДиБгд， CC BY-SA 4.0 ）

Reconstruction of Allenypterus montanus（Image source：DiBgd， CC BY-SA 4.0 ）

The cheek region played a particularly important functional role in sarcopterygians. It contributed to jaw suspension and biting mechanics, while also influencing respiration and feeding. Variation in this region is evident across different evolutionary lineages. Coelacanths typically exhibit shorter, deeper cranial and cheek structures, whereas tetrapodomorph lineages, including elpistostegalians, display more flattened and elongated skulls. These differences reflect contrasting ecological strategies: the former are associated with aquatic predation and maneuverability, while the latter show adaptations to shallow-water or transitional environments, where a flattened skull may facilitate substrate-oriented feeding or surface-level prey detection.

Reconstruction of Panderichthys (Elpistostegalia)（Image source：Nobu Tamura， CC BY-SA 3.0 ）

The Late Devonian was marked by a series of major biological crises, most notably the Kellwasser and Hangenberg events. These episodes were associated with widespread marine anoxia, the collapse of coral reef ecosystems, and mass extinctions among marine invertebrates. Previously complex ecological networks were dismantled, and numerous ecological niches disappeared. Sarcopterygians were heavily impacted, with many lineages experiencing sharp declines in species richness.

This ecological collapse was mirrored in morphological patterns. Disparity in the cheek region and skull decreased significantly during this interval. As available ecological niches contracted and functional demands became more constrained, the range of viable morphological variation was reduced. In contrast, overall body shape disparity was less affected, indicating that different anatomical systems responded unevenly to environmental stress.

With the onset of the Carboniferous, recovery began. As environmental conditions stabilized, coral reef ecosystems re-emerged, and marine habitats regained structural complexity. This recovery provided new ecological opportunities for surviving sarcopterygian lineages. Morphological disparity increased once again, particularly in cranial and cheek structures, showing a clear rebound in diversification.

Morphological disparity of body (blue), cheek (red), and skull roof (green) across different time intervals（Image source：Vanhaesebroucke O et al. (2025)， CC BY 4.0 ）

The study also reveals that skull roof morphology in sarcopterygians was strongly influenced by phylogenetic relationships. Certain structural features, such as the presence or absence of an intracranial joint, played a decisive role. Lungfishes and tetrapods lack this joint, resulting in a fused neurocranium and a distinct pattern of skull organization. In morphospace, these groups form separate clusters, illustrating that not all morphological variation arises directly from environmental pressures. Instead, inherited anatomical frameworks can constrain or channel evolutionary trajectories.

Ultimately, some sarcopterygian lineages began to acquire features characteristic of early tetrapods and ventured onto land. The boundary between aquatic and terrestrial ecosystems was gradually eroded during this time, opening new evolutionary pathways. From this turning point onward, vertebrate evolution expanded into a far broader array of ecological possibilities.

Author: Shui-Ye You

Reference:

Vanhaesebroucke O et al. (2025). Early sarcopterygian morphological disparity through the Devonian-Carboniferous crisis. Scientific Reports.