Sharks have captivated scientists and the public alike for centuries for many reasons. Their predator prowess. Their diversity. Their evolutionary tale spanning millions of years. And recent research gives scientists a further glimpse into their lifespans, indicating that selachians (the group that includes all sharks) originated in benthic or benthopelagic habitats before expanding into the pelagic zone around 122.6 million years ago. This shift coincided with significantly higher sea surface temperatures than we experience today. Fossil records and prior studies suggest that Lamniformes were the first group to venture into these open waters, leading to increased rates of diversification. Following them, Carcharhiniformes also expanded into the pelagic zone. Both groups experienced increased diversification rates, a trend that continues with the majority of extant pelagic sharks belonging to these lineages.
A key to understanding how sharks navigate their environments lies in their pectoral fins. Yup, it turns out they play a crucial role in the swimming mechanics of sharks, and their morphology is closely linked to ecological niches. The study found evidence supporting the idea that pectoral fin shape is an adaptive trait shaped by ecological demands. Benthic sharks, which can rest on the ocean floor, tend to have shorter, rounded pectoral fins, while pelagic sharks, which are obligate ram ventilators that must continuously swim to breathe, have longer, more streamlined fins. This adaptation likely helps reduce energy expenditure while swimming, a vital factor for survival in the energy-demanding pelagic environment. While the data do not show a direct correlation between fin aspect ratio and body size across all species, larger pelagic sharks tend to possess higher aspect ratios in their pectoral fins. Larger body sizes translates to several advantages for these predators, such as increased buoyancy, energy storage, and the ability to undertake long migrations. However, larger bodies require more energy to move, reinforcing the selection for higher pectoral fin aspect ratios.
The Cretaceous period, specifically from the Barremian to the Cenomanian ages, marks critical phases in shark evolution. Phylogenetic analyses indicate that by the Barremian, selachians had begun to occupy open-water habitats. During this time, both the body size of sharks and the aspect ratio of their pectoral fins increased significantly. In fact, fossil records reveal that Lamniform sharks grew to impressive sizes, reaching lengths exceeding six meters by the Albian period, marking them as formidable pelagic predators. One of the most notable species from this time is Cretoxyrhina mantelli, a shark akin to the modern great white and often referred to as the "ginsu shark." Cretoxyrhina dominated its marine environment during a time characterized by high sea temperatures and ecological complexity. The fossil record also indicates that during the Late Jurassic, many sharks had lower aspect ratios, which raises questions about the evolutionary processes at play. Could environmental conditions and fossil preservation biases explain the predominance of these traits in early selachian species? Perhaps. For now, we just don't know.
The mid-Cretaceous period was a pivotal moment for shark evolution, driven by volatile climate conditions and exceptionally high SSTs, averaging around 23 degrees Celsius. Warmer sea surface temperatures during the mid-Cretaceous, particularly the Cenomanian-Turonian Thermal Maximum, likely influenced muscle-driven performance in sharks. As temperatures increased, the swimming performance of sharks also improved, with pelagic species capable of higher speeds than benthic ones. This performance advantage likely facilitated their successful adaptation to pelagic life. However, while pelagic sharks thrived, some species faced physiological limits in extremely warm waters.
Despite the strong correlation between temperature and the evolution of pectoral fin morphology, the scientists argue that it is crucial to recognize that SST alone cannot account for the continued diversification and adaptation of sharks. Even during periods of cooling over the last 30 million years, the disparities in aspect ratios among different shark lineages persisted! This suggests the influence of other factors, such as the emergence of coral reefs and changes in prey availability, which could have driven the evolutionary trajectory of sharks. "We posit that integrative studies that combine ecomechanical and ecophysiological approaches to chondrichthyan macroevolution will improve our understanding of vertebrate radiations in the oceans," the authors conclude.