Understanding what sharks looked like 400 million years ago requires examining the Early Devonian Period, often called the “Age of Fishes.” This era saw an explosion of diversity among marine vertebrates, laying the groundwork for all subsequent fish groups. The earliest stem-sharks belong to the Chondrichthyes, the class of cartilaginous fishes that includes modern sharks, rays, and chimaeras. The fossil record reveals a creature recognizable as a shark but significantly more primitive than any species swimming today.
Establishing the Earliest Shark Lineage
The ancestors of cartilaginous fish split from bony fish (Osteichthyes) sometime in the late Silurian or Early Devonian. The fossil record from this period is notoriously incomplete because the skeletons of these ancient creatures were made of cartilage, which rarely mineralizes and preserves well. Consequently, evidence for the 400-million-year-old shark lineage often consists only of isolated teeth, fin spines, and microscopic dermal denticles.
These fragments confirm the presence of early jawed fish belonging to the Chondrichthyes group, such as the 410-million-year-old Doliodus problematicus. Paleontologists rely on these sparse remains, along with younger Devonian sharks like Cladoselache, to reconstruct the anatomy of the 400-million-year-old ancestor. This approach provides a coherent picture of the first true shark-like forms to emerge.
Key Anatomical Features
The Early Devonian shark was defined by its cartilaginous skeleton, which provided a lightweight, flexible body plan. Unlike bony fish, these creatures lacked calcified vertebral centra; their main support was a persistent notochord, a flexible rod of cartilage. This primitive skeleton contributed to the rarity of complete fossil specimens, as the tissue decayed rapidly after death.
A defining difference from modern species was the terminal mouth position, located at the front of the snout. This arrangement gave the shark a blunt, torpedo-like head shape, unlike the underslung, ventral mouth found in modern species like the Great White. Their teeth were simple and multi-cusped, characterized as “cladodont” teeth, designed primarily for hooking and grasping prey rather than tearing or crushing.
The fins were broad-based and relatively rigid, lacking the complex skeletal supports found in modern species. This limited maneuverability, suggesting they were likely fast, straight-line swimmers rather than adept at precise, tight turns. A prominent, stout spine was often present just in front of the dorsal fin, likely offering a degree of defense against larger predators. The skin was covered in dermal denticles, which are tiny, tooth-like scales, though some forms like Cladoselache had a nearly naked body with denticles clustered around the eyes and fin edges.
The Devonian Environment and Niche
The Early Devonian oceans were warm and extensive, hosting a wide array of life forms in a period of intense evolutionary competition. Early sharks shared these waters with armored placoderms, a dominant group of jawed fish that included massive predators like Dunkleosteus. They also coexisted with rapidly diversifying bony fish and various forms of jawless fish that were beginning to decline.
These early sharks occupied a niche as active, pelagic predators, relying on speed to hunt. Their terminal mouth and grasping teeth suggest a feeding strategy of capturing and swallowing smaller prey whole, rather than taking large, tearing bites. Their diet consisted of smaller, early bony fish and marine invertebrates like cephalopods and trilobites. This role placed them in the middle to upper levels of the marine food web, competing with other burgeoning groups of jawed fish.
Ancient Structure Versus Modern Form
The 400-million-year-old stem-shark represented a successful blueprint, but it lacked several major evolutionary innovations that define modern sharks. The most significant difference lay in the jaw structure, as ancient forms possessed jaws firmly attached to the cranium. This fixed structure prevented the protrusible, or slingshot-like, jaw action that allows many modern sharks to extend their mouth forward to snatch prey.
Their fixed, broad fins provided less sophisticated control compared to the highly mobile pectoral fins of a modern shark, which allow for precision steering and braking. The lack of a highly calcified skeleton and the simpler tooth design demonstrate a less specialized form optimized for general predation. While fast and streamlined, these early sharks lacked the advanced mechanics for feeding and maneuverability that allowed their descendants to dominate marine ecosystems.