Geoscience Reference
In-Depth Information
cient fish fossils preserved at Miguasha in 1842, and the cliffs were made a unesco World Heritage Site in
1999.
It now appears that Miguasha was part of a large drainage system on the southwest margin of the Euramer-
ican continent, which was flowing into the Rheic Ocean. The young Appalachians towered above it and were
shedding their sediments into it, forming an impressive delta that may have rivaled that of today's great river
basins, such as the Amazon, Mississippi, and Nile. During the Devonian, the warm estuarine waters at
Miguasha were overflowing with fish species, large and small.
Devonian fish species fall into two large groups: agnathans, or jawless, fishes, and the gnathostomes, the
jawed fishes. Only two of the agnathan group have survived: the lampreys and hagfish. These eel-like fishes
have sucker-like mouths furnished with a rasp. Lampreys attach themselves to other fishes, like salmon, which
they feed on parasitically, whereas hagfish scavenge dead and dying animals, such as whales that fall to the
ocean bottom. The rest of the jawless fishes disappeared at the end of the Devonian, as well as the placoderms,
a formerly dominant fish type that were armored with bony plates and had primitive jaws.
Three types of jawed fishes survived. Among them were the sharks and rays (chondrichthyans), which
thrive to this day. Another was the ray-finned fishes (actinopterygians), which, at 29,000 species, is the most
successful vertebrate group on the planet. They were the first true bony fishes, with a bony vertebral column
rather than the cartilaginous one that sharks and rays have. But perhaps the most important group, from an
evolutionary viewpoint, is the lobe-finned fishes, which gave rise to the first four-legged, air-breathing ter-
restrial vertebrates, the tetrapods. The highest number—some 3,000—and best-preserved specimens are
known from Miguasha. The most important of these lobe-finned fishes, Eusthenopteron foordi, has been nick-
named the Prince of Miguasha.
It has also been called “the fish with legs,” because its fin bones are so similar in structure to those of the
four-legged tetrapods, including amphibians, which would soon adapt to living part of their life on land. (Sub-
sequently, reptiles, birds, and mammals would also join the tetrapod assemblage.) This primitive fish shared
other traits with tetrapods, having teeth made of folded sheets of dentine and, perhaps most intriguingly, in-
ternal nares (called choanae) opening into the mouth, which would have allowed it to breathe air. It is not
known for sure that it was an air breather, but examination of the skeleton indicates that “the Prince” quite
likely had lungs. It also sported sail-like fins far back on the body, which allowed it to accelerate rapidly in
pursuit of prey. As a top predator, it was equipped with formidable fangs, which it obviously put to good use,
as whole fish have been found fossilized within its abdomen, including those of its own species.
It is its lobe fins, however, that mark its appearance as a milestone in the story of evolution. Eusthenopteron
had a powerful pair of pectoral fins that closely match the bone structure in the arms of early tetrapods. The
fins featured an upper arm bone (humerus), two forearm bones (radius and ulna), and primitive wrist bones.
Modeling experiments show that it could swing its muscular forelimb back and forth in a 20 to 25-degree arc.
Most of the movement was at the shoulder joint, but there was some flexibility at the elbow. It was not able to
raise itself off the ground and support its own weight as later amphibians could do, but it could drag itself
through the soft mud between drying pools in a primitive walking movement and in doing so take a first tentat-
ive step onto land.
By the end of the Devonian, invertebrates were moving onto the land. It is likely that leeches, flatworms,
and earthworms had come ashore, even though there is no fossil evidence of their presence. Snails and insects
also probably were now terrestrial. Fossil evidence discovered near Dalhousie Junction in northern New Brun-
swick demonstrates that a centipede-like creature Eoartheopleura was crawling on land, as was an air-breath-
ing scorpion. These two fossils are the earliest evidence of life on land in North America. It is not surprising
that they were arthropods, equipped to cope with water loss and the need for structural support—two of the
main challenges of life on land.
