Othniel Charles Marsh discovered the first fossils of this bird in the 1870s. This was, alongside the Archaeopteryx, one of the first Mesozoic birds to become known to science.
The 1 m (3 ft 4 in) long Baptornis had lost the ability to fly, possessing only vestigial wings. Unlike its larger relative Hesperornis, the manual bones were not fully reduced, and it seems to have retained a tiny, finger-like stub. Its legs were powerful and the feet were very large with long toes that could not rotate well, allowing the animal to swim and dive at considerable speed. Its foot, altogether, was rather similar to that of a large loon which it also resembled in overall bulk.
Thus the toes of Baptornis were probably webbed as in loons or ducks, rather than lobed as in grebes and Hesperornis: for birds with lobed toes, rotating the toes is necessary to reduce drag when pulling the foot forward for a new stroke. Still, no skin impressions have been found of webs or lobes, leaving the matter open to debate. What is known from fossil skin impression is that the tarsometatarsus was covered by larger scutes in front and smaller scales behind like in loons, rather than all-scaled as in grebes.
Like other Hesperornithes, Baptornis probably had teeth in its beak which allowed it to grab fish and other slippery prey. The skull is still unknown, but some teeth of the right size and shape have been found in the same rocks as Baptornis bones, and these are likely to be from the bird rather than a non-avian theropod dinosaur. Its neck was unusually long, further extending its reach.
The pygostyle was long, high and narrow. The tail thus probably was laterally compressed and served as rudder; a similar adaptation though less extreme is found in today's loons.
As its relatives, the bones of Baptornis were dense, much like in mammals. This helped the animal to dive by reducing its buoyancy. It also made the bones fossilize better, making them more common than those of flying birds like Ichthyornis which, compared to other vertebrate bones, are rare as fossils.
More material evidence exists for the ecology of B. advenus than for any other member of the Hesperornithes, with the possible exception of Hesperornis regalis, but still much is left to conjecture. The loon-sized bird was of middle size among its relatives and had a markedly elongated neck. Presumably, it thus behaved in a manner similar to today's darters, hunting smaller, more mobile prey than its larger relatives. Unlike a darter however, it could not spear its prey, but instead held it with its beak like today's mergansers.
The waters which it inhabited were fairly shallow epicontinental or shelf seas. Remains found far off the prehistoric shore suggest that it either ventured far out and/or that it bred on islands. A considerable number of juvenile specimens are known. These tend to be from the northern part of its range - today's Canada and Alaska - though they have also been found in Kansas. This suggests that the birds were migratory like some penguins are today, moving polewards in summer to breed. The Cretaceous had a much warmer climate than today; the waters inhabited by Baptornis were subtropical to temperate.
The long neck also provided extra mobility necessary since its feet were probably webbed rather than lobed, and thus optimized towards speed rather than maneuverability. With the dense bones, the animals probably swam half-submerged as darters and cormorants do. The wings played no major role in locomotion, but may have been helpful in changing depth and/or direction, similar to a submarine's diving planes or a fish's pectoral fins. It is even possible that it gained some additional maneuverability by the hand-stub, which despite being much reduced still could be moved independently at the wrist joint.
While it was excellently adapted to swimming and diving, Baptornis is thought to have been clumsy on land, pushing itself along the rocks with its feet rather than actually walking. The natural position of the lower legs was flush against the body, with the feet stretched out sideways and thus it would have been unable to move upright without toppling over. As opposed to Hesperornis which almost certainly had to slide on its belly or galumph like an earless seal, Baptornis's lower leg was not as firmly placed along the body sides. Thus, it would have found it more easy to place its feet under its body with the toes pointing forwards and might have managed small hops or even an awkward waddle, body held low to the ground.
The only certain record of Hesperornithes' food found so far comes from Baptornis: Specimen UNSM 20030 was found associated with some coprolites. These are small round lumps - maybe a centimeter in diameter or so - and contain the remains of a small species of the sabre-toothed "herring" Enchodus, possibly E. parvus. Baptornis had powerful gastric juices and/or regurgiated most indigestible parts of its prey as a pellet like most living fish-eating birds do, because the Enchodus remains make up only a small fraction of the coprolites' mass, most of which was nondescript feces.