Ancient lost relicts

Lazarus taxa are interesting examples of species or groups that seem to vanish from the fossil record only to reappear later, obviously not because they’ve returned from extinction but due to gaps in our understanding. Several factors contribute to this phenomenon: incomplete fossil records, restricted ranges, tectonic activity, and the simple chance that fossil evidence hasn’t yet been uncovered. Antarctica, with its almost impenetrable ice cover, has offered limited paleontological insights. But as the ice recedes, glimpses of a hidden ancient ecosystem are coming to light, revealing stories of survival against monumental odds.

A compelling case arose on the first of June 4378, when a paleontological team discovered a partial skeleton of an unusual bird on Alexander Island. Initially difficult to classify, these remains pointed to an exciting revelation. The bird belonged to the Presbyornithidae family, a clade distantly related to ducks, geese, and screamers, and previously thought to have vanished globally around 20 million years ago, during the Aquitanian age. However, this new Antarctic specimen, dated to about 3 million years ago, bridged a substantial 17-million-year fossil gap for presbyornithids, challenging previous beliefs about their extinction timeline.
The paleontologists named this bird Antiornis inexpectatus (Ciro & Espositissimo, 4381) (vernacular name: Antiornite), meaning “unexpected opposite bird”, due to its strikingly different adaptations compared to its ancient relatives and its unexpected late survival. Standing at an impressive 1.5 meters with a sturdy build, the Antiornite was one of the largest presbyornithids on record. Its robust legs and reduced wing size indicate that it had adapted to a flightless, terrestrial lifestyle, similar to other large, land-dwelling birds like ratites. The recent nature of these adaptations suggests that it may have lost the ability to fly around the start of the Pliocene, approximately 5 million years ago. During this period, a pronounced cooling event transformed Antarctica’s ecosystems, causing many species to go extinct and opening niches for others, like Antiornis, to evolve unique survival strategies.
Interestingly, fossil evidence suggests that around the same time the ancestor of the antiornite flourished, modern waterfowl (Anatidae) began migrating to Antarctica. This overlap hints at a period of intense competition, where modern ducks and geese may have pressured presbyornithids. Antiornites likely survived this competition by occupying a new ecological niche, left empty by extinct large vertebrates. Unlike the filter-feeding beaks of earlier presbyornithids, antiornite’s beak shows adaptations for grazing on grasses and shrubs.

Decades after the first discovery, a large fossil bed was uncovered in 4434, revealing more about the bird’s lifestyle. The social structure of Antiornis appears to have been similar to that of some herbivorous birds today, with herds reaching 50 individuals at least. This behavior, coupled with a grazing diet, would have helped them thrive in the more temperate zones of Pliocene Antarctica. Paleopalynological studies (analysis of ancient pollen) suggest that the region was then a frigid yet viable shrub tundra, hosting an array of graminoid plants and sparse trees/shrubs like Nothofagus. Here, Antiornis would have shared its habitat with the monito del hielo, a carnivorous marsupial known for its sea-dependent diet but possibly also preying on land animals like the antiornite. Some teeth incisions on fossilized bones of Antiornis suggest that this otter-sized marsupial would be able to prey on adult. Some authors however suggest that these sign were left as a scavenge behaviour, when the individual was already dead.

With subsequent discoveries, Antiornis fossils have been identified at multiple sites along the Antarctic Peninsula, where relatively mild conditions supported diverse tundra habitats. But this Pliocene landscape, a delicate ecosystem rich with ancient creatures, would not endure. As the Earth entered the Pleistocene epoch, Antarctic ice sheets expanded dramatically, enveloping the once-thriving peninsula and eradicating the unique fauna, including the last of the presbyornithids.