Tytthostonyx is a genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of prehistoric seabird

Seabird

Seabirds are birds that have adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same environmental problems and feeding niches have resulted in similar adaptations...

. Found in the much-debated Hornerstown Formation

Hornerstown Formation

The Hornerstown Formation is a Paleogene or latest Mesozoic geologic formation. The age of these deposits have been controversial. While most fossils are of animals types known from the earliest Cenozoic era, several fossils of otherwise exclusively Cretaceous age have been found...

 which straddles the Cretaceous-Paleogene boundary 65 million years ago, this animal was apparently closely related to the ancestor of some modern birds, such as Procellariiformes

Procellariiformes

Procellariiformes is an order of seabirds that comprises four families: the albatrosses, petrels and shearwaters, storm petrels, and diving petrels...

 and/or "Pelecaniformes

Pelecaniformes

The Pelecaniformes is a order of medium-sized and large waterbirds found worldwide. As traditionally—but erroneously—defined, they encompass all birds that have feet with all four toes webbed. Hence, they were formerly also known by such names as totipalmates or steganopodes...

". A single species is placed herein, Tytthostonyx glauconiticus.

It has been placed into a family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

of its own, Tytthostonygidae.