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Description
With the year 2024, it's been a full decade since Gastornis turned vegan, so I decided to draw this in honor of it.

Gastornis is a relatively diverse genus of large flightless birds (having at least 5-6 species) that inhabited the Holarctic during the early Paleogene and it's perhaps the most iconic fossil taxon from that time. Their position in the bird family tree has been controversial, but nowadays, they are generally considered to be members of Anserimorphae, making extant waterfowl such as ducks, geese and swans their closest living relatives, but even closer to the gastornithids are the Australian mihirungs (dromornithids), which is easy to see from their appearance alone , and mihirungs outlasted their northern cousins by a considerable margin, surviving until just 45,000 years ago (as Australia stayed warm and tropical way past the lower Eocene). Together, these two families form the gastornithiforms.

The type species of the genus, Gastornis parisiensis (60-55 mya), was described in 1955 but as a chimera, leading to it originally being restored as a nearly 3-meter-tall crane-like bird , based on fossils from Thanetian-Ypresian strata in France (hence its species name), though some of the fossils attributed to it have recently been split into a new species ; Gastornis laurenti (2018). Meanwhile, the infamous Edward Cope of Bone Wars fame described Diatryma gigantea in 1876, based on another fragmentary set of fossils from the lower Ypresian Wasatch Formation of New Mexico (55-50 mya). Later, in 1916, a complete skull and accompanying skeleton was uncovered in the contemporary Willwood Formation of Wyoming, which finally made the big bird famous . For many decades, it was debated if Gastornis and Diatryma were synonymous, but once it was realized that the original restoration of G. parisiensis was a chimera, by the end of the 20th century, D. gigantea and purported Diatryma material from Europe were all sunk into Gastornis.

Equally controversial was the diet of these Cenozoic dinosaurs, since like all Cenozoic birds, Gastornis lacked teeth (oviraptorosaurs and ornithomimosaurs have the same issue). Throughout the 20th century, Gastornis were popularly depicted as apex predators of the Early Eocene jungles, hunting tiny horses like Eohippus and Propalaeotherium (or rather Eurohippus) but among paleontologists, this was a controversial topic, since these birds didn't posses any clear predatory features such as a hooked beak or sharp talons (further confirmed later by footprints attributed to gastornithids in 2012), and others theorized that they were herbivores, or at most generalized omnivores. In 2013 and 2014, two different isotope analyses on Gastornis fossils were performed, both coming to the conclusion that these avians were indeed herbivores . The powerful bite of Gastornis, which some workers in the past (most notably Lawrence Witmer) used as evidence that Gastornis were bone-crushing predators, seems to have evolved to help the birds crack nuts and other tough vegetation while foraging the jungle floor and/or using their long necks and great height to browse.

At the time Gastornis was alive, the Earth was going through the Paleocene-Eocene Thermal Maximum, leading to the whole globe being covered in tropical jungles. In that context, Gastornis was most analogous to today's cassowaries, who are native to Australia and New Guinea; a large, flightless, jungle-dwelling herbivorous bird, but on steroids and with a much larger head. Their great size, robust build, and powerful beaks meant that only the largest land predators from the time, such as Boverisuchus and Pachyaena , could have posed a threat to adult Gastornis.

All Gastornis species were large by bird standards, but G. gigantea (the sole North American species) was by far the largest, standing up to 2.1 meters tall, with the largest skull being half a meter long, and weighing up to 180 kg. Other species were smaller, like G. geiselensis, who is known from a single femur from the Messel Pits, and more complete material from the adjacent Geiseltal site , as it reached 1.7 meters in height and about 135 kg in weight. Based on the current fossils record, Gastornis seems to have emerged in Europe during the Paleocene and then radiated across the rest of the Holarctic by the dawn of the Eocene (lower Ypresian). The ancestors of G. gigantea likely arrived over the Turgai Strait (along with other animals like early horses and panzer crocs), and it's the most completely known species of Gastornis, while the token Asian species, G. xichuanensis (formerly Zhongyuanus), is known only from a single tibiotarsus from Henan, China, first described in 1980. By the upper Ypresian-lower Lutetian (50-45 mya), these big birds seem to have become restricted to Europe once more, with no fossils being known from Mid Eocene strata (50-45 mya) outside of Europe, with the German Gastornis geiselensis being the youngest known species, vanishing about 45 million years ago.

In their ancient rainforest home that stretched across the now temperate Northern Hemisphere, Gastornis ranked among the largest animals, and are found alongside many smaller mammals but also several early examples of mammalian megafauna. At Willwood (55-50 mya), G. gigantea was sympatric with a rich menagerie of early mammals; primitive bats (Icaronycteris), miacids like Vulpavus (stem-carnivorans), non-bipedal leptictids (Palaeictops, Prodiacodon), small marsupials (like Mimoperadectes), very basal, lemur-like primates/stem-primates ( Carpolestes, Cantius, Bownomomys, etc.), oxyaenids (Oxyaena and Palaeonictis), fox-sized to coyote-sized basal hyaenodonts (Prolimnocyon, Tritemnodon), early equids (Eohippus) and other early ungulates (Diacodexis, Phenacodus, Meniscotherium, Homogalax, Ectocion), taeniodonts (Ectoganus), the tapir-sized pantodont Coryphodonn (some specimens grew as big as small rhinos), and several species of the mesonychid Pachyaena, from the lynx-sized P. gracilis to the tiger-sized P. gigantea (the likely apex predator of this biome). Gastornis has so far not been found in later, upper Ypresian-lower Lutetian strata like the Bridger Formation (50-45 mya), where we first see the rhino-sized and utterly bizarre Uintatherium and Eobasileus show up (the oldest known multi-ton land mammals), though more basal dinoceratans such as the tiger-sized Prodinoceras already inhabited North America by the end of the Paleocene.

Likewise, G. geiselensis was sympatric with a rich menagerie of animals in early Lutetian Germany (48-45 mya), such as numerous types of bats (Hassianycteris, Archaeonycteris, Palaeochiropteryx, etc.), the apatemyid Heterohyus, the dichobunids Messelobunodon and Aumelasia, the otter-like pantolestid Buxolestes, the squirrel-like paroxyclaenid Kopidodon, the amphilemurids Macrocranion and Pholidocercus, three species of the hopping (or at least bipedal) leptictid Leptictidium, the miacids Messelogale and Paroodectes, the ferret-sized hyaenodont Lesmesodon, the stem-pangolins Eurotamandua and Eomanis, the basal equids Eurohippus and Propalaeotherium (though the latter might be a palaeotheriid, as its name originally implied), stem-lemur adapids (Europolemur, Godinotia and Darwinius), ancient rodents (Masillamys, Ailuravus), and the fully terrestrial planocraniid crocodile Boverisuchus magnifrons, along with the more conventional croc "Asiatosuchus" germanicus and smaller Diplocynodon, Hassiacosuchus, and Bergisuchus. Another predator known from the same time (48-46 mya) is the fragmentary Eleutherornis, an early phorusrhacid whose fossils come from France, and it's estimated to have stood up to 1.5 meters tall.

Anatomical Reference: twitter.com/skeletaldrawing/st…

Description
With the year 2024, it's been a full decade since Gastornis turned vegan, so I decided to draw this in honor of it.

Gastornis is a relatively diverse genus of large flightless birds (having at least 5-6 species) that inhabited the Holarctic during the early Paleogene and it's perhaps the most iconic fossil taxon from that time. Their position in the bird family tree has been controversial, but nowadays, they are generally considered to be members of Anserimorphae, making extant waterfowl such as ducks, geese and swans their closest living relatives, but even closer to the gastornithids are the Australian mihirungs (dromornithids), which is easy to see from their appearance alone , and mihirungs outlasted their northern cousins by a considerable margin, surviving until just 45,000 years ago (as Australia stayed warm and tropical way past the lower Eocene). Together, these two families form the gastornithiforms.

The type species of the genus, Gastornis parisiensis (60-55 mya), was described in 1955 but as a chimera, leading to it originally being restored as a nearly 3-meter-tall crane-like bird , based on fossils from Thanetian-Ypresian strata in France (hence its species name), though some of the fossils attributed to it have recently been split into a new species ; Gastornis laurenti (2018). Meanwhile, the infamous Edward Cope of Bone Wars fame described Diatryma gigantea in 1876, based on another fragmentary set of fossils from the lower Ypresian Wasatch Formation of New Mexico (55-50 mya). Later, in 1916, a complete skull and accompanying skeleton was uncovered in the contemporary Willwood Formation of Wyoming, which finally made the big bird famous . For many decades, it was debated if Gastornis and Diatryma were synonymous, but once it was realized that the original restoration of G. parisiensis was a chimera, by the end of the 20th century, D. gigantea and purported Diatryma material from Europe were all sunk into Gastornis.

Equally controversial was the diet of these Cenozoic dinosaurs, since like all Cenozoic birds, Gastornis lacked teeth (oviraptorosaurs and ornithomimosaurs have the same issue). Throughout the 20th century, Gastornis were popularly depicted as apex predators of the Early Eocene jungles, hunting tiny horses like Eohippus and Propalaeotherium (or rather Eurohippus) but among paleontologists, this was a controversial topic, since these birds didn't posses any clear predatory features such as a hooked beak or sharp talons (further confirmed later by footprints attributed to gastornithids in 2012), and others theorized that they were herbivores, or at most generalized omnivores. In 2013 and 2014, two different isotope analyses on Gastornis fossils were performed, both coming to the conclusion that these avians were indeed herbivores . The powerful bite of Gastornis, which some workers in the past (most notably Lawrence Witmer) used as evidence that Gastornis were bone-crushing predators, seems to have evolved to help the birds crack nuts and other tough vegetation while foraging the jungle floor and/or using their long necks and great height to browse.

At the time Gastornis was alive, the Earth was going through the Paleocene-Eocene Thermal Maximum, leading to the whole globe being covered in tropical jungles. In that context, Gastornis was most analogous to today's cassowaries, who are native to Australia and New Guinea; a large, flightless, jungle-dwelling herbivorous bird, but on steroids and with a much larger head. Their great size, robust build, and powerful beaks meant that only the largest land predators from the time, such as Boverisuchus and Pachyaena , could have posed a threat to adult Gastornis.

All Gastornis species were large by bird standards, but G. gigantea (the sole North American species) was by far the largest, standing up to 2.1 meters tall, with the largest skull being half a meter long, and weighing up to 180 kg. Other species were smaller, like G. geiselensis, who is known from a single femur from the Messel Pits, and more complete material from the adjacent Geiseltal site , as it reached 1.7 meters in height and about 135 kg in weight. Based on the current fossils record, Gastornis seems to have emerged in Europe during the Paleocene and then radiated across the rest of the Holarctic by the dawn of the Eocene (lower Ypresian). The ancestors of G. gigantea likely arrived over the Turgai Strait (along with other animals like early horses and panzer crocs), and it's the most completely known species of Gastornis, while the token Asian species, G. xichuanensis (formerly Zhongyuanus), is known only from a single tibiotarsus from Henan, China, first described in 1980. By the upper Ypresian-lower Lutetian (50-45 mya), these big birds seem to have become restricted to Europe once more, with no fossils being known from Mid Eocene strata (50-45 mya) outside of Europe, with the German Gastornis geiselensis being the youngest known species, vanishing about 45 million years ago.

In their ancient rainforest home that stretched across the now temperate Northern Hemisphere, Gastornis ranked among the largest animals, and are found alongside many smaller mammals but also several early examples of mammalian megafauna. At Willwood (55-50 mya), G. gigantea was sympatric with a rich menagerie of early mammals; primitive bats (Icaronycteris), miacids like Vulpavus (stem-carnivorans), non-bipedal leptictids (Palaeictops, Prodiacodon), small marsupials (like Mimoperadectes), very basal, lemur-like primates/stem-primates ( Carpolestes, Cantius, Bownomomys, etc.), oxyaenids (Oxyaena and Palaeonictis), fox-sized to coyote-sized basal hyaenodonts (Prolimnocyon, Tritemnodon), early equids (Eohippus) and other early ungulates (Diacodexis, Phenacodus, Meniscotherium, Homogalax, Ectocion), taeniodonts (Ectoganus), the tapir-sized pantodont Coryphodonn (some specimens grew as big as small rhinos), and several species of the mesonychid Pachyaena, from the lynx-sized P. gracilis to the tiger-sized P. gigantea (the likely apex predator of this biome). Gastornis has so far not been found in later, upper Ypresian-lower Lutetian strata like the Bridger Formation (50-45 mya), where we first see the rhino-sized and utterly bizarre Uintatherium and Eobasileus show up (the oldest known multi-ton land mammals), though more basal dinoceratans such as the tiger-sized Prodinoceras already inhabited North America by the end of the Paleocene.

Likewise, G. geiselensis was sympatric with a rich menagerie of animals in early Lutetian Germany (48-45 mya), such as numerous types of bats (Hassianycteris, Archaeonycteris, Palaeochiropteryx, etc.), the apatemyid Heterohyus, the dichobunids Messelobunodon and Aumelasia, the otter-like pantolestid Buxolestes, the squirrel-like paroxyclaenid Kopidodon, the amphilemurids Macrocranion and Pholidocercus, three species of the hopping (or at least bipedal) leptictid Leptictidium, the miacids Messelogale and Paroodectes, the ferret-sized hyaenodont Lesmesodon, the stem-pangolins Eurotamandua and Eomanis, the basal equids Eurohippus and Propalaeotherium (though the latter might be a palaeotheriid, as its name originally implied), stem-lemur adapids (Europolemur, Godinotia and Darwinius), ancient rodents (Masillamys, Ailuravus), and the fully terrestrial planocraniid crocodile Boverisuchus magnifrons, along with the more conventional croc "Asiatosuchus" germanicus and smaller Diplocynodon, Hassiacosuchus, and Bergisuchus. Another predator known from the same time (48-46 mya) is the fragmentary Eleutherornis, an early phorusrhacid whose fossils come from France, and it's estimated to have stood up to 1.5 meters tall.

Anatomical Reference: twitter.com/skeletaldrawing/st…