Gomphotherium land bridge

The Gomphotherium land bridge was a land bridge that connected Eurasia to Afro-Arabia between approximately 19 and 15 million years ago (Ma) during the Burdigalian age.^[1] More recent analysis found that the Gomphotherium land bridge may have formed slightly earlier, around 20 Ma.^[2] It is believed that the connection between the Mediterranean and Indian oceans was temporarily re-established during the Langhian stage of the Middle Miocene, before reclosing, and has remained closed to present day.^[1]

Before the interchange

Isolation of Afro-Arabia

Apterodon langebadreae, a apterodontine hyaenodont

During the Early Cretaceous (Aptian-Albian), Africa became isolated from the rest of Gondwana and Laurasia, spending the rest of the Cretaceous as an island continent.^[3] Mammalian fauna endemic within the continent primarily consisted of Afrotheria (proboscideans, sirenians, embrithopods, hyracoids, and several other groups), metatherians (herpetotheriids), and hyaenodonts (hyainailouroids and several unranked clades).^[4][5][6][3]

Barytherium, a basal proboscidean

Proboscideans, the order that contains modern elephants, first evolved in Africa around 60 Ma, during the Selandian stage of the Paleocene; with Eritherium being the earliest known proboscidean, which weighed 3–8 kilograms (6.6–17.6 lb).^[7] By the Late Eocene, some taxa began to reach notable sizes such as Barytherium, which could’ve weighed 2 tonnes (2.0 long tons; 2.2 short tons).^[8] Some proboscideans such as Moeritherium were semi-aquatic.^[9]

Within the carnivore guild, hyaenodonts were the dominant carnivores of Africa.^[10] Hyaenodonts were consisted of groups such as hyainailouroids, boualitominae, and the Lahimia clade. Hyainailouroids, the most notable group, were one of the three superfamilies of the order Hyaenodonta and were thought to have originated from Afro-Arabia, evolving during the late Paleocene, despite the oldest fossils of hyainailourids being found in Asia.^[5][11][12]

Early Dispersals

Pterodon dasyuroides

Afro-Arabian origin

Hyainailourids had dispersed into Europe at least three times during the Paleogene via the Iberian Peninsula.^[13][14] Hyainailourines made their first appearance in at MP16, with the appearance of Paroxyaena galliae and Kerberos.^[15][13] The second dispersal was during the Late Eocene, being represented by Pterodon and Parapterodon.^[13] However, hyainailourines would later go extinct in Europe as the result of the Grande Coupure, the result of the turnover saw a 60% extinction rate for West European mammalian lineages.^[15] The third dispersal was during the Oligocene, represented by Apterodon intermedius.^[14] However, the last occurrence of Apterodon was at MP23, with their extinction likely being the result of global cooling.^[16] Hyainailourines also dispersed into Asia during the Eocene, which led to the evolution of Maocyon and Orienspterodon.^[11][12]

Embrithopods dispersed into Eurasia during the Paleocene, with additional evidence of dispersal into Europe during the Eocene.^[17]

Significance

Passage of fauna between Eurasia and the Arabian Plate and thus Africa was largely hindered before the Early Miocene, as animals could not cross the open Tethyan seaway. However, during the mid-Burdigalian, the tectonic plates of Afro-Arabia and Eurasia collided, creating a terrestrial isthmus connecting the two landmasses. This faunal exchange that resulted is known as the Proboscidean Datum Event.^[1] The land bridge allowed the elephantine gomphotheres and other proboscideans to migrate out of Africa and into Eurasia,^[18] which caused an increased in diversity for proboscideans.^[19]

Fossil evidence also showed hyainailouroids such as Hyainailouros, Dissopsalis, Metapterodon, and possibly Megistotherium dispersed into Eurasia.^{[13][20][12][21]} On the other hand, carnivorans, such as amphicyonids, dispersed from Eurasia into Afro-Arabia.^[12][22]