Overview
Pterodon (Ancient Greek: (wing) + (tooth) meaning "wing tooth") is an extinct genus of hyaenodont in the family Hyainailouridae, containing two species. The type species Pterodon dasyuroides is known exclusively from the late Eocene to the earliest Oligocene of western Europe. The genus was first erected by the French zoologist Henri Marie Ducrotay de Blainville in 1839, who said that Georges Cuvier presented one of its fossils to a conference in 1828 but died before he could make a formal description of it. It was the second hyaenodont genus with taxonomic validity after Hyaenodon, but this resulted in taxonomic confusion over the validities of the two genera by other taxonomists. Although the taxonomic status of Pterodon was revised during the late 19th and early 20th centuries, it became a wastebasket taxon for other hyaenodont species found in Africa and Asia.
Today, only the type species is recognized as belonging to the genus while just one is pending reassessment to another genus. P. dasyuroides had cranial and dental characteristics typical of the Hyainailouridae such as an elongated, narrow, and proportionally large skull which measures ~ in length and dentition for hypercarnivorous diets and bone-crushing similar to modern hyenas. Due to the scarcely known postcranial materials of the species, its overall anatomy is unknown, although it likely weighed and may have been similar to another Eocene-aged European hyainailourine Kerberos. The hyainailourine made its appearance in western Europe back when it was a semi-isolated archipelago, likely originating from a ghost lineage from Afro-Arabia. It was one of the larger-sized carnivores of the continent, a typical trait of hyainailourines.
It coexisted largely with faunas that were adapted to tropical to subtropical environments and grew strong levels of endemism, becoming a regular component based on fossil evidence from multiple localities. Pterodon went extinct by the Grande Coupure extinction and faunal turnover event in the earliest Oligocene of Europe, which was caused by shifts towards seasonality plus glaciation as well as closing seaway barriers that allowed for large faunal dispersals from Asia. Its extinction causes are uncertain but may have been the result of rapid habitat turnover, competition with immigrant faunas, or some combination of the two. However, competition has been questioned as experts found no evidence of carnivorans competitively replacing hyaenodonts in Europe.
Early history
In 1838, French zoologist Henri Marie Ducrotay de Blainville made a review of palaeontological history and taxonomy as built upon from previous decades. Blainville recognized the importance of dentition in determining the affinities of fossil animals but criticized the overreliance of dental systems as automatically indicating taxonomic affinities. He then went on to examine reported fossil "didelphids" (the taxonomic group now known as "marsupials") in European land. He conducted reviews of fossils as previously described by Laizer and Parieu, concluding that Hyaenodon is a valid genus based on its dentition but rejected the idea of it being a didelph based on its dental system and its molars being closer in affinity to modern carnivorans.
Blainville also mentioned a fossil of an upper jaw that the late Georges Cuvier found and previously thought was close in affinity to the Tasmanian devil (Sarcophilus harrisii) of New Holland (Australia). Previously, Cuvier presented the fossil to the French Academy of Sciences in 1828 and thought that it was a large species of thylacine (Thylacinus cynocephalus) but died before he could make a formal description of it. Of note is that although the article of Hyaenodon by Blainville was first written and published in 1838, it was not until a year later in 1839 when it was republished that Blainville added a footnote to the paragraph about the upper jaw. In it, he said that he restudied that fossil from the Paris Basin and again was certain that it was a "monodelphian" (today placental) predator, which he named Pterodon dasyuroides.
The genus name means "wing tooth" and is a combination of the Ancient Greek words "" (wing) and "" (tooth). The etymology of the type species name derives from the Australian marsupial genus Dasyurus and the Greek suffix "" meaning "like" due to apparent initial confusion of the genus affinity by Blainville. In 1841-1842, Blainville mentioned the genus Pterodon but replaced the previous species name with P. parisiensis. He also stated that despite thinking that the mammal did not have close affinities with Dasyurus, he did not have sufficient skull material to prove what mammal group it was closest to. The species name is in reference to Paris where its first fossil was found.
Taxonomic disputes
The taxonomic positions of Pterodon and Hyaenodon for much of the 19th century were disputed by many palaeontologists. In 1846, Auguste Pomel argued that he was unsure if Pterodon belonged to the didelph clade based on its dentition being apparently similar to thylacines but having different skull structures from them and other marsupials. However, he rejected the position that Pterodon was more closely related to the monodelphs due to thinking that the skull did not closely resemble its members. Additionally, he said that it, Taxotherium, and Hyaenodon are functionally the same genus if not the same species, therefore stating that the latter two genera are synonymous with Pterodon.
Pomel defined four species for the genus Pterodon: P. parisiensis, P. cuvieri (erected previously for Taxotherium as T. parisiense by Blainville), P. leptorhynchus (erected for Hyaenodon by Lazier and Parieu), and P. brachyrhynchus (previously named for Hyaenodon by Blainville). In 1848-1852, however, he reclassified the species as Hyaenodon requieni (having recognized the validity of the genus) and listed P. dasyuroides as the only species of Pterodon. In 1853, Pomel changed his position by recognizing the validity of Hyaenodon, restoring the taxonomic affinities of species previously classified as belonging to it and therefore establishing that they no are no longer classified under Pterodon (H. leptorhynchus and H. brachyrhynchus). He also reclassified Taxotherium as a junior synonym of Hyaenodon instead of Pterodon.
Within the genus Pterodon, he recognized the species name P. dasyuroides instead of P. parisiensis and created two additional species based on dentition shapes and sizes: P. cuvieri and P. coquandi.
European hyaenodont revisions
Gervais erected the species P. exiguum in 1873 based on dentition with some similarities to both Pterodon and Hyaenodon but noted that it may constitute a new genus once he has more fossil material. The palaeontologist then corrected himself in 1876 by stating that the species belongs to Hyaenodon as H. exiguum, not Pterodon. In 1876, Filhol recognized only P. dasyuroides among all species previously erected for the genus and named a new species P. biincisivus from the phosphorite deposits of Escamps, France. He, in addition to confirming the taxonomic validities of the two species, erected a third named P. quercyi in 1882.
The French naturalist named a new species Oxyaena galliæ based on dental remains being apparently similar to that of species of Oxyaena previously described by Edward Drinker Cope in the Eocene deposits of New Mexico, United States. in The same year, Filhol also reported that an individual whose last name was Pradines recently discovered anterior portions of the skull of P. dasyuroides from the phosphate deposits of Limogne-en-Quercy. English naturalist Richard Lydekker made a review of known pan-carnivoran genera in 1884, classifying them within the order Carnivora and rejecting Cope's classification of the members into the suborder Creodonta within the order Bunotheria.
While Cope originally assigned Hyaenodon as the sole member of Hyaenodontidae and Pterodon plus Oxyaena into Oxyaenidae, Lydekker felt that Pterodon was close in affinity to Hyaenodon and therefore belonged in Hyaenodontidae. For H. brachyhynchus, he listed P. brachyhynchus, P. requieni, and H. requieni as junior synonyms. He also listed P. leptorhynchus as a junior synonym of H. leptorhynchus as well as H. exiguus and P. exiguus as synonyms of H. vulpinus. For Pterodon, he recognized P. dasyuroides as the main valid species of the genus and listed P. parisiensis as a definite synonym as well as P. cuvieri and P. coquandi as possible synonyms, although he did not invalidate P. biincisivus. He also listed the species Oxyæna galliæ but thought that the genus could be merged into Pterodon due to minor dental differences and similarities to P. biincisivus.
Swiss palaeontologist Ludwig Rütimeyer erected another species named P. magnus in 1891 based on the larger dentition sizes compared to typical species of Pterodon. However, in 1906, German scientist Rudolf Martin said that he wanted to synonymize P. magnum and the questionable P. quercyi with P. dasyuroides. He also listed P. biincisivus as a synonym of P. dasyuroides and stated that only one species is valid within Pterodon. Additionally, he revalidated the species O. galliæ but created the genus Paroxyaena for it, arguing that because oxyaenids are very weakly represented in Europe compared to North America and that therefore the species' similarities to oxyaenids may have an instance of convergent evolution. In 1979, Brigitte Lange-Badré made a systematic review of known hyaenodonts from Europe including Pterodon.
She listed P. parisiensis, P. cuvieri, P. coquandi, P. biincisivus, and P. quercyi as synonyms of the only European species P. dasyuroides. In addition, she listed P. magnum as a synonym of Paroxyaena galliae and listed Hyaenodon exiguus as taking taxonomic priority over H. vulpinus, therefore making the latter name and P. exiguum synonyms.
Wastebasket history
For much of its history, Pterodon was a wastebasket taxon for middle to late Paleogene and Miocene hyainailourines that lacked unique or advanced dental traits that Hyaenodon had. Many of the species classified or formerly classified to Pterodon were of African or Asian origins. Akhnatenavus leptognathus, "Hyainailouros" bugtiensis, Orienspterodon dahkoensis, and Neoparapterodon rechetovi. Several species names previously assigned to Pterodon were later considered to be synonyms of Hyaenodon species, namely "P. exploratus" (= H. incertus), "P. californicus" (= H. vetus), and "P. mongoliensis" (= H. mongoliensis). Additionally, Hemipsalodon was made a synonym of Pterodon by Robert Joseph Gay Savage in 1965 while Metapterodon was synonymized with it by Leigh M. Van Valen in 1967, but the synonymies were unsupported by later authors.
As a result of the synonymies, only one species assigned to Pterodon remains pending reassessment to another genus: P. hyaenoides, which is classified as belonging to the Hyaenodontinae and is located in Asia. In 1999, A.V. Lavrov mentioned in his dissertation paper a genus he erected named "Epipterodon" for which "Pterodon" hyaenoides would have been reclassified to. However, as the genus name has not yet been referenced and taxonomically validated in any peer-reviewed source, its name currently remains invalid.
Classification
horridus of North America. The Hyaenodontidae was recently determined to have been a separate lineage from the Hyainailouridae. Pterodon has historically been classified undisputedly as at least being within the clade of hyaenodonts within the later 20th century, later being included within the subfamily Hyainailourinae. In 2016, Matthew Borths et al. suggested that the Hyainailouridae had a close relationship with the Teratodontinae and therefore arranged the superfamily Hyainailouroidea to include them and include the Proviverrinae, Hyaenodontidae, and North American hyaenodont groups. Compared to the proviverrines which never exceeded , the hyainailourines were much larger in size. This is because P. dasyuroides does not form a natural clade with non-European species classified within the genus, therefore meaning that they are pending reassessments to other genera.
Pterodon sensu stricto (in a strict sense) made its appearance in western Europe by MP18 (late Eocene) in the form of P. dasyuroides and lasted up to MP20. P. dasyuroides was likely part of a ghost lineage of dispersing hyainailourines from Africa, and Kerberos did not appear to have descended into Pterodon or Parapterodon.
Skull
The order Hyaenodonta is diagnosed as having an elongated and narrow skull with a narrow cranial base (basicranium), a high and narrow occipital bone, and a transversely constricted interorbital region. Within the order, members of the Hyainailouridae share traits including a proportionally massive skull with an absence of any suture between the parietal bone and frontal bone, and a weak postorbital process (or projection), an extended pterygoid bone in its underside and side areas, the presence of a preglenoid crest, and side expansions of the squamosal relative to the back position of the zygomatic arch. and it is similar to that of Kerberos in being elongated and having a long rostrum and neurocranium. It is one of the two only known Eocene hyainailourine genera to be known by skull material, the other being Kerberos.
In the upper view of the skull, the nasals are convex dorsally (top area in the case of the skull) and extend far into the back beyond the front edge of the orbit. A short tubercle emerges from the nasals into the lacrimal bone, and then the nasals come into contact with the frontal bone by a W-shaped suture. The frontal eminences of the frontal bone, or the rounded elevations of the front of the upper skull, are not very prominent based on the weak depressions that are widely open. The skull lacks any postorbital process projections within its front area. The blunt brow ridges arise back from the upper edge of the orbits as triangular-shaped roughnesses and arrive at the sagittal plane area in front of the post-orbital constriction. The coronal suture is destroyed in known specimens and therefore has an unknown position relative to the post-orbital constriction.
The cranial vault is formed by the two parietal bones, and is gradually detached from the high and thin sagittal crest, which meets with the supraoccipital crest and forms a small triangular facet. The maximum width of the cranium barely exceeds that of the frontal bones at the upper edges of the orbits. In a lateral (or side) view, the premaxillae are short and extend a small distance between the nasals and maxillae, slightly preceding the canines. Their thick upper front edges form a roughly heart-shaped and subvertical nasal opening with a maximum width at the level of the nasal premaxillary suture. There is a slight projection of the root of the canine on the upper maxilla which is followed by an elongated depression that precedes that the long, narrow opening of the infraorbital canal.
The lacrimal bone extends widely onto the face in a large semicircle shape which restricts frontal-maxillary contact. There is no lacrimal tubercle present in the skull. The squamosal suture is low in position, not very convex, and is very inclined towards the zygomatic arches, explaining the triangular cross section of the skull. The squamosal roots of the zygomatic arches are found in a completely marginal position and, unlike modern carnivorans, are not under the temporal wall of the skull but outside it. The mastoid part of the temporal bone is between the retrotympanic part of the squamosal and the paroccipital process of the exoccipital region. While the zygomatic arches were not completely preserved in P. dasyuroides, those of the related Apterodon macrognathus and Kerberos langebadreae are dorsoventrally deep for robustness to support masseter muscles.
The face of the occipital bone is narrow and has a fan-like shape due to the strong narrowing at the level of the supraoccipital-squamosal suture followed by a strongly concave squamous part of the occipital bone. Due to this development, it differs strongly from that of Hyaenodon but is comparable to that of Apterodon. Typical of mammals with elongated snouts, the lower edge of the mandible (specimen Qu8636, National Museum of Natural History, France) is slightly curved. After the receding mandibular symphysis region, the mandible is rectilinear without any change in shape in the preangular region. The angle of the mandible is short and stocky for which the medial pterygoid muscle would have connected to.
Endocast anatomy
P. dasyuroides is known by a brain endocast, which was first described by Jean Piveteau in 1935. The natural endocast is stored at the National Museum of Natural History, France with no catalogue number. It is estimated to have an endocast volume of . Only limited morphologies can be observed as that of the surface was not well-preserved. The cerebellum is bulky compared to the front portion of the cerebrum because the former's cavity is not as large as the cerebral fossa and lack of coverage by the neocortex. The cerebellum appears higher in position than the cerebrum, and the cerebellar vermis strongly projects between the cerebellum's two hemispheres. The primary fissure of the cerebellum, located on the paleocerebellum-neocerebellum boundary, is in a backwards position, more so than certain "condylarths" such as Arctocyon and Pleuraspidotherium.
There is no other known transverse furrow on the cerebellar vermis.
Dentition
The subfamily Hyainailourinae within the family Hyainailouridae is diagnosed in the upper dentition as having high and secant-shaped paraconid cusps and cone-shaped metacone and paracone cusps on the M 1 -M 2 molars, a weak to absent P 3 lingual cingulum, and a lack of any continuous lingual cingulum on P 4 . For lower dentition, the M 3 talonid cusp is reduced compared to those on M 1 -M 2 , and the protoconid and paraconid cusps are roughly equal in length in the molars. P. dasyuroides differs from Kerberos by smaller-sized P 1 and P 1 premolars, a reduced number of upper incisors, a smaller protocone cusp on P 3 , and smaller size. P. dasyuroides is also described as having a dental formula of .
There is no evidence of any sexual dimorphism based on dentition. typica (A-E) compared to the upper left M 2 (F-G) and lower left M 3 (H-J) of Pterodon dasyuroides in different views In the upper dentition, the two incisors, conical in shape, are unequal in size, the central incisor being twice or thrice as large as the lateral incisor. Their shapes suggest that they play a role in piercing through food rather than cutting them. The upper canines, although much larger than other teeth of the upper dental row, are not as strong as those of extant hyenas, as they are less robust in relation to the skull. P 1 is small-sized, lacks any cingulate prominence except the anterior-lingual region where it connects with a ridge from the top of a cusp, and is formed by a conical cusp with a back half that is stretched into a lower edge.
P 2 is similar to P 1 in shape; its hook shape in its main cusp is prominent, and the cingulum runs from the ridges over the entire lingual surface. P 3 differs from the two other premolars by the top of the main cusp occupying a median position. The P 4 has a lingual expansion of the median area of the paraconid cusp but is difficult to distinguish by itself from the P 4 of Hyaenodon requieni. The first two upper molars have compressed and stretched metastyle ridges for a bladelike appearance that makes carnivorous diets unambiguous. The shapes of the two molars are of a right triangle with a front (anterior) base, and the first is smaller and more conservative in cuspid development than the second.
M 3 is wider than it is long and comes in two variants: it is either as wide as the two molars that precede it or is narrower than the two other molars and is stockier than the other variant. In terms of the lower dentition, there are 8-9 teeth, meaning a reduction in dentition compared to primitive placental mammals. The known individual specimens from Quercy only show one pair of lower incisors, which are rarely preserved and are thick with flat wear. Similar to the upper canines, the lower canines are long, slender, and not very curved. The P 1 is most often absent in development, although one specimen reveals one on the left side but not right of a mandible while another has them on both sides. It is a conical, short, and thick tooth with no distal stretching and two roots fused into one.
P 2 -P 3 have low and stubby crowns while P 4 is high and long, its height being 1.5 to 2 times that of P 3 . The M 1 , rarely adequately preserved, is subject to early and severe wear that levels the trigonid and talonid cusps, likely caused by frequent maximum-force occlusion with P 4 . The M 2 and M 3 , despite the disappearance of the metaconid cusp and the receded talonid cusp, are thick and not very sharp. The protoconid cusp is slender and is the dominant cusp within the last two molars. M 3 differs from M 2 by being larger in size and the smaller, triangular-shaped talonid cusp.
Postcranial remains
P. dasyuroides is known by very few postcranial remains, which themselves currently have no known whereabouts. Blainville illustrated postcranial remains designated to "Taxotherium parisiense" in 1841, but only the ulna and fibula were referenced as belonging to P. dasyuroides by later palaeontologists like Léonard Ginsburg in 1980, leaving the taxonomic statuses of the humerus, carpals + metacarpals, astragalus, and calcaneus ambiguous, especially since Taxotherium was synonymized with Hyaenodon. According to Ginsburg, the ulna of Hyainailouros sulzeri is arched and has a high plus well-developed olecranon (bony prominence on the elbow) and a long and strong diaphysis up to the distal end of the bone. The olecranon of H. sulzeri is long compared to those of carnivorans and it in a back position similar to artiodactyls.
The antero-external tubercle is diminished while the antero-internal tubercle well-developed, a trait Ginsburg said was also found to be similar to a sketched ulna of P. dasyuroides by Blainville in 1841. The fibula of H. sulzeri was described as being thicker in comparison to carnivorans, with the diaphysis being twice as antero-posteriorly elongated in its distal area as its proximal area. The fibula's astragalus surface being wide and long is similar to that of P. dasyuroides as also depicted by Blainville.
Size
In 1977, Radinsky made early size estimates for hyaenodonts with known skeletons and no known complete which he based size estimates off of other "creodonts." He estimated that P. dasyuroides had an estimated body length range of - and an estimated body weight range of -.
Palaeobiology
The order Hyaenodonta occupied a wide range of body sizes/body masses and ranged from mesocarnivorous to hypercarnivorous diets. The Hyainailourinae, which includes P. dasyuroides, was one hyaenodont lineage that gained hypercarnivorous adaptations given its various specific dental configurations. Due to an overall lack of postcranial remains known for P. dasyuroides in the modern day, the locomotion of the Paleogene hyainailourid is unknown. In comparison, adequate postcranial remains are known for Hyainailouros sulzeri, Kerberos langebadreae, and Simbakubwa kutokaafrika, allowing for determinations of the locomotion methods of the hyainailourids. The elbow of H. sulzeri reveals incapability of flexible pronation-supination movements compared to typical cursorial mammals.
The angulations and lengths of the fingers in relation to the metapodial bones, and the relation of the radius to the ulna suggest digitigrade movements of the forelimbs. The hindlimbs indicate similar results of digitigrade movement but, according to Ginsburg, have remnant traits of ancestral plantigrade movement. He theorized that it may have been semi-digitigrade overall with capabilities of leaping and occasional plantigrade movement. Similar traits of semi-digitigrady were also observed in S. kutokaafrika, with no capability of full digitigrade movement. Such adaptations of Miocene hyainailourines were likely the result of responses to more open environments. In comparison, however, the Paleogene hyainailourid Kerberos shows plantigrade stances and terrestrial locomotion based on known postcranial evidence.
The locomotion method made it differ from the more cursorial hyaenodontid Hyaenodon as well as hyaenids and borophagine canids, which all also displayed degrees of ossiphageous (bone-crushing) adaptations. The locomotion method of Kerberos suggests that plantigrady was a primitive trait of the order Hyaenodonta, including the hyainailourids, while digitigrady/semi-digitigrady adaptations were derived traits within the order. Kerberos is thought to have been an active predator and opportunistic scavenger based on its fossil evidence.
Early pre–Grande Coupure Europe
of Europe and Asia during the middle Eocene with possible artiodactyl and perissodactyl dispersal routes. For much of the Eocene, the world's environments were shaped by warm and humid climates, with subtropical to tropical closed forests being the dominant habitats. Multiple carnivorous mammal groups arose in Europe, Asia, Afro-Arabia, and North America, namely mesonychians, hyaenodonts, oxyaenids, and carnivoramorphs, dispersing between the continents. Land-based connections to the north of the developing Atlantic Ocean were interrupted around 53 Ma, meaning that North America and Greenland were no longer well-connected to western Europe.
From the early Eocene up until the Grande Coupure extinction event (56 Ma to 33.9 Ma), the western Eurasian continent was separated into three landmasses, the former two of which were isolated by seaways: western Europe (an archipelago), Balkanatolia, and eastern Eurasia (Balkanatolia was in between the Paratethys Sea of the north and the Neotethys Ocean of the south). The Holarctic mammalian faunas of western Europe were therefore mostly isolated from other continents including Greenland, Africa, and eastern Eurasia, allowing for endemism to occur within western Europe. By MP16, a faunal turnover event occurred that marked the extinctions of lophiodonts, European tapiroids, and all crocodylomorphs except for the alligatoroid Diplocynodon.
The causes of the faunal turnover have been attributed to a shift from humid and highly tropical environments to drier and more temperate forests with open areas and more abrasive vegetation. The surviving herbivorous faunas shifted their dentitions and dietary strategies accordingly to adapt to abrasive and seasonal vegetation. The environments were still subhumid and full of subtropical evergreen forests, however. The Palaeotheriidae was the sole remaining European perissodactyl group, and frugivorous-folivorous or purely folivorous artiodactyls became the dominant group in western Europe. During and after the faunal turnover event in western Europe, the Hyainailourinae made its first appearance in the continent by MP16 while the Hyaenodontinae appeared by MP17a and the carnivoran family Amphicyonidae by MP18.
Hyaenodonts remained the dominant carnivorous mammal group compared to the carnivoraforms.
Late Eocene
P. dasyuroides first appears in the western European fossil record by MP18, with a significant ghost lineage of probably Afro-Arabian origins that makes its exact evolutionary history unknown. non-endemic artiodactyls (Dichobunidae, Tapirulidae, and Anthracotheriidae), perissodactyls (Palaeotheriidae), primates (Adapoidea and Omomyoidea), eulipotyphlans (Nyctitheriidae), chiropterans, and endemic rodents (Pseudosciuridae, Theridomyidae, and Gliridae). The alligatoroid Diplocynodon, present only in Europe since the upper Paleocene, coexisted with pre-Grande Coupure faunas as well, likely consuming insects, fish, frogs, and eggs due to prey partitioning previously with other crocodylomorphs that had since died out by the late Eocene.
In addition to snakes, frogs, and salamandrids, rich assemblage of lizards are known in western Europe as well from MP16-MP20, representing the Iguanidae, Lacertidae, Gekkonidae, Agamidae, Scincidae, Helodermatidae, and Varanoidea, most of which were able to thrive in the warm temperatures of western Europe.
The MP19 locality of Escamps, for instance, indicates that P. dasyuroides coexisted with the herpetotheriids Peratherium and Amphiperatherium, pseudorhyncocyonid Pseudorhyncocyon, bats (Hipposideros, Vaylatsia, Vespertiliavus, Stehlinia), primates (Microchoerus, Palaeolemur), rodents (Blainvillimys, Theridomys, Plesiarctomys, Glamys), hyaenodont Hyaenodon, amphicyonid Cynodictis, palaeotheres Palaeotherium and Plagiolophus, dichobunid Dichobune, anoplotheriids Anoplotherium and Diplobune, cainothere Paroxacron and Oxacron, xiphodonts (Xiphodon, Dichodon, Haplomeryx), and amphimerycid Amphimeryx.
Extinction
in the Isle of Wight, from which Anoplotherium material has been collected. The stratigraphy of it and the Bouldnor Formation led to better understandings of faunal chronologies from the Late Eocene up to the Grande Coupure. The Grande Coupure extinction and faunal turnover event of western Europe, dating back to the earliest Oligocene (MP20-MP21), is one of the largest and most abrupt faunal events in the Cenozoic record, which is coincident with climate forcing events of cooler and more seasonal climates. The result of the event was a 60% extinction rate of western European mammalian lineages while Asian faunal immigrants replaced them. The Grande Coupure is often marked by palaeontologists as part of the Eocene-Oligocene boundary as a result at 33.9 Ma, although some estimate that the event began 33.6-33.4 Ma.
The event correlates directly with or after the Eocene-Oligocene transition, an abrupt shift from a greenhouse world characterizing much of the Paleogene to a coolhouse/icehouse world of the early Oligocene onwards. The massive drop in temperatures stems from the first major expansion of the Antarctic ice sheets that caused drastic pCO 2 decreases and an estimated drop of ~ in sea level. The seaway dynamics separating western Europe from other landmasses to strong extents but allowing for some levels of dispersals prior to the Grande Coupure are complicated and contentious, but many palaeontologists agreed that glaciation and the resulting drops in sea level played major roles in the drying of the seaways previously acting as major barriers to eastern migrants from Balkanatolia and western Europe.
The Turgai Strait is often proposed as the main European seaway barrier prior to the Grande Coupure, but some researchers challenged this perception recently, arguing that it completely receded already 37 Ma, long before the Eocene-Oligocene transition. Alexis Licht et al suggested that the Grande Coupure could have possibly been synchronous with the Oi-1 glaciation (33.5 Ma), which records a decline in atmospheric CO 2 , boosting the Antarctic glaciation that already started by the Eocene-Oligocene transition. The Oi-1 glaciation, similar to the first glaciation event, caused large drops in sea level and pushed the global climate towards a coolhouse/icehouse environment.
The Grande Coupure event also marked a large faunal turnover marking the arrivals of later anthracotheres, entelodonts, ruminants (Gelocidae, Lophiomerycidae), rhinocerotoids (Rhinocerotidae, Amynodontidae, Eggysodontidae), carnivorans (later Amphicyonidae, Amphicynodontidae, Nimravidae, and Ursidae), eastern Eurasian rodents (Eomyidae, Cricetidae, and Castoridae), and eulipotyphlans (Erinaceidae). MP20 also marks the last known appearance of Hyaenodon requieni while Hyaenodon gervaisi and Cynodictis extended beyond the extinction event. The extinction of Pterodon and survival of Hyaenodon by the Grande Coupure extinction event are notable but have no clear explanation available. The extinction causes have been attributed to climate deterioration (subsequent losses of suitable habitats and food), competition with dispersing carnivorans, or some combination of the two.
However, carnivorans playing a role in its extinction has been questioned as there’s no strong evidence of carnivorans competitively displacing hyaenodonts. Additionally, the newcomers didn’t appear in Europe until the MP21, while hyainailourines were last record in MP19-20.