Wikipedia

Anoplotherium

Anoplotheriumis thetype genusof the extinctPalaeogeneartiodactylfamilyAnoplotheriidae,which was endemic toWestern Europe.It lived from the lateEoceneto the earliestOligocene.It was the fifth fossil mammal genus to be described with official taxonomic authority, with a history extending back to 1804 when its fossils fromMontmartreinParis,France were first described by the French naturalistGeorges Cuvier.Discoveries of incomplete skeletons ofA. communein 1807 led Cuvier to thoroughly describe unusual features for which there are no modern analogues. His drawn skeletal and muscle reconstructions ofA. communein 1812 were amongst the first instances of anatomical reconstructions based on fossil evidence. Cuvier's contributions to palaeontology based on his works on the genus were revolutionary for the field, not only proving the developing ideas ofextinctionand ecological succession but also paving the way for subfields such aspalaeoneurology.Today, there are four known species.

Anoplotherium
Temporal range: LateEocene– earlyOligocene37–33.4Ma
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Anoplotherium communeincomplete skeleton from the commune ofPantin,National Museum of Natural History, France
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Family: †Anoplotheriidae
Subfamily: †Anoplotheriinae
Genus: †Anoplotherium
Cuvier,1804
Type species
†Anoplotherium commune
Cuvier, 1804
Other species
  • †A. laurillardiPomel, 1851
  • †A. latipesGervais, 1852
  • †A. pompeckjiDietrich, 1922
Synonyms
Genus synonymy
  • EurytheriumGervais, 1852
Synonyms ofA. commune
  • Anoplotherium duvmoyiPomel, 1851
Synonyms ofA. latipes
  • Anoplotherium platypusPomel, 1851
  • Anoplotherium tridactylumKovalevsky, 1873
Synonyms ofA. laurillardi
  • Anoplotherium cuvieriPomel, 1851

Anoplotheriumwas amongst the largest non-whippomorphartiodactyls of the Palaeogene period, weighing on average 115 kg (254 lb) to 271 kg (597 lb) and measuring at least 2.5 m (8 ft 2 in) in head and body length and 1.25 m (4 ft 1 in) in shoulder height. It was an evolutionarily advanced and unusual artiodactyl, sporting three-toed feet in certain species likeA. latipes,a long and robust tail, and a highly-developed brain with strong support for both sense of smell andsensory perception.Its overall robust build may have allowed it to stand bipedally to browse on plants at greater heights, reaching approximately 3 m (9.8 ft) tall, effectively competing with the few other medium to large herbivores it lived with. The full extent of its bipedalism needs to be confirmed by more research, however. The larger, two-toedA. communeand slightly smaller, three-toedA. latipesmay be sexual dimorphs in that the former is female and the latter male, but this idea remains speculative. Its closest relative wasDiplobune,which similarly is hypothesized to have had specialized behaviours.

The artiodactyl lived in western Europe back when it was anarchipelagothat was isolated from the rest of Eurasia, meaning that it lived in an environment with various other faunas that also evolved with strong levels of endemism. Its exact origins are unknown, but it arose long after a shift towards drier but still subhumid conditions that led to abrasive plants and the extinctions of the large-sizedLophiodontidae,achievinggigantismand establishing itself as a dominant herbivore throughout the entirety of the western European region given its abundant fossil evidence.

Its success was abruptly halted by theGrande Coupureextinction and faunal turnover event in the earliest Oligocene of western Europe, which was caused by shifts towards further glaciation and seasonality. Tropical and subtropical forests were rapidly replaced by more temperate environments, and most ocean barriers previously separating western Europe from eastern Eurasia closed, allowing for large faunal dispersals from Asia. Although the specific causes are uncertain, Anoplotherium was likely unable to adapt to these major changes and succumbed to extinction.

Taxonomy

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Research history

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Main article:Research history ofAnoplotherium

Identifications

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Sketches of dental, cranial, and limb remains ofAnoplotherium communebyGeorges Cuvier,1804

While Georges Cuvier knew about fossil bones from the gypsum quarries of the outskirts ofParis(known as theParis Basin) as early as at least 1800, it was not until 1804 that he would describe them. After describingPalaeotherium,he wrote about the next set of fossils that he was able to discern as being different fromPalaeotheriumbased on dentition form, including the apparent lack ofcaninesthat left a large gap between theincisorsandpremolars.He observed that the hemimandible (half amandible) had three lower incisors instead of four incisors or none which he said characterized other "pachyderms".Cuvier, basing the name on its apparent lack of suitable arms and canines for offensive attacks, erected the nameAnoplotherium.[1][2]

Thegenus nameAnoplotheriummeans "unarmed beast" and is a compound of theGreekwordsαν-(an,'not'),ὅπλον(hóplon,'armor, large shield'), andθήρ(thēr,'beast, wild animal').[3]

Cuvier named three species ofAnoplotheriumin the same year, the first of which was the "sheep-sized"A. communeand the other three of which were "smaller species" that he namedA. medium,A. minus,andA. minimum.The etymology of the species nameA. communerefers to how "common" fossils of the species were while the etymologies of the other two species were based on sizes compared toA. commune.[a]He also attributed acloven hoof(or didactyl hoof) toA. communesince the specimen appeared to be large-sized. He thought thatAnoplotheriumhad didactyl hooves instead of tridactyl hooves, which would have separated it fromPalaeotherium.Based on the hooves and dentition, he concluded thatAnoplotheriumwas similar toruminantsorcamelids.[4][5]However, in 1807, Cuvier found out thatAnoplotherium communehad three toes on its hind limbs, although the third index toes were of smaller sizes compared to the other two.[6]

Skeletons

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Sketches of two incomplete, embedded skeletons ofAnoplotherium commune,found in the communes ofPantin(left skeleton) andAntony(right skeleton)

In 1807, Cuvier wrote about two incomplete skeletons that were recently uncovered, although the first was partially damaged because it was not collected carefully (which he expressed as having frustrated his understanding of the skeletal anatomy ofAnoplotheriuminitially). The first skeleton, found in the quarries ofMontmartrein the commune ofPantin,helped to confirm Cuvier's earlier diagnoses ofAnoplotheriumas correct. The embedded skeleton was the size of a small horse and helped to confirm the large didactyl feet and the 44 total teeth that it had (11 in each side of its jaw). It also had 11 complete ribs and a fragment of a 12th, matching with the number of ribs of camelids. The most surprising element to Cuvier, however, was the enormous tail with 22 vertebrae in the skeleton, a feature that he said he would not have known about previously, as there are no modern analogues of the elongated and thick tail in any large quadrupedal mammal.[7]

The second incomplete skeleton came fromAntony,this time more carefully removed with supervision from experts than the first skeleton. In it, he was able to confirm sixlumbar vertebraeand threesacral vertebrae,all of which were extremely strong and probably supported the long tail. Most notable to Cuvier was the confirmation thatAnoplotheriumhad two large fingers and one small finger on its front legs, which was unusual for mammals related to it.[7]

Significance in palaeontological history

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Anoplotherium communeskull,National Museum of Natural History, France

AlthoughPalaeotheriumandAnoplotheriumare not well-recognized compared to fossil animals of other periods (i.e.Mesozoicdinosaurs andNeogene-Quaternarymammals), their fossil discoveries in Montmartre and formal descriptions by Cuvier are recognized as critical moments that pioneered palaeontology to the modern era. UnlikePleistocenefossil genera in the Americas in early palaeontological history such asMegatheriumandMammut,PalaeotheriumandAnoplotheriumwere not found in surface-level deposits but embedded in deeper, harder rock deposits dating to theEocene.People in Paris had been previously familiar with animal skeletons being in their area for centuries, some of which were later kept and formally described. However, it was Cuvier who formally erected two fossil genera that came from older deposits, and from his homeland in the continent of Europe instead of the Americas whereMegatheriumandMammutwere found.[8]ThePalaeogene-aged fossils left no evidence of any later descendants, extinct or extant, although the similarities ofPalaeotheriumto tapirs made proving the theory more difficult. He noticed that below the gypsum was older sediments of seashells and reptiles like what Cuvier described as a giant "crocodile", which would later be known asMosasaurus.Cuvier knew then that the world thatAnoplotheriumandPalaeotheriumcame from was a different span of time before that of the preceding time of sea reptiles and the proceeding times ofMegatheriumandMammut,thereby proving the concept of natural extinction.[9]

Cuvier's descriptions of anendocast(fossilized brain case) of acerebral hemispherebelonging to a broken skull ofA. communefrom Montmartre, starting from 1804 up to 1822, are recognized as the first true instance ofpalaeoneurology,the study of brain evolution. The very first definition of an "endocast" dates back to 1822 when Cuvier described a mould of the brain ofA. commune,noticing that it offered hints to the true shape of the brain of the now-extinct mammal (although it was later found to be a portion of the brain rather than the entirety of it). Since the first endocast study, many other brain studies were conducted for other fossil mammals throughout the second half of the 19th century onward.[10][11][12]An 1822 description by Cuvier of a healed fractured femur ofA. communeis cited as an early instance ofpalaeopathology,the study of ancient diseases and injuries on prehistoric organisms.[13][14][15]

Early depictions

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Georges Cuvier's published sketch (left) and unpublished sketch with outline (right) of an incomplete skeletal reconstruction ofAnoplotherium commune,~1812.

In 1812, Cuvier published his drawing of a skeletal reconstruction ofA. communebased on known fossil remains of the species including the aforementioned incomplete skeletons. Based on the robust build of the mammal species, he hypothesized that its body structure was similar toottersexcept for its legs, that it was adapted for semi-aquatic life by swimming for consumption of aquatic plants, lacking long ears similar to semi-aquatic mammals, and living in marshy environments. Cuvier suggested that its lifestyle was therefore similar to semi-aquatic quadrupedal mammals likehippopotamusesandmuroidrodents. He thought that in comparison, other species ofAnoplotheriumsuch asA. mediumandA. minuswere adapted for terrestrial behaviours and mixed feeding (browsing and grazing).[16][17]Today, the reconstruction for the skeletal anatomy has aged well, mostly standing the test of time since 1812.[18]AnoplotheriumandPalaeotheriumwere also depicted in 1822 drawings by the French palaeontologistCharles Léopold Laurillardunder the direction of Cuvier, although the restorations were not as detailed as Cuvier's.[19]

The reconstruction ofAnoplotheriumas an aquatic swimmer was supported by multiple 19th century European palaeontologists and persisted for over a century[20][21]until 1938 when M. Dor rejected the theory of the genus as being aquatic-adapted based on anatomical differences from otters and hippopotamuses that contradict semi-aquatic behaviours and are more consistent with terrestrial life.[22]This rejection was supported by Jerry J. Hooker in 2007 and Svitozar Davydenko et al. in 2023 based on anatomical traits, although the former disagreed with Dor's observations on the tail. Hooker argued that although the distal caudal vertebrae of the anoplothere are less prominent than those of kangaroos (Macropus), the vertebrae patterns of Anoplotherium are more similar toMacropusthan ungulates likeBosorEquus.Today,Anoplotheriumis thought to be a terrestrial browser with specialized behaviours.[23][24]

Sculptures of anAnoplotherium communeherd as part of theCrystal Palace Dinosaurssculptures on the Tertiary Island of theCrystal Palace Park,United Kingdom

A. communewas notably depicted in theCrystal Palace Dinosaursattraction in theCrystal Palace Parkin theUnited Kingdom,open to the public since 1854 and constructed by English sculptorBenjamin Waterhouse Hawkins.More specifically, three statues ofA. communewere made, two of which are standing and the third of which is in a reposed position. These statues resemble hybrids of deer andbig catsand measure 3.6 m (12 ft) long. Its inclusion in the Crystal Palace Park reflects the popularity and public interest inAnoplotheriumin the 19th century, as it was an icon of palaeontology, geology, and natural history that it was regularly incorporated in palaeontological texts and classrooms (its popularity diminished since the 20th century).[25][26]

The sculptures ofA. communewere overall based on Hawkins closely following Cuvier's description of the genus based on known remains, including Cuvier's unpublished robust muscle speculations which are seen as accurate by modern-day standards. Hawkins did also deviate outside of Cuvier's descriptions, however, likely basing its facial designs and the inaccurate presence of tetradactyl limbs (four toes on each foot) instead of didactyl or tridactyl limbs on extant camelids. Besides these errors, the statues have largely been accurate to modern-day depictions ofAnoplotherium.[26]

Confusions with other mammal groups

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Illustrations ofAnoplotherium(foreground left),Xiphodon(background left), andPalaeotherium(right)

For much of the 19th century, palaeontologists confused mammals of other families withAnoplotheriumlargely due to palaeontology being at its early stages. One of the earlier examples is 1822, when Cuvier erected the namesA. gracile,A. murinum,A. obliquum,A. leporinum,andA. secundaria,replacing earlier species names withinAnoplotheriumoutside ofA. commune.InA. gracile,he noticed differences in themolarsthat he erected thesubgenusXiphodon.ForA. leporinum,A. murinum,andA. obliquum,the subgenusDichobunewas created by Cuvier based on its small size.[11]In 1848, French palaeontologistAuguste Pomelpromoted the 2 subgenera to genus ranks and erected an additional genusAmphimeryxforA. murinusandA. obliquus.The revised taxonomies were followed by subsequent palaeontologists like anotherPaul Gervais.Therefore, the species are no longer classified asAnoplotheriumbut distant genera.[27][28][29]

Other mammals initially confused with the genusAnoplotheriumbut eventually reclassified within the 19th century represented the endemic European artiodactyl familyCainotheriidae(Cainotherium[30][31][29][32]), European and Indian subcontinental members of the perissodactyl familyChalicotheriidae(Anisodon[33][34][35]andNestoritherium[36][37][38]), and even endemic South American members of the orderLitopterna(ScalabrinitheriumandProterotherium[39]).

Revisions within the Anoplotheriidae

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A. communejaw fragment,Teylers Museum

In 1851, Pomel observed thatAnoplotheriumspecies could be determined as having either didactyl hooves (lessened third index) or tridactyl hooves (greater-developed third index) and that the only previously erected species that are valid areA. communeandA. secundaria.In addition, he erected three new species based on additional remains:A. duvmoyi(based on Cuvier's fossil illustrations ofA. commune),A. platypus,A. laurillardi(convex incisors on the anterior surface), andA. cuvieri.A. laurillardiderives as a species name from Charles Laurillard.[31]

French palaeontologist Paul Gervais in 1852 named the genusEurytheriumbased on its presence of tridactyl hooves instead of didactyl hooves, for he made the new speciesE. latipisthe type species andA. platypusa synonym of the former.[20]Henri Filholwould follow Gervais by erectingE. quercyiandE. minusbased on dental sizes and reclassifyingA. secundarium(orA. secundaria) toEurytherium.[40]

In 1862,Ludwig Rütimeyererected the subgenusDiplobunefor the genusDichobuneon the basis that it was an evolutionary transition betweenAnoplotherium secundariumand the dichobunid.[41]It was promoted to a distinct genus with one speciesD. bavaricumbeing placed into the genus byOscar Fraasby 1870, however.[42]

In 1883,Max SchlossermadeEurytheriuma synonym ofAnoplotheriumbecause he argued that the limb anatomies and dentitions were specific differences in characteristics rather than major ones that defined an entire genus. Sclosser pointed out that all species ofAnoplotheriumin some form had three indexes despiteA. communehaving less developed third indexes thanA. latipes.He also reinforced the idea that "A. platypus"is a synonym ofA. latipes.The nameA. latipestakes priority overA. platypusto the modern day because Pomel in 1851 did not list any specimen for the species, effectively making it anomen dubium.He also mentioned that the status ofA. duvmoyiwas not stable due to being based on illustrations, which he considered to be a "hopeless effort". He also supportedDiplobunebeing a valid genus in that he argued thatA. secundariashould be renamed toD. secundariabased on dentition and smaller sizes. Schlosser also said thatA. cuvieriwas an invalid species because the diagnosis based on isolatedmetatarsal boneswas valid-enough.[21][29][23]

Richard Lydekkererected the speciesA. cayluxensein 1885 based on its smaller size and unique variations in the molar cusps. He also demoted the genusDiplobuneas a synonym ofAnoplotherium,meaning that the former's species were added/readded toAnoplotheriumasA. secundarium,A. quercyi,A. modicum,A. bavaricum,andA. minus(=A. minor,Filhol 1877).[29]The synonymy ofDiplobunewithAnoplotheriumwas not supported byHans Georg Stehlinin 1910, as he argued that the former was generically distinct from the latter despite their close relations, thus restoring the previous species intoDiplobune(with the exception ofD. modicum,which he synonymized withD. bavarica) and adding "A. secundarium"intoDiplobuneasD. secundaria.He also wrote thatA. cayluxensewas a synonym ofD. secundaria.Stehlin also tentatively referred "A."obliquumto the genusHaplomeryxasH? obliquum.As a result of the revisions, the only valid species ofAnoplotheriumwereA. commune,A. latipes,andA. laurillardi.[43]

In 1922,Wilhelm Otto Dietricherected the fourth speciesA. pompeckjifrom the locality ofMähringenin Germany, named in honor of German palaeontologistJosef Felix Pompeckj.The species was described as a medium-sized tridactyl species with 4-fingered front limbs and 3-toed hind limbs with slimmer hand bone proportions and a smallerastragalus.[44]A. pompeckjiis the least characterized species and has similar dentition toA. laurillardi,making its status less certain compared to the three other species.[23][45]

In 1964, palaeontologistLouis de Bonisreviewed briefly the taxonomic synonyms ofAnoplotherium,considering thatA. duvernoyiwas based on a young individual with incisor characteristics that Pomel did not specify and thatA. cuvieridoes not differ in metacarpal dimensions fromA. laurillardi.He followed Stehlin in recognizing the three main species ofAnoplotherium,although he did not mentionA. pompeckjiin his review.[46]

Classification

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Portrait ofGeorges Cuvier,the French naturalist who describedPalaeotheriumandAnoplotheriumin 1804

Anoplotheriumis thetype genusof the Anoplotheriidae, a Palaeogene artiodactyl family endemic to western Europe that lived from the middle Eocene to the early Oligocene (~44 to 30 Ma, possible earliest record at ~48 Ma). The exact evolutionary origins and dispersals of the anoplotheriids are uncertain, but they exclusively resided within the continent when it was anarchipelagothat was isolated by seaway barriers from other regions such asBalkanatoliaand the rest of eastern Eurasia. The Anoplotheriidae's relations with other members of the Artiodactyla are not well-resolved, with some determining it to be either a tylopod (which includes camelids andmerycoidodontsof the Palaeogene) or a close relative to the infraorder and some others believing that it may have been closer to the Ruminantia (which includestragulidsand other close Palaeogene relatives).[47][45]

The Anoplotheriidae consists of two subfamilies, theDacrytheriinaeandAnoplotheriinae,the latter of which is the younger subfamily thatAnoplotheriumbelongs to. The Dacrytheriinae is the older subfamily of the two that first appeared in the middle Eocene (since theMammal Palaeozone Zonesunit MP13, possibly up to MP10), although some authors consider them to be a separate family in the form of the Dacrytheriidae.[48][49]Anoplotheriines made their first appearances by the late Eocene (MP15-MP16), or ~41-40 Ma, within western Europe withDuerotheriumandRobiatherium.By MP17a-MP17b, however, there is a notable gap in the fossil record of anoplotheriines overall as the former two genera seemingly made their last appearances by the previous MP level MP16.[50]

By MP18,AnoplotheriumandDiplobunemade their first appearances in western Europe, but their exact origins are unknown. The two genera were widespread throughout western Europe based on abundant fossil evidence spanning from Portugal, Spain, United Kingdom, France, Germany, and Switzerland for much of pre-Grande Coupure Europe (prior to MP21), meaning that they were typical elements of the late Eocene up until the earliest Oligocene.[51][50][45]The earlier anoplotheriines are considered to be smaller species whereas the later anoplotheriines were larger.AnoplotheriumandDiplobuneare considered the mostderived(or evolutionarily recent) anoplotheriids based on dental morphology and achieved gigantism amongst non-whippomorphartiodactyls, making them some of the largest non-whippomorph artiodactyls of the Palaeogene as well as amongst the largest mammals to roam western Europe at the time (all species ofAnoplotheriumwere large to very large whereas not all species ofDiplobunewere large).[45][52][53][12]

Reconstructed skeleton of the relatedCainotherium

Conducting studies focused on the phylogenetic relations within the Anoplotheriidae has proven difficult due to the general scarcity of fossil specimens of most genera.[50]The phylogenetic relations of the Anoplotheriidae as well as theXiphodontidae,Mixtotheriidae,and Cainotheriidae have also been elusive due to theselenodontmorphologies of the molars, which were convergent with tylopods or ruminants.[53]Some researchers considered the selenodont families Anoplotheriidae, Xiphodontidae, and Cainotheriidae to be within Tylopoda due to postcranial features that were similar to the tylopods from North America in the Palaeogene.[23]Other researchers tie them as being more closely related to ruminants than tylopods based on dental morphology. Different phylogenetic analyses have produced different results for the "derived" selenodont Eocene European artiodactyl families, making it uncertain whether they were closer to the Tylopoda or Ruminantia.[54][55]

In an article published in 2019, Romain Weppe et al. conducted a phylogenetic analysis on theCainotherioideawithin the Artiodactyla based on mandibular and dental characteristics, specifically in terms of relationships with artiodactyls of the Palaeogene. The results retrieved that the superfamily was closely related to the Mixtotheriidae and Anoplotheriidae. They determined that the Cainotheriidae,Robiacinidae,Anoplotheriidae, and Mixtotheriidae formed a clade that was the sister group to the Ruminantia while Tylopoda, along with theAmphimerycidaeand Xiphodontidae split earlier in the tree.[55]The phylogenetic tree used for the journal and another published work about the cainotherioids is outlined below:[56]

In 2020, Vincent Luccisano et al. created a phylogenetic tree of the basal artiodactyls, a majority endemic to western Europe, from the Palaeogene. In one clade, the "bunoselenodont endemic European" Mixtotheriidae, Anoplotheriidae, Xiphodontidae, Amphimerycidae, Cainotheriidae, and Robiacinidae are grouped together with the Ruminantia. The phylogenetic tree as produced by the authors is shown below:[54]

Artiodactyla

In 2022, Weppe created a phylogenetic analysis in his academicthesisregarding Palaeogene artiodactyl lineages, focusing most specifically on the endemic European families. The phylogenetic tree, according to Weppe, is the first to conduct phylogenetic affinities of all anoplotheriid genera, although not all individual species were included. He found that the Anoplotheriidae, Mixtotheriidae, and Cainotherioidea form a clade based onsynapomorphicdental traits (traits thought to have originated from their most recent common ancestor). The result, Weppe mentioned, matches up with previous phylogenetic analyses on the Cainotherioidea with other endemic European Palaeogene artiodactyls that support the families as a clade. As a result, he argued that the proposed superfamily Anoplotherioidea, composing of the Anoplotheriidae and Xiphodontidae as proposed by Alan W. Gentry and Hooker in 1988, is invalid due to thepolyphylyof the lineages in the phylogenetic analysis. However, the Xiphodontidae was still found to compose part of a wider clade with the three other groups.AnoplotheriumandDiplobunecompose a clade of the Anoplotheriidae because of their derived dental traits, supported by them being the latest-appearing anoplotheriids.[53][57]

Description

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Skull

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Upper part of the skull ofAnoplotherium commune,National Museum of Natural History, France

The Anoplotheriidae is characterized in part by low-proportioned skulls with elongatedmuzzles(the muzzle aligns with the top of thecraniumin the case ofAnoplotherium), and a wide-open skull orbit.[58][48][45]Anoplotheriumlacks bony processes andlacrimal fossae.It has large paroccipital processes and shorterpostorbital processprojections of thelacrimal bone.[59][60]

The skull ofAnoplotheriumis narrow and elongated, with a constricted postorbital bone indicating poor brain development. It features robustsagittalandnuchal crests,the former having high elevations and emerging from low postorbital ridges and the latter having complicated elevation shifts. The back has a circular foramen magnum and large occipital condyles. The underside has an elongatedpalatewith glenoid surfaces and strong post-glenoid processes of thesquamosal bone.[61]

The skull's bones are robust, with the spongydiploëbone being greatly developed. The skull's strength is attributed to massivetemporal musclesas part of an overall strong body build. The skull has a shallowsella turcica,a pear-shapedcranial fossa,extensiveparietal bones,large squamosal bone, narrowoccipital bone,and two smalloccipital bunsfor muscle attachment. Many cranial traits seen inAnoplotheriumare also found in the closely relatedDiplobune.[61][62]

In the auditory region (including the temporal bones), theperiotic boneof the inner ear is extensive, theinternal auditory meatusandfacial canalopenings of the temporal bone being visible in the lower triangular area of the periotic bone. Thetympanic part of the temporal boneis connected partially to the squamosal bone, remains separate from the periotic bone, and consists of a small but thick auditory bulla (hollow bony structure of the auditory region), which projects underneath thepetrous part of the temporal bone.[61]

In a skull fragment ofA. laurillardiwith incisors and caninealveoli,the known length of thenasalregion is large, measuring 38.1 mm (1.50 in).[45]The trait of large nasals is similar to what was observed in a skull ofDiplobune secundaria,which are recorded to be massive, elongated, and connected to each other and themaxilla.Cyril Gagnaison and Jean-Jacques Leroux proposed in the case ofD. secundariathat the elongated nasal region supports the presence of a very tapered tongue, which similar togiraffesmay have allowed it to pull plant branches.[63]

Endocast anatomy

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Illustrations of a brain cast ofAnoplotheriumin dorsal (top) and ventral (botton) views, 1913

In 1913, R.W. Palmer conducted studies on the brain cast from a cranium ofAnoplotherium commune,originating from thePhosphorites of Quercywithin theBritish Museumcollections (the endocast is now in theNational Museum of Natural History, Franceas the specimen BMNH 3753). The individual in question was estimated to have weighed 80 kg (180 lb) by its death similar to extantllamas,weighing considerably less than typical estimates of adultAnoplotherium.The total length of the brain is under 10 cm (3.9 in), its volume measuring approximately 230 ml (8.1 imp fl oz; 7.8 US fl oz).[61][64][45]

The form of the brain is naturally narrow and elongated.[61][12]The cerebellum and cerebrum are both at high positions compared to modern ungulates that have brain hemispheres located above the cerebellum. Palmer noticed that the brain was similar to the modernaardvark(Orycteropus afer). The highly-developed cerebrum that enables a strong sense of smell fromAnoplotheriummakes it macrosmatic (derived in sense of smell), as also indicated by the enlarged olfactory bulbs and the small size of theneocortex.[61]In bothAnoplotheriumandDiplobune,therhinal fissuredivides the brain hemisphere horizontally and equally in half. Thecerebellar vermisof the cerebellum is divided almost equally by theprimary fissure of cerebellum(or "fissura prima" ).[65]

Additionally, theolfactory bulbsare thick, and theolfactory tuberclestake the form of smooth circular elevations that are curved more backwards than the aardvark and are easily noticeable.[61]In another endocast forAnoplotherium,the olfactory bulbs compose 7.5% of the total volume of the brain, above average for both extinct and extant artiodactyls.[12]

Theneocortexarea of the brain, responsible forsensory perceptionand other sensory brain functions, covers 28% of the medium-sizedA. communeendocast's surface area.[64]Another endocast, which belongs toAnoplotheriumsp., measures 7,173.92 mm (282.438 in)2in the cerebrum surface, 4,419.56 mm (173.998 in)2in neopallium surface, and 416.09 cm (163.81 in)3in endocranial volume. The former two data when calculated together (neopallium surface/cerebrum surface) compose 61.6% in the total neocortical surface area of the brain, meaning that adultAnoplotheriumhas massive brain and neocortical surface area measurements compared to most Palaeogene artiodactyls, the latter measurement being on par with or less than those of modern artiodactyls.[12]

Anoplotheriumand other anoplotheriids share traits of generally elongated and parallelsulci(shallow furrows) in thecerebral cortex,as well as a vertical (cordial) sulcus corresponding to the lateral (side) sulcus. The fissures (deep furrows) on the surface of the central area of the brain show clear formations of a complexlateral sulcus(also known as the Sylvian fissure) in a process known as operculization.[12]The operculization of the brain of anoplotheriids is similar to theAnthracotheriidaebut does not indicate any close phylogenetic relation, which means that the similarities are an instance ofparallel evolution.The measurements of the endocasts ofAnoplotheriumare larger than those of other Palaeogene artiodactyls in a 2015 study by Ghislain Thiery and Stéphane Ducrocq.[66]

Dentition

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Anoplotherium laurillardilower dental remains,Natural History Museum of Basel

Unlike most mammal fossil genera,Anoplotheriumis diagnosed mainly based on postcranial morphology than dental morphology, but it does have diagnoses based on the latter.[45]Thedental formulaofAnoplotheriumand other anoplotheriids is3.1.4.33.1.4.3for a total of 44 teeth, consistent with the primitive dental formula for early-middle Palaeogeneplacentalmammals.[58][67]Anoplotheriids have selenodont or bunoselenodont premolars and molars made for folivorous/browsing diets. The canines of the Anoplotheriidae are premolariform in shape, meaning that the canines are overall undifferentiated from other teeth like incisors. The lower premolars of the family are piercing and elongated. The upper molars are bunoselenodont in form while the lower molars have selenodont labial cuspids and bunodont lingual cuspids. The subfamily Anoplotheriinae, of whichAnoplotheriumis the type genus, differs from the Dacrytheriinae by the lower molars lacking a third cusp between the metaconid and entoconid as well as molariform premolars with crescent-shaped paraconules.[45]

The upper molars ofAnoplotheriumare characterized by trapezoidal outlines in occlusal views (or top views of the tooth enamel), W-shaped ectolophs (crests or ridges of upper molar teeth), and specific differences in cusps. More specifically, the upper molars of the genus contain near-central and conical protocone cusps closely aligned with the mesostyle cusps, conical paraconules that are connected to the parastyle by posterior crests, and compressed parastyles and mesostyles. The lower molars of the anoplotheriid contains the paraconid and metaconid cusps which have pronounced separations by a valley between them.[58][45]

Vertebrae and ribs

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A. communetail, National Museum of Natural History, France

Anoplotheriumhas 7 totalcervical vertebraefor a series of C1-C7, typical of most mammals. Theatlas(C1) is similar to those of camelids such asLamain form as well as the position of the "alar foramina" in association withfacet jointconnections involving theaxis(C2).[11][23]An axis that was attributed toA. commune(but also possibly belonging to its close relativeDiplobune secundaria) is elongated in length and has a diminished spinous process. The vertebrae C3-C7 are analogous toCainotherium.The C4 vertebra appears slanted, which hints towards the neck changing in orientation from vertebra C3 to C4 as a potential bending in the front area of the neck, similar to modern bears. As a result of the neck vertebrae morphology,Anoplotheriumlikely had a sloped, upward position of the neck.[23]

Anoplotheriumalso had 12thoracic vertebrae,6 lumbar vertebrae, and 3 sacral vertebrae. The lumbar vertebrae, especially L4-L6, contain transverse processes that are wide, long, and point slightly towards a forward direction. The 3 sacral vertebrae are robust and containapophysesfor strong attachments to the long tail. The vertebrae of the anoplotheriid genus are built for typical ungulate movement.[11][23]

The most unusual postcranial aspect ofAnoplotheriumcompared to other artiodactyls is the long and thick tail, which is made up of 22 caudal vertebrae for strong muscle support. The frontal vertebrae had well-pronounced process, and all vertebrae except for the farthest distal ones havehaemal archeson them.[23]

Like the chalicothereChalicotheriumand unlike other mammals likecaprinesof the genusOvisandCainotherium,the ribs curve in wider areas and their tubercles do not project as much in the dorsal direction. The ribs ofAnoplotheriumform a barrel-shaped trunk, meaning that the rib cage is much wider than those of modern ruminants. The ribs generally project sideways due to the very curved positions of them, the position of the tubercle, and the thoracic vertebrae projecting on the upper sides.[23]

Limbs

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Foot ofA. communeat the National Museum of Natural History, France

Anoplotheriumhas short limbs and is thought to have beenunguligradein limb positions, with most species having three toes on both their front and hind limbs.A. communeis differentiated from the similarA. latipesby its didactyl ( "two-toed" ) as opposed to tridactyl ( "three-toed" ) digits.[23][45][68]

Front limbs

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Thescapula(or shoulder blade) has a convexcoracoidborder and is similar to that ofDiplobune.Similar to camels (Camelus), thesupraspinous fossais broader than theinfraspinous fossa,but camels have narrower scapulae, especially in distal (back) ends of the supraspinous fossa. Thescapular spineis robust, thick, and gradually rises in height distally up until it reaches the edge of the glenoid cavities like camels but unlike most other modern artiodactyls. The coracoid process (normally resembling a small hooklike structure) is reduced to a blunt knob that only slightly projects. The wide supraspinous fossa and broadly curved coracoid edge of the scapulae ofAnoplotheriumare unlikeCainotheriumandMerycoidodonbecauseAnoplotheriumshares neither any triangular shape of the shoulder blades nor narrow supraspinous fossae.[23]

The elbow morphology ofAnoplotherium,based on the structures and articulations of elbow bones like thehumerus,radius,andulna,shows evidence of adaptations to moving the elbow up and down in supination-pronation rotations by 13° maximum. A fully extended elbow could make an angle between the ulna and humerus that measures approximately 135°, indicating high flexibility compared to other artiodactyls, including the already semi-flexible elbows ofCainotherium.[23]

Similar in wrist morphology to pigs of the genusSus,the hooves ofAnoplotheriumspread out by ~16° when downward, supported by footprint morphology. The wrist may have been able to rotate up and down but only to a limited degree and nowhere near the flexible wrist morphologies of primates, suggesting that the adaptation was not a main feature of the artiodactyl genus but the result of regaining a primitive trait.[23]

The carpus consists of thescaphoid,lunate,triquetrum,andpisiformin its first row and thetrapezium,trapezoid,capitate,andhamatein its second.Anoplotheriumhas four digit bones, but those of digit V and, in the case ofA. commune,digit II are poorly developed.[69]The second finger (digit II) ofAnoplotheriumhas no capability of rotation or flexible movements, which signifies that it does not play any thumb-like role like in primates or thegiant panda.[23]

Hind limbs

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A. communefoot, Teylers Museum

Theilium,part of thehip boneof the greaterpelvisbone, is broad and has a firmly roundediliac crestthat meets with the concave underside edge at a sharp angle. The ilium ofAnoplotheriumcan be differentiated fromPalaeotheriumby the shorter iliac body, the longerischium(the lower and back area of the hip bone), and a straighter back edge of pelvis that results in a longerpubic symphysis.Theacetabular fossaregion of theacetabulumsurface of the pelvis is large, itsacetabular notchbeing in a posterior (or back) position similar to that inChalicotherium.[23]

The femur is larger than thetibia,has only two trochanters similar to other basal artiodactyls, has a narrow gap between itsfemoral headandgreater trochanter,and has a longfemoral neck.Thetrochanteric fossa,a hollow at the surface of the greater trochanter, is wide in depth and narrow in shape, deepening by the sides. The tibia is robust, strongly supporting muscle attachments based on its crests and processes. The distal end of thefibulaplus the medialmalleolusprominence of the tibia enclose the center area of the astragalus in order to prevent it from moving sideways.[23]

Anoplotheriids with known postcranial fossils have proportionally wide, stocky, and oblique astragali (or talus or ankle bone), differing widely from other artiodactyls.A. latipesdiffers fromA. communein part by morphologies of the facets plus fossae of the astragalus and a shorter and more robustcalcaneum(heel bone).[23][52]The astragali of anoplotheres share levels of elevations and positions of specific facets with the merycoidodonts that no modern artiodactyls share, possibly an instance ofconvergent evolution.[70][71]The medial (sustentacular) facet ofAnoplotheriumandDiplobuneis concave, contrasting with the flat to slightly convex facet ofDacrytherium.

The tarsus consists of the navicular, three cueniform bones, and a cuboid bone. The foot ofA. communeconsisted of two toes, as indicated by the relatively small outermost and middle cuneiform bones.[69]

Footprints

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Large-sized footprints from southern France and north Spain that date to the late Eocene[72]may have been fromAnoplotherium.Theichnogenusis namedAnoplotheriipusand was first described from the department ofGardin France by Paul Ellenberger in 1980. The derivation of the genus name refers to the ichnotaxon being closest in affinity to the Anoplotheriidae. The ichnogenus is diagnosed as belonging to a very large artiodactyl, theautopodarea exceeding that ofA. communeby ~33%, the subparallel position of the two hooves, and the posterior area of the pedal sole being as transversely wide as the anterior area of the pedal sole.[73]Anoplotheriipusis round to rectangular in shape with broad and anteriorly-pronounced cloven digit imprints that resemble poorly-preserved camel tracks.[74]The similar artiodactyl ichnogenusDiplartiopusdiffers from it by the parallelism of the two fingers that are more elongated.[75]

The type species isAnoplotheriipus lavocati,which Ellenberger named in honor of palaeontologistRené Lavocatand considered the "most majestic" of the three ichnospecies due to the displayed specific mobility of the metatarsals. It measures 170 mm (6.7 in) to 180 mm (7.1 in) in length and 120 mm (4.7 in) in width, is stocky in shape, and measures 12° in toe divergence. The two fingers are nearly equal in length and, at minimum, measure 115 mm (4.5 in) without the metatarsal bones being taken into account and 225 mm (8.9 in) with the metatarsals. The measurements are considerably higher than typical measurements of the toes ofA. commune,which are 85 mm (3.3 in) without the metatarsals and 170 mm (6.7 in) with.[73]

Anoplotheriipus similicommunis,deriving in species etymology from "similis" (similar in Latin) andA. commune,is similar to the type ichnospecies but is smaller, corresponding more directly to typical foot measurements ofA. communeby its length of 140 mm (5.5 in) and width of 105 mm (4.1 in). The angle of divergence between the two main toes is 10°, and the minimum lengths of the fingers are 90 mm (3.5 in) without the metatarsals and 180 mm (7.1 in) with.[73]

Anoplotheriipus compactusis the third ichnospecies, which in species etymology derives from the Latin word "compactus" meaning "compact" in English due to the short and rounded autopod. It has a less definitive diagnosis compared to the other two ichnotaxa but is similar in size toA. similicommunisand has a nearly circular pedal sole for supporting slightly shorter fingers. Its length is 120 mm (4.7 in) while the width is 100 mm (3.9 in), and the finger lengths measure 70 mm (2.8 in) - 80 mm (3.1 in) without the metatarsals and 140 mm (5.5 in) - 150 mm (5.9 in) with. The footprints may have been produced byA. latipesalthough the answer is still uncertain.[73]

Size

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EstimatedAnoplotheriumsize comparisons based on known fossil material

Anoplotheriumspecies were particularly large in the late Eocene, reaching sizes unusual for most artiodactyl groups in the Palaeogene. The large size estimates began in 1995 when Martinez and Sudre made weight estimates of Palaeogene artiodactyls based on the dimensions of their astragali and M1teeth. The astragali are common bones in fossil assemblages due to their reduced vulnerability to fragmentation as a result of their stocky shape and compact structure, explaining their choice for using it. The two measurements forA. communeyielded different results, with the M1giving the body mass of 312.075 kg (688.01 lb) and the astragalus yielding 265.967 kg (586.36 lb). These estimates are far larger than those of most other Palaeogene artiodactyls in the study, although the researchers pointed out that the M1measurements could be overestimated compared to the astragalus estimate.[52]

In 2014, Takehisa Tsubamoto reexamined the relationship between astragalus size and estimated body mass based on extensive studies of extant terrestrial mammals, reapplying the methods to Palaeogene artiodactyls previously tested by Sudre and Martinez. The researcher used linear measurements and their products with adjusted correction factors. The recalculations resulted in somewhat lower estimates compared to the 1995 results (with the exception ofDiplobune minor,which as a shorter astragalus proportion than most other artiodactyls), displayed in the below graph:[76]

Estimated body masses (kg) of Palaeogene artiodactyls based on recalculated trochlear widths (Li1) in comparison to estimates from Martinez and Sudre (1995)

In 2022, Weppe calculated the body mass ofA. commune,yielding 360 kg (790 lb).[53]In 2023, Ainara Badiola et al. estimated that the weight ofAnoplotheriumranges between 115 kg (254 lb) and 271 kg (597 lb). In their calculations,A. laurillardiwas the smaller anoplotheriid that weighed on average 157 kg (346 lb).A. latipeswas larger and has an average weight estimate of 229 kg (505 lb), andA. communehas the heaviest weight estimates at 271 kg (597 lb).[45]

In 2007, Hooker made size estimates ofA. latipesbased on an incomplete skeleton of an immature individual from the Hamstead Member of theBouldnor Formationin theIsle of Wight,United Kingdom. The reconstructed Hamstead level 3 individual gave size measurements of 2 m (6 ft 7 in) in head and body length. The immatureAnoplotheriumindividual's humerus measures 330 mm (13 in) long, so the humeri of mature individuals may have measured about 410 mm (16 in) long. As a result, adultA. latipesmay have measured 2.5 m (8 ft 2 in) in head and body length and 1.25 m (4 ft 1 in) in shoulder height. When standing up bipedally on its hind limbs with the back, neck and head at an angle of about 15°, the Hamstead level 3 individual might have reached 2.5 m (8 ft 2 in) when browsing while more matureA. latipesindividuals might have stood just over 3 m (9.8 ft).[23]

Palaeobiology

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Reconstruction ofA. communein a quadrupedal position

Since 2007,Anoplotheriumis thought to have been a quadruped that could have stood on its hind legs as a bipedal browser thanks to the strong pelvis, long and robust tail for balance, and splayed hind legs. The bipedal adaptations show some instance of convergence with other animals like chalicotheres, various genera ofground sloths,giant pandas (Ailuropodamelanoleuca),gorillas(Gorilla), and thegerenuk(Litocranius walleri). Otherwise, the general body form appears to resemble those of theCanidae.As a result of the bend C3-C4 cervical vertebrae, the neck and head could have maintained horizontal orientations while standing bipedally. The forelimbs could have extended horizontally beyond the snout while the individual stood bipedally, although it could not have reached upward and did not have claws or prehensile organs on themanusunlikeChalicotherium.Therefore, the forearms may have not been used for ripping and tearing plants but as bipedal support. It may have browsed while standing up at a steep angle more comparable to the gerenuk than toChalicotherium.[23]

Its large size and ability to bipedally browse may have givenAnoplotheriumfew sources of terrestrial competition other than fromPalaeotherium magnum,a large-sizedpalaeotherewith a long neck that may have reached 240.3 kg (530 lb) in body mass.[23][77]The subspeciesP. magnum magnumwould have reached just over 2 m (6 ft 7 in) in browsing height in quadruped stance, and there is no evidence for any bipedal adaptation in palaeotheres.[23]Anoplotheriumlikely engaged in degrees ofniche partitioningwith the late Eocene palaeotheres andDiplobune.While all were folivorous browsers, the palaeotheresPlagiolophusandPalaeotheriummay have had small degrees of frugivory whileDiplobunewas likely adapted toarborealism.[78][79][68]How well-adaptedAnoplotheriumwas to abrasive leaves and drier but still subhumid conditions in the late Eocene is not well-known and requires future research in dentition for answers.[23]

Reconstruction ofA. communein a bipedal position, most likely to browse on taller vegetation

Hooker proposed the possibility that the didactylA. communeandA. latipesmay have been sexual dimorphs of the same species (in whichA. latipeswould be a synonym ofA. commune). There are little consistent differences in dental morphology between the two species, with any small differences potentially accounting for individual variations. The differences in toe number between the species may have reflectedA. latipesbeing three-toed andA. communebeing two-toed. The palaeontologist explained that while there is no evidence for the extra digit touching the ground while the individual was walking, the extra digit ofA. latipesmay have served as extra balance while browsing bipedally.[23]

The third digit might have also served as part of sparring inintraspecific competitionbetween male individuals. However, he noted that despite the apparent "advantage" ofA. latipesin bipedal browsing, there is no evidence of sexual differences in dietary behaviours or preferences. In addition, both species are found in the same localities of Bouldnor in the United Kingdom plus La Débruge and Montmartre in France, that althoughA. latipesis more common in La Débruge than Montmartre, this may be the results of behavioural and/or taphonomic factors.[23][45]Grégoire Métais expressed being unconvinced that the third toe ofA. latipesis a sexually dimorphic adaptation for bipedal browsing, instead suggesting that they were used in male sparring ifA. latipesandA. communewere sexual dimorphs.[68]

Some evidence of the morphologies ofAnoplotheriumhave been criticized by some sources. In their study of the morphology of the gerenuk that allows for bipedal, researchers Matt Cartmill and Kaye Brown argued that several postcranial features that were supposedly adaptations ofLitocraniusand other bipedal genera does not distinguish the gerenuk from other bovids.[80]Ciaran Clark et al. (including J.J. Hooker) found from micro-CT scansthatAnoplotheriumbeing a facultative bipedal browser was not supported by thetrabeculararchitecture of the proximal area of the femur. This may have been the result of poor data results from the micro-CT scans and the smaller sample size, which higher-contrast micro-CT data may better answer in postural information.[81]

The footprint track patterns ofAnoplotheriipussuggest thatAnoplotheriumwalked in very similar movement speeds as each other. Based on groupings of the footprint ichnotaxon within the locality of Fondota in the municipality ofAbiegoin Spain,Anoplotheriummay have commonly walked in small groups which may imply some gregarious (or sociable) behaviour.[82]

Palaeoecology

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Further information:Mammal Palaeogene zones

Early pre–Grande Coupure Europe

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Palaeogeographyof Europe and Asia during the middle Eocene with possible artiodactyl and perissodactyl dispersal routes.

For much of the Eocene, ahothouse climatewith humid, tropical environments with consistently high precipitations prevailed. Modern mammalian orders including the Perissodactyla, Artiodactyla, andPrimates(or the suborder Euprimates) appeared already by the early Eocene, diversifying rapidly and developing dentitions specialized for folivory. Theomnivorousforms mostly either switched to folivorous diets or went extinct by the middle Eocene (47–37 million years ago) along with the archaic "condylarths".By the late Eocene (approx. 37–33 mya), most of the ungulate form dentitions shifted from bunodont (or rounded) cusps to cutting ridges (i.e. lophs) for folivorous diets.[83][84]

Land connections between western Europe and North America were interrupted around 53 Ma. From the early Eocene up until theGrande Coupureextinction event (56–33.9 mya), western Eurasia was separated into three landmasses: western Europe (an archipelago), Balkanatolia (in-between theParatethys Seaof the north and theNeotethys Oceanof the south), and eastern Eurasia.[47]TheHolarcticmammalian faunas of western Europe were therefore mostly isolated from other landmasses including Greenland, Africa, and eastern Eurasia, allowing for endemism to develop.[84]Therefore, the European mammals of the late Eocene (MP17–MP20 of the Mammal Palaeogene zones) were mostly descendants of endemic middle Eocene groups.[85]

The appearances of derived anoplotheriines by MP18 occurred long after the extinction of the endemic European perissodactyl familyLophiodontidaein MP16, including the largest lophiodontLophiodonlautricense,likely the result of 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.[86][87]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.[88][89]MP16 also marked the last appearances of most Europeancrocodylomorphs,of which thealligatoroidDiplocynodonwas the only survivor due to seemingly adapting to the general decline of tropical climates of the late Eocene.[90][91][92]

Late Eocene

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Restoration ofPalaeotheriummagnum,which coexisted withAnoplotherium

After a considerable gap in anoplotheriine fossils in MP17a and MP17b, the derived anoplotheriinesAnoplotheriumandDiplobunemade their first known appearances in the MP18 unit.[50]They were exclusive to the western European archipelago, but their exact origins and dispersal routes are unknown. By then,AnoplotheriumandDiplobunelived in Central Europe (then an island) and the Iberian Peninsula, only the former genus of which later dispersed into southern England by MP19 due to the apparent lack of ocean barriers.[45][23]

Anoplotheriumcoexisted with a wide diversity of artiodactyls in western Europe by MP18, ranging from the more widespreadDichobunidae,Tapirulidae,and Anthracotheriidae to many other endemic families consisting of the Xiphodontidae,Choeropotamidae(recently determined to be polyphyletic, however),Cebochoeridae,Amphimerycidae,and Cainotheriidae.[48][54][93][94]Anoplotheriumalso coexisted with the Palaeotheriidae, the remaining perissodactyl family of western Europe.[85]Late Eocene European groups of the cladeFeraerepresented predominantly theHyaenodonta(Hyaenodontinae,Hyainailourinae,andProviverrinae) but also containedCarnivoramorpha(Miacidae) andCarnivora(small-sizedAmphicyonidae).[88]Other mammal groups present in the late Eocene of western Europe represented theleptictidans(Pseudorhyncocyonidae),[95]primates (AdapoideaandOmomyoidea),[96]eulipotyphlans(Nyctitheriidae),[97]chiropterans,[84]herpetotheriids,[98]apatotherians,[99]and endemicrodents(Pseudosciuridae,Theridomyidae,andGliridae).[100]The alligatoroidDiplocynodon,present only in Europe since the upper Paleocene, coexisted with pre-Grande Coupure faunas as well.[101]In addition to snakes, frogs, andsalamandrids,rich assemblage of lizards are known in western Europe as well from MP16-MP20, representing theIguanidae,Lacertidae,Gekkonidae,Agamidae,Scincidae,Helodermatidae,andVaranoidea.[102]

In the MP18 locality of Zambrana in Spain,A. laurillardiandA.sp. remains were found with undetermined frog and squamate groups, alligatoroidDiplocynodon,the herpetotheriidPeratherium,rodents (Theridomys,Elfomys,Pseudoltinomys,Remys), omomyidMicrochoerus,carnivoraformesQuercygaleandParamiacis,dichobunidDichobune,xiphodontsXiphodonandHaplomeryx,and palaeotheres (Palaeotherium,Leptolophus,Iberolophus,Pachynolophus,Paranchilophus).[103]

As part of a separate landmass at the time, La Débruge of France, dating to MP18, yielded slightly different faunas that coexisted withA. commune,A. latipes,andA. laurillardi,namely the herpetotheriidPeratherium,rodents (Blainvillimys,Theridomys,Plesiarctomys,Glamys), hyaenodonts (HyaenodonandPterodon), amphicyonidCynodictis,palaeotheres (Plagiolophus,Anchilophus,Palaeotherium), dichobunidDichobune,choeropotamidChoeropotamus,cebochoeridsCebochoerusandAcotherulum,anoplotheriidsDacrytheriumandDiplobune,tapirulidTapirulus,xiphodontsXiphodonandDichodon,cainothereOxacron,amphimerycidAmphimeryx,and anthracothereElomeryx.[104]

Extinction

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A panorama of theHeadon Hill Formationin the Isle of Wight, from which Anoplotherium material has been collected. The stratigraphy of it and theBouldnor Formationled to better understandings of faunal chronologies from the Late Eocene up to the Grande Coupure.

The Grande Coupure event during the latest Eocene to earliest Oligocene (MP20-MP21) is one of the largest and most abrupt faunal turnovers in the Cenozoic of Western Europe and coincident withclimate forcingevents of cooler and more seasonal climates.[105]The event led to the extinction of 60% of western European mammalian lineages, which were subsequently replaced by Asian immigrants.[106][107][108]The Grande Coupure is often dated directly to the Eocene-Oligocene boundary at 33.9 Ma, although some estimate that the event began slightly later, at 33.6–33.4 mya.[109][110]The event occurred during or after theEocene-Oligocene transition,an abrupt shift from a hotgreenhouse worldthat characterised much of the Palaeogene to a coolhouse/icehouse world from the early Oligocene onwards. The massive drop in temperatures results from the first major expansion of the Antarcticice sheetsthat caused drasticpCO2decreases and an estimated drop of ~70 m (230 ft) in sea level.[111]

Many palaeontologists agree that glaciation and the resulting drops in sea level allowed for increased migrations between Balkanatolia and western Europe. TheTurgai Strait,which once separated much of Europe from Asia, 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. In 2022, 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 atmosphericCO2,boosting the Antarctic glaciation that already started by the Eocene-Oligocene transition.[47][112]

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,andUrsidae), eastern Eurasian rodents (Eomyidae,Cricetidae,andCastoridae), and eulipotyphlans (Erinaceidae).[113][114][106][115]

The Eocene-Oligocene transition of western Europe, as a result of the global climatic conditions, is marked by a transition from tropical and subtropical forests to more open, temperate or mixed deciduous habitats with adaptations to increased seasonality. WhileAnoplotheriumdid not last long in the earliest Oligocene, there are disagreements as to whether it survived the Grande Coupure or went extinct at the event.[89][48]While evidence points towardsAnoplotheriumbeing extirpated from areas like France and the United Kingdom by the Grande Coupure (last occurrences MP20),[104][106][53]the perception is complicated by the apparent last survival ofA. communein the MP21 locality of Möhren 19 in southern Germany (the edge of western Europe) along withPalaeotherium mediumandDiplobune quercyi(slightly younger localities indicate their extinctions and replacements by Grande Coupure immigrants such as the anthracothereAnthracotheriumand the rhinocerotidEpiaceratherium).[116]

Hooker pointed out that localities like Möhren 19 span earlier times where the surviving endemic faunas are accompanied by some Grande Coupure immigrants but otherwise were not yet joined by certain immigrants such asAnthracotherium.Additionally, the surviving endemics of the locality are missing from other areas dating to MP21. Therefore, he argued that certain older MP21 localities with surviving endemic faunas fill the long gap between the youngest pre-Grande Coupure Lower Hamstead Member and the younger post-Grande Coupure Upper Hamstead Member within the Bouldnor Formation. This interpretation, Hooker explained, means that the localities represented very brief moments of survival of endemic faunas during the Grande Coupure, therefore supporting the idea of a major and rapid faunal extinction and immigration event, including the extinction ofAnoplotheriumin the event.[106][117]

The extinctions of a majority of endemic artiodactyls, includingAnoplotherium,have been attributed to competition with immigrant faunas, environmental changes from cooling climates, or some combination of the two.[109]Sarah C. Joomun et al. determined that certain faunas may have arrived later and therefore may have not played roles in the extinctions. They concluded that climate change, which led to increased seasonality and changes in plant food availability, led the artiodactyls to become unable to adapt to the major changes and go extinct.[118]Weppe made similar arguments towards climate change being the main cause of the Grande Coupure extinction event, arguing that the cooling climates displaced the previously stable subtropical environments of western Europe and caused a collapse in the artiodactyl community, which after their extinctions left empty ecological niches that were passively filled by immigrant faunas.[53]

Notes

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  1. ^The French adjectivecommunetranslates in English to 'common'.

References

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  1. ^Cuvier, Georges (1804)."Suite des Recherches: Sur les espèces d'animaux dont proviennent les os fossiles répandus dans la pierre à plâtre des environs de Paris".Annales du Muséum National d'Histoire Naturelle, Paris(in French).3:364–387.Archivedfrom the original on 2023-07-27.Retrieved2023-08-30.
  2. ^Rudwick, Martin J. S. (2022)."Georges Cuvier's appeal for international collaboration, 1800".History of Geology.46(1): 117–125.doi:10.18814/epiiugs/2022/022002.S2CID246893918.
  3. ^Roberts, George (1839).An etymological and explanatory dictionary of the terms and language of geology.London: Longman, Orme, Brown, Green, & Longmans. p. 8.Retrieved29 December2021.
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