Proboscidea

Morphology and Anatomy

Living proboscideans are the largest surviving terrestrial animals on Earth, with body masses ranging from 2 to over 6 tonnes and shoulder heights between 2 and 4 metres.^[2][6][7] In the fossil record, members of the order displayed substantial anatomical variation.^[8] The earliest forms, such as Eritherium azzouzorum, were small cursorial mammals weighing only 3 to 8 kilograms with a shoulder height of around 20 centimetres.^[4][9][10] In contrast, several extinct taxa evolved into massive forms.^[5] The mammutid Zygolophodon borsoni and the elephantid Palaeoloxodon namadicus reached or surpassed body masses of 16 tonnes, with speculative estimates based on fragmentary limb bones suggesting weights of up to 22 tonnes and shoulder heights exceeding 5 metres for Palaeoloxodon namadicus.^{[11][12][13][14]} Modern species exhibit pronounced sexual dimorphism in body mass, height, and tusk development, a pattern corroborated by fossil remains of extinct species.^[15] Shared derived traits uniting all proboscideans include enlarged second upper incisors, the evolutionary loss of the first premolar, and keyhole-shaped enamel prisms in tooth cross-sections.^[16][17]

Skull and Mandible

The cranial architecture of proboscideans is distinguished by extreme structural pneumatisation of the cranial vault.^[2][18][19] The frontal, parietal, nasal, and intermaxillary bones are perforated by a honeycomb-like network of air cells separated by thin bony lamellae.^[2][18] This pneumatic construction lightened the massive skull while expanding its outer surface area to provide sufficient attachment space for the heavy nuchal musculature that supported the head and tusks, as well as the robust masticatory muscles operating the lower jaw.^[2][18] Cranial pneumatisation evolved very early in proboscidean phylogeny, being demonstrably present in Barytherium during the Oligocene and late Eocene.^[2][18] Over evolutionary time, the cranium became shorter, taller, and more vaulted, while the cervical vertebral column shortened to bring the center of mass closer to the shoulders.^[20][21]

In primitive proboscideans, the mandible was characterized by an elongated mandibular symphysis connecting the two jaw halves anteriorly.^[22] This mandibular elongation was coupled with the housing of lower tusks in alveoli running parallel along the symphysis, producing a long-snouted, longirostrine jaw condition.^[22] In several subsequent lineages, including Mammutidae, Gomphotheriidae, and Elephantidae, the symphysis underwent evolutionary abbreviation accompanied by the reduction or total loss of the lower tusks.^[22] This independent transition to a short-jawed, brevirostrine condition shifted food-gathering functionality entirely to the muscular trunk and changed the biomechanical loading of the lower jaw.^{[22][23][24][25]}

Dentition and Tusks

The primitive placental dental formula of ancestral proboscideans consisted of 44 permanent teeth: three incisors, one canine, four premolars, and three molars in each quadrant (3.1.4.3 / 3.1.4.3).^[4] Throughout proboscidean evolution, this complement was progressively reduced through the loss of canines, posterior incisors, and anterior premolars.^[26] Extant elephants possess a permanent dental formula containing only one upper incisor tusk and three molars per quadrant, alongside three deciduous milk premolars (1.0.3.3 / 0.0.3.3).^[2][26] Because of their close morphological resemblance to molars, the three deciduous premolars are frequently termed milk molars.^[27] Basal members of the crown elephantid genera Loxodonta, Elephas, and Mammuthus retained two permanent premolars, which were subsequently lost independently in each lineage.^[26]

Proboscidean tusks are hypertrophied incisors that grow continuously from open pulp cavities throughout the life of the animal.^{[17][19][28][29]} In Elephantiformes, the upper tusks derive from the second upper incisor (I2), while the lower tusks of longirostrine forms derive from the first lower incisor (I1).^[30] Exceptions occur in Moeritherium, where lower tusks formed from the second lower incisors, and Barytherium, which developed eight short tusks consisting of two incisor pairs in each quadrant.^[31] In Deinotheriidae, tusks formed exclusively from the lower jaw and hooked downward, lacking upper counterparts entirely.^[32][33] Tusk dimensions varied dramatically across taxa. Primitive forms bore short, straight, or chisel-like tusks under 50 centimetres in length.^[34] Late Miocene and Pliocene taxa developed colossal tusks: Zygolophodon borsoni produced nearly straight tusks reaching 5.02 metres in length, and advanced mammoth tusks frequently weighed over 130 to 200 kilograms.^{[34][35][36][37]} Amebelodontids evolved flattened, shovel-shaped lower tusks, with Konobelodon reaching lengths of 1.61 metres, whereas Stegotetrabelodon possessed lower tusks extending up to 2.2 metres.^[35]

Structurally, proboscidean tusks consist of a central vascularized pulp cavity encased in thick dentine and enveloped by a thin outer layer of cementum.^[38][39] The dentine, which constitutes true ivory, is composed of carbonated hydroxyapatite crystals embedded in an organic collagen matrix, conferring hardness and elasticity.^[38][39] Dentine is secreted within microscopic tubules that radiate outward from the central axis in acute-angled branching patterns.^[38] In transverse sections, the tusks of elephantoids display intersecting curvilinear lines that produce an engine-turned or checkerboard pattern termed Schreger lines.^[39][40] The angles at which these rhombic Schreger patterns intersect differ consistently between taxa, providing diagnostic value to distinguish mammoth ivory from modern elephant ivory in forensic wildlife trade monitoring under CITES.^[39][40] Schreger lines are documented in Anancus, Stegodon, Gomphotherium, and Mammut, but are absent in basal forms like Deinotherium.^[41][42][43] In ancestral proboscideans, a longitudinal band of enamel covered part of the tusk shaft, but in Elephantidae this enamel coating was lost, remaining only as a temporary cap on the tips of juvenile tusks that wears off after early life.^{[35][39][44][45]}

Cheek Teeth and Tooth Replacement

The morphology of the cheek teeth forms the primary basis for proboscidean systematics.^[46][47][48] Proboscidean molars derived from the ancestral tribosphenic molar pattern of placental mammals, comprising an elevated anterior trigon or trigonid bearing the main cusps and a lower posterior talon or talonid.^{[46][47][48][49]} On upper molars, the paracone and protocone formed the anterior transverse crest (protoloph), while the metacone and hypocone formed the second crest (metaloph), with additional crests developing sequentially from the talon.^[46][47][48] A median longitudinal groove divides each transverse ridge into lingual and buccal half-crests, known as the entoloph and ectoloph, creating distinct wear facets termed pretrite (the more heavily worn half) and posttrite (the less worn half).^{[46][47][48][49]} In the upper jaw the pretrite half is lingual, whereas in the lower jaw it is buccal.^[46][47][48]

Four principal molar patterns occur across the order: bunodont, zygodont, lophodont, and lamellodont.^{[22][46][47][48]} The bunodont type represents the basal condition, found in Eritherium, Moeritherium, and gomphotheres, characterized by conical cusps paired transversely and flanked by accessory conules that wear into cloverleaf shapes.^{[4][46][47][48]} Modifications of bunodonty include offset half-crests (anancoidy), V-shaped crests (chevroning), and numerous grooves and small cusps (ptychodonty or choerodonty). The zygodont pattern evolved in Mammutidae, where sharp, straight ridges link the cusps without obstructing the intermediate valleys.^[46][50] The lophodont type occurs in Numidotheriidae, Barytheriidae, Deinotheriidae, and Stegodontidae, featuring continuous transverse shearing crests formed by fused cusps.^[46][47][51] The lamellodont pattern characterizes Elephantidae, where cheek teeth consist of numerous parallel, flattened enamel plates packed with cementum, reaching up to 30 plates on the third molars of advanced mammoths.^[46][52][53] Molars evolved from low-crowned (brachyodont) structures adapted to browsing into high-crowned (hypsodont) grinding batteries capable of processing abrasive silica-rich grasses.^[52][53][54]

Primitive proboscideans exhibited typical vertical tooth replacement, in which permanent cheek teeth erupted from below and all functional teeth were used simultaneously in the jaw.^[26][55] Within Elephantimorpha, this mechanism was replaced by horizontal tooth replacement, or lateral progression, first evidenced in the late Oligocene genus Eritreum.^{[19][26][55][56]} Under horizontal displacement, only one or two functional teeth are in active occlusion in each jaw quadrant at any given time.^[26][55][57] As the abrasive diet wears down the functional crown, new teeth developing in the rear of the jaw migrate forward to push out the worn remnants.^[19][26][55] This sequence operates through six successive tooth generations (three milk premolars and three molars), providing up to 24 cheek teeth across an animal's lifetime.^[26][55][58] When the final, sixth tooth generation wears down completely, typically around 60 to 65 years of age, the animal can no longer grind vegetation and faces starvation.^[55][56][58]

Microstructural examination of proboscidean enamel reveals a specialized three-layered architecture in advanced forms.^[59][60] This comprises an outer zone of radial enamel prisms, an intermediate zone of alternating light and dark Hunter-Schreger bands, and an inner zone composed of a three-dimensional decussation of interwoven prism bundles.^[59][60] Basal taxa, including Phosphatherium, Moeritherium, and Palaeomastodon, possessed a simpler two-layered enamel structure.^[59][60] The complex three-dimensional prism decussation provided high resistance against shear stresses generated during grinding, evolving well before the emergence of hypsodont crowns in an apparent evolutionary preadaptation.^[60]

Postcranial Skeleton

The postcranial skeleton of proboscideans exhibits graviportal adaptations to carry a high body mass.^{[2][19][21][61]} The vertebral column is rigid, characterized by a arched thoracic and lumbar curvature with interlocking vertebral processes that strictly restrict rotational, vertical, and horizontal flexibility.^[62] The neck is shortened, composed of compressed cervical vertebrae with tall anterior thoracic neural spines providing anchorage for the ligaments supporting the heavy cranium.^[20] Clavicles and the baculum are entirely absent across the order.^[29] The limb bones are positioned vertically beneath the body in a columnar arrangement, forming a 180-degree angle between the upper and lower segments during standing, which contrasts with the flexed limb posture of typical cursorial mammals.^{[19][21][63][64]}

The long bones of proboscideans lack hollow medullary cavities, their interior lumens being filled with dense cancellous spongy bone within which haematopoiesis occurs.^[2] Limb bones fall into two distinct structural morphotypes across extinct lineages: a gracile type with slender shafts and compact articular ends, found in Deinotherium and Elephantidae, and a robust type with broad shafts and massive articular ends, typical of Gomphotheriidae and Mammutidae.^[65][66] The ankle joint demonstrates parallel divergence: gomphotheres and deinotheres possess an elongated calcaneus and an asymmetrical astragalar facet favoring powerful low-speed leverage, whereas elephantids, mammutids, and stegodontids developed a shortened calcaneus with a symmetrical astragalar facet facilitating faster, cushioned movement.^[67]

Proboscidean autopodia are taxeopod, meaning the carpal and tarsal bones are arranged in serial rows where individual elements lie directly beneath one another rather than alternating or overlapping across joints.^[68] The radius and ulna remain unfused, crossing one another to stabilize the forelimb.^[2] Both forefeet and hindfeet are functionally pentadactyl, but the skeletal posture is modified digitigrade.^[61][68] The phalanges are arranged in a semicircular arch supported by a fibrocartilaginous cushion pad of fatty connective tissue that acts as a shock absorber.^[57][61][68] Beneath the pad, a specialized sixth false digit, known as the prepollex on the forefoot and the prehallux on the hindfoot, articulates with the first metacarpal or metatarsal.^[68] Originating as cartilaginous structures that partially ossify with age, these elements first evolved in Deinotheriidae during the early Miocene, stabilizing the feet as early proboscideans transitioned from amphibious environments to fully terrestrial megaherbivory.^[68]

Soft-Tissue Anatomy

The hallmark organ of the order is the proboscis, or trunk, formed by the embryonic fusion of the elongated upper lip and external nose.^{[1][29][69][70][71]} Containing no bones or internal cartilage, the trunk comprises up to 150,000 distinct muscle fascicles arranged in longitudinal, radiating, and helical orientations.^[2] The two nasal cavities extend through its entire length to the distal tip, which is equipped with sensitive finger-like projections used to grasp objects with fine motor precision.^[2][29] The trunk acts as a tactile, olfactory, and sound-producing organ, as well as a prehensile appendage that bridges the physical distance between the elevated mouth and the ground caused by the shortened neck and columnar limbs.^{[2][19][20][29]} In modern Asian elephants, the trunk can hold 8.5 litres of water, and an adult bull can siphon up to 212 litres in under five minutes to spray into the mouth.^[29] In fossil taxa, retracted nasal regions of the skull in members of Elephantiformes and Deinotheriidae indicate the development of a trunk.^[32][72] In deinotheres, however, the skull indicates a short, broad muscular structure similar to that of tapirs rather than the classic elephant trunk.^[73][74][75]

Proboscideans have large brains, with modern elephant brains reaching volumes of 2,900 to 9,000 cubic centimetres.^[16][17][76] The encephalisation quotient of living elephants ranges from 1.1 to 2.2 with a mean of 1.7.^[16][17][76] The brain contains approximately 257 billion neurons, roughly three times the number found in the human brain, although about 98 percent of these neurons are concentrated in the cerebellum.^[77] The temporal lobes are folded and enlarged, which correlates with complex memory retention, cognitive mapping, and vocal learning.^[16][17] Fossil endocasts reveal that Moeritherium had an estimated brain volume of 240 cubic centimetres and an encephalisation quotient of 0.2, whereas Palaeomastodon possessed a volume of 740 cubic centimetres and an encephalisation quotient of 0.3.^[78][79] Mammut americanum possessed brain volumes between 3,860 and 4,630 cubic centimetres with an encephalisation quotient of 0.30 to 0.74, while the insular dwarf species Palaeoloxodon falconeri developed an encephalisation quotient of approximately 3.75.^[78][79][80]

Modern proboscideans have sparse hair coats, retaining sensory bristles primarily around the chin, trunk tip, and tail.^[6][7] Their sparse pelage is an adaptation to facilitate thermal dissipation in warm tropical environments, supplemented by heat radiation through thin, highly vascularized ear pinnae and bathing behaviors.^{[81][82][83][84]} In contrast, woolly mammoths (Mammuthus primigenius) evolved a dense double coat composed of fine, crimped underwool and coarse guard hairs up to 13 centimetres long to survive Pleistocene permafrost biomes.^[85][86] Guard hairs of mammoths lacked internal medullary canals, and genetic studies indicate natural polymorphisms in wool color ranging from dark brown to blond.^[85][86][87] Preserved hair patches from the American mastodon indicate that Mammut americanum similarly bore a two-layered coat suited to temperate and subarctic coniferous woodlands.^[88] Unique soft-tissue features of Elephantidae also include the temporal gland, a modified subcutaneous gland situated between the eye and ear that discharges during physiological excitement and musth.^[2][6][7]

Evolutionary History

Proboscidean evolutionary history is documented by an extensive fossil sequence encompassing over 180 described extinct species.^[3][4][89] While early paleontologists debated affinities with South Asian anthracobunids, recent anatomical and phylogenetic analyses place Anthracobunidae within Perissodactyla or basal Tethytheria rather than true Proboscidea.^{[90][91][92][93][94][95]} Early Paleogene African taxa like Ocepeia and Abdounodus exhibit dental and cranial similarities to paenungulates, indicating that proboscideans branched off within Afrotheria during the Late Cretaceous or early Paleocene.^{[96][97][98][99]} Proboscidean evolution unfolded through three successive adaptive radiations marked by progressive anatomical specialization, body size enlargement, and shifts in ecological niche.^{[16][17][100][101]}

First Radiation: Paleocene to Oligocene

The first proboscidean radiation spanned from approximately 61 to 24 million years ago and was largely restricted to the African continent and the Arabian Peninsula, which formed a single landmass.^{[17][102][103]} The most primitive known genus is Eritherium from the late Paleocene (Thanetian) phosphate beds of the Ouled Abdoun Basin in Morocco, followed closely by Phosphatherium escuilliei.^{[4][9][104][105][106][107]} These early animals were small, digitigrade or plantigrade quadrupeds lacking elongated trunks, with bunodont molars exhibiting incipient transverse crests.^{[4][9][51][105]} During the Eocene, diversity expanded in North Africa with Numidotheriidae, Barytheriidae, and Moeritheriidae.^{[31][108][109]} Moeritherium was a tapir-like, semi-aquatic browser that inhabited estuarine swamps, using its flexible upper lip to consume aquatic vegetation.^{[31][110][111][112]} Barytherium marked the first large increase in body size within the proboscidean line, reaching a shoulder height of 2.5 to 3 metres and a body mass of around 2 tonnes, and it had eight short tusks, two on each jaw branch.^[10][11][31]

Deinotheriidae diverged during the late Oligocene in Africa, represented first by Chilgatherium from Ethiopia.^[113][114] Deinotheres possessed bilophodont and trilophodont molars and down-curved lower tusks, with Deinotherium giganteum later attaining shoulder heights over 4 metres and weights exceeding 12 to 14 tonnes.^{[11][13][114]} Unlike other advanced proboscideans, deinotheres retained vertical tooth replacement and lacked upper tusks throughout their evolutionary tenure.^[32][55] In the Fayum Basin of Egypt, late Eocene and early Oligocene strata yielded Palaeomastodon and Phiomia, which displayed intermediate dental characters, developing trilophodont cheek teeth and four short tusks while continuing to utilize vertical tooth replacement.^[115][116] From this ancestral stem emerged the family Mammutidae in the late Oligocene of Africa, characterized by zygodont molars.^[50][117] The earliest mammutid, Losodokodon losodokius, lived in Kenya around 26 million years ago.^[50][117]

Second Radiation: Miocene Expansion

The second radiation was initiated during the early Miocene, around 18 to 22 million years ago, triggered by the collision of the Afro-Arabian tectonic plate with Eurasia and the closure of the Tethys Seaway.^{[101][102][103][109]} This tectonic contact established overland connections that allowed proboscideans to disperse across Europe and Asia, an event historically known as the Proboscidean Datum Event.^[101][102] Rather than a single wave, recent stratigraphy demonstrates that this expansion occurred in at least half a dozen distinct migration pulses.^[102] Zygolophodon and Gomphotherium reached Eurasia first, subsequently crossing the Bering Land Bridge into North America between 16 and 15 million years ago.^{[101][102][118][119]} Deinotheres dispersed across Eurasia during the early Miocene as well, persisting there until the Pliocene while remaining absent from the Americas.^{[101][114][120]}

Gomphotheriidae diversified into a broad array of morphologies during the Miocene, as the climate cooled and open landscapes spread.^{[17][101][103][109]} Basal gomphotheres retained four tusks and bunodont molars with cloverleaf wear patterns.^{[46][103][109]} Specialized subfamilies arose rapidly: Amebelodontinae developed elongated lower jaws with spade-like lower incisors, exemplified by Platybelodon and Amebelodon, which were utilized to shovel and scrape tough vegetation; Choerolophodontinae possessed heavily folded molar enamel and reduced lower tusks; and Rhynchotheriinae resembled Gomphotheriinae but had laterally flattened lower tusks.^{[33][101][121][122][123]} Advanced tetralophodont gomphotheres, including Tetralophodon and Anancus, developed four transverse crests on intermediate molars and lost functional lower tusks.^{[115][122][124]} In eastern and southeastern Asia, Stegodontidae branched off from bunodont gomphotheres around 15 million years ago, producing taxa like Stegolophodon and Stegodon with multi-crested, roof-shaped molars.^[125][126]

Third Radiation: Pliocene to Holocene

The third proboscidean radiation began in Africa around 7 to 10 million years ago with the emergence of the family Elephantidae from gomphothere stock.^{[17][109][127]} Early representatives included Stegotetrabelodon, which still bore long lower tusks, and Primelephas, which was ancestral to the modern elephant lineages.^{[17][128][129]} Molecular dating and fossil evidence demonstrate that Loxodonta diverged first, around 7.6 million years ago, remaining confined to the African continent throughout its evolutionary history.^{[117][130][131][132]} Elephas and Mammuthus separated around 6.7 million years ago.^{[117][132][133]} Both lineages expanded out of Africa into Eurasia during the late Pliocene, between 3.6 and 3.2 million years ago.^[131][134] Mammuthus subsequently crossed Beringia into North America approximately 1.5 million years ago, evolving into endemic taxa such as the Columbian mammoth (Mammuthus columbi) and the woolly mammoth (Mammuthus primigenius).^{[34][109][119][135]}

Around 3 million years ago, the formation of the Isthmus of Panama facilitated the Great American Biotic Interchange, allowing gomphotheres to enter South America.^{[16][54][103][115][136]} In South America, gomphotheres evolved into brevirostrine genera including Notiomastodon and Cuvieronius, the latter adapting to high-elevation Andean mountain zones.^[103][123] Meanwhile, Palaeoloxodon dispersed across Eurasia approximately 800,000 years ago, giving rise to large mainland browsers such as the straight-tusked elephant (Palaeoloxodon antiquus) and Palaeoloxodon namadicus.^{[131][137][138]} By the beginning of the Late Pleistocene, proboscideans were represented by around 23 species.^[101]

Insular Dwarfism

During the Pliocene and Pleistocene, multiple proboscidean populations became isolated on oceanic islands following sea-level fluctuations, undergoing severe insular dwarfism.^[139][140] Island dwarfism likely developed because insular environments lacked large or viable predator populations and offered limited resources, while small mammals under identical conditions often developed insular gigantism.^[139] In the Mediterranean Basin, isolated populations of Palaeoloxodon antiquus evolved into dwarf forms across Sicily, Malta, Cyprus, Crete, the Cyclades, and the Dodecanese.^[139] The Sicilian dwarf elephant, Palaeoloxodon falconeri, stood only 1 metre tall at the shoulder and weighed approximately 190 kilograms, representing a body mass reduction of over 95 percent compared to its mainland ancestor.^[80][139] Dwarf mammoths also evolved on Sardinia (Mammuthus lamarmorae) and Crete (Mammuthus creticus).^[139][141]

In North America, Columbian mammoths colonized the California Channel Islands during the late Pleistocene, giving rise to the pygmy mammoth (Mammuthus exilis), which reached shoulder heights of 1.2 to 1.8 metres and body masses between 200 and 2,000 kilograms.^[139] In Southeast Asia, Stegodon underwent pronounced dwarfism on Flores, Sulawesi, and Timor, producing diminutive species such as Stegodon sondaari and Stegodon florensis.^[140][141] A population of woolly mammoths remained isolated on Wrangel Island in the Arctic Ocean until approximately 4,000 years ago.^[139][142] Although initially described as dwarf mammoths upon their discovery in 1993, subsequent re-evaluation at the Second International Mammoth Conference in 1999 concluded that the Wrangel Island specimens were small-bodied relict woolly mammoths that experienced severe genetic bottlenecks and inbreeding rather than true insular dwarfs.^{[139][143][144][145][146][147]}

Quaternary Extinctions and Human Interactions

Proboscidean diversity declined severely during the Late Pleistocene megafaunal extinctions between 50,000 and 10,000 years ago.^{[101][122][148]} During this interval, all remaining non-elephantid proboscideans, including American mastodons (Mammut americanum), Stegodon, and the American gomphotheres Cuvieronius and Notiomastodon, vanished, alongside mainland mammoth species and straight-tusked elephants.^{[101][122][148]} Only the three living elephant species in Africa and tropical Asia survived into historical times.^[1][16] The causes of this extinction wave remain debated, with two main hypotheses that do not exclude each other: nutritional stress from reduced plant diversity due to climate change, and population decline through human hunting.^{[54][122][148]} In South and North America, some gomphothere populations may have experienced competitive displacement or localized ecological stress prior to intensive human arrival.^[149][150]

Archeological sites across Africa, Eurasia, and the Americas confirm sustained interactions between hominins and proboscideans extending back to Homo erectus around 2 million years ago.^[151][152] Early humans systematically butchered proboscidean carcasses for meat, marrow, hide, and ivory, as documented at butchery localities in Europe, the Levant, and North America.^{[151][153][154]} Direct proof of active hunting is demonstrated by rare artifact associations, such as the Eemian Lehringen spear in Germany, where a worked yew-wood spear was found embedded between the ribs of a Palaeoloxodon antiquus skeleton.^[155] During the Upper Paleolithic, proboscideans were central subjects of portable art and parietal cave art in Franco-Cantabrian sites, appearing as carved ivory figurines and engraved wall paintings across western and central Europe.^[156][157] In historical cultures, elephants were integrated into Asian religious ceremonies and state warfare, though they were never fully domesticated in the manner of livestock.^[158] Today, surviving populations face severe endangerment driven by illegal ivory poaching, habitat loss, and agricultural fragmentation.^{[159][160][161][162][163]}

Behavior and Ecology

Living elephants maintain complex, female-led matriarchal societies composed of related adult cows and their dependent offspring, while adult males lead solitary lives or congregate in loose bachelor groups.^[6][7][164] Fossil trackway evidence indicates that similar herd structures existed millions of years ago.^[165] At the late Miocene Baynunah Formation in the United Arab Emirates, an assemblage of at least 13 parallel proboscidean trackways spanning a 20 to 30 metre band over 190 metres records a herd of varying body sizes travelling together, which is intersected by a solitary 260-metre stride line produced by a single massive bull.^[165] Similar social trackways belonging to Palaeoloxodon antiquus have been documented from late Pleistocene littoral sediments at Matalascañas in southwestern Spain.^[166] Furthermore, fossil footprints of Deinotherium in the late Miocene of Romania suggest that deinotheres also congregated in social herds.^[167]

Communication in modern proboscideans relies on multi-modal sensory channels, including visual posturing, tactile trunk interaction, chemical signaling via urine and temporal gland secretions, and low-frequency acoustics.^{[6][7][168][169]} Social rumbles and contact calls utilize infrasound between 10 and 200 Hertz, propagating over several kilometers through ground and air.^{[168][169][170]} The physiological capacity for infrasonic hearing correlates with inner ear anatomy: modern elephants possess a cochlea with two full turns (670 to 790 degrees) lacking a secondary spiral lamina at the base, which allows the basilar membrane to extend widely.^[171][172] In contrast, early Paleogene proboscideans like Eritherium and Phosphatherium had cochleae with only 1.5 turns and retained a secondary lamina, which suggests that they were probably sensitive only to higher frequencies.^[171][172] Derived cochlear morphology suited to infrasonic hearing was possibly present in early Mammutidae and was in place at the latest with the emergence of the relatives of modern elephants.^{[170][173][174]}

Reproductive physiology in male proboscideans features musth, an annual period of heightened testosterone secretion accompanied by heavy temporal gland discharge, urine dribbling, and extreme aggression during intra-sexual dominance contests.^[6][7][175] Preserved permafrost mammoth mummies confirm the presence of temporal glands in extinct Elephantidae.^[17] In modern elephants and mammoths, fluctuations in testosterone levels during musth leave distinct annual chemical markers within the incremental growth dentine rings of their tusks.^[176] Tusk growth anomalies appearing in early summer on fossil tusks of Notiomastodon from South America suggest that musth also occurred in gomphotheres.^[177][178] In American mastodons (Mammut americanum), tusk impact fractures, rib breaks, and puncture wounds consistent with conspecific combat during spring breeding seasons suggest that musth-like sexual competition may have been inherited from the last common ancestor of elephantimorphs.^{[179][180][181]} Female mastodon tusks show growth interruptions every three to four years, matching the four-year calving intervals observed in modern elephant cows.^[182]

Proboscidean diets shifted dramatically over geological time, transitioning from generalized browsing to specialized grazing.^{[17][148][183][184]} Basal Paleogene taxa like Moeritherium and Barytherium fed predominantly on aquatic vegetation and soft C3 browse, as evidenced by low-crowned bunolophodont molars and carbon isotope values.^[185][186] Miocene Mammutidae retained a browser ecology, utilizing zygodont crests to shear twigs, leaves, and conifer foliage, a diet corroborated by mastodon coprolites and gastrointestinal contents containing spruce and larch needles.^{[187][188][189]} Gomphotheres maintained mixed-feeding strategies, though several lineages pioneered grass consumption as open savannas expanded.^[190][191] With the rise of Elephantidae in the late Miocene, the shift to lamellodont, hypsodont molars permitted intensive grazing on abrasive C4 grasses.^{[17][53][192]} In East Asia, Stegodon lived more in woodland than early Elephas, which used more open landscapes.^[193]

Through their foraging behaviors, proboscideans function as key ecosystem engineers.^[6][7] By stripping bark, breaking branches, and uprooting bushes and small trees, elephants can open up closed areas, push back forest edges, and keep open landscapes clear, which is important for the savannas of eastern and southern Africa.^[6][7] They transport seeds across vast landscapes, facilitating plant dispersal, while their dung provides nutrient-rich microhabitats for coprophagous insects and fungi.^[6][7] A similar role can be assumed for extinct forms such as the mammoths of the Mammoth Steppe.^[194][195] In the Miocene, wear marks and damage on the downward-curved tusks of Deinotherium point to bark scraping or the splitting of trees, suggesting that this behaviour may have appeared early in proboscidean history.^[196]

Taxonomic History

The scientific name Proboscidea was coined in 1811 by the German zoologist Johann Karl Wilhelm Illiger, derived from the Latin proboscis, which originates from the Ancient Greek proboskis (composed of pro, meaning 'before', and boskein, meaning 'to feed').^{[1][75][197][198][199]} Carl Linnaeus had initially placed elephants in 1758 within his order Bruta alongside sloths, anteaters, and manatees based on the absence of front incisors.^[200] In the late 18th century, Johann Friedrich Blumenbach transferred elephants to the order Belluae with hippopotamuses, rhinoceroses, and pigs, describing them as large, thick-skinned quadrupeds.^[201] In 1795, Georges Cuvier and Étienne Geoffroy Saint-Hilaire grouped these thick-skinned herbivores into Pachydermata.^[202][203] Henri Marie Ducrotay de Blainville broke apart Pachydermata in 1816, assigning elephants to Gravigrades, and Richard Owen formalized the division of ungulates into Artiodactyla and Perissodactyla in 1848.^{[204][205][206]}

In 1870, Theodore Nicholas Gill noted a closer relationship between sea cows and hyraxes, although he gave it no name.^[207] Edward Drinker Cope grouped these taxa within Taxeopoda based on serial foot carpal architecture, while Richard Lydekker and Max Schlosser used the term Subungulata.^[206] In 1945, George Gaylord Simpson established the superordinal clade Paenungulata to encompass Proboscidea, Sirenia, Hyracoidea, and extinct allies within Protungulata.^[206] At the turn of the 21st century, molecular sequencing and the discovery of shared retrotransposons (AfroSINEs) firmly integrated Paenungulata into the superorder Afrotheria, demonstrating that proboscideans share a deep African ancestry with aardvarks, elephant shrews, and afrosoricids.^{[96][208][209][210][211]} Within Paenungulata, proboscideans and sirenians are frequently united in the clade Tethytheria.^[208][212]

Fossil proboscidean classification was revolutionized in the early 20th century by Henry Fairfield Osborn, who led an American Museum of Natural History expedition to the Fayum in 1907.^[213][214] Osborn published a massive two-volume monograph, The Proboscidea (1936, 1942), describing over 350 species and proposing four major suborders: Moeritherioidea, Deinotherioidea, Mastodontoidea, and Elephantoidea.^{[215][216][217]} Simpson, who criticized Osborn for disregarding the rules of nomenclature, revised Osborn's scheme in 1945, recognizing four superfamilies: Moeritherioidea, Barytherioidea, Deinotherioidea, and Elephantoidea.^[206] Further comprehensive revisions were executed by Vincent Maglio and John M. Harris in 1978, Malcolm McKenna and Susan K. Bell in 1997, and Jeheskel Shoshani and Pascal Tassy in 1996 and 2005, which established the modern framework of Elephantiformes and Elephantimorpha.^{[122][218][219][220]} Recent phylogenomic and proteomic analyses have clarified relationships among extinct species, revealing that the straight-tusked elephant (Palaeoloxodon) is genetically closer to Loxodonta than to Elephas, with evidence of extensive hybridization during early elephantid diversification.^{[221][222][223]}

Systematics and Classification

The order Proboscidea is divided into basal stem grades (sometimes assembled as Plesielephantiformes) and the advanced suborder Elephantiformes.^{[113][122][224]} Below is the consensus taxonomy of described proboscidean families and genera based on systematic revisions up to the 2020s:^{[23][113][116][122][225][226][227]}

Taxonomic List

Order Proboscidea Illiger, 1811 * Basal stem taxa: * †Eritherium Gheerbrant, 2009 * †Moeritherium Andrews, 1901 (Family †Moeritheriidae Andrews, 1906) * †Saloumia Tabuce et al., 2019 * Suborder †Plesielephantiformes Shoshani et al., 2001 (paraphyletic): * Family †Phosphatheriidae Gheerbrant et al., 2005 * †Phosphatherium Gheerbrant et al., 1996 * Superfamily †Barytherioidea Andrews, 1906: * Family †Numidotheriidae Shoshani & Tassy, 1992 * †Numidotherium Mahboubi et al., 1986 * Family †Barytheriidae Andrews, 1906 * †Barytherium Andrews, 1901 * †Arcanotherium Delmer, 2009 * †Daouitherium Gheerbrant & Sudre, 2002 * †Omanitherium Seiffert et al., 2012 * Superfamily †Deinotherioidea Bonaparte, 1845: * Family †Deinotheriidae Bonaparte, 1845 * Subfamily †Chilgatheriinae Sanders et al., 2004: †Chilgatherium Sanders et al., 2004 * Subfamily †Deinotheriinae Bonaparte, 1845: †Prodeinotherium Ehik, 1930; †Deinotherium Kaup, 1829

* Suborder Elephantiformes Tassy, 1988: * †Dagbatitherium Hautier et al., 2021 * †Hemimastodon Pilgrim, 1912 * Family †Palaeomastodontidae Andrews, 1906: †Palaeomastodon Andrews, 1901 * Family †Phiomiidae Kalandadze & Rautian, 1992: †Phiomia Andrews & Beadnell, 1902 * Infraorder Elephantimorpha Tassy & Shoshani, 1997: * †Eritreum Shoshani et al., 2006 * Parvorder †Mammutida Tassy & Shoshani, 1997: * Superfamily †Mammutoidea Hay, 1922: * Family †Mammutidae Hay, 1922: †Losodokodon Rasmussen & Gutierrez, 2009; †Eozygodon Tassy & Pickford, 1983; †Zygolophodon Vacek, 1877; †Sinomammut Mothé et al., 2016; †Mammut Blumenbach, 1799

* Parvorder Elephantida Tassy & Shoshani, 1997: * Family †Choerolophodontidae Gaziry, 1976: †Afrochoerodon Pickford, 2001; †Choerolophodon Schlesinger, 1917 * Family †Amebelodontidae Barbour, 1927: †Progomphotherium Pickford, 2003; †Archaeobelodon Tassy, 1984; †Afromastodon Pickford, 2003; †Protanancus Arambourg, 1945; †Serbelodon Frick, 1933; †Amebelodon Barbour, 1927; †Konobelodon Lambert, 1990; †Torynobelodon Barbour, 1929; †Eurybelodon Lambert, 2016; †Platybelodon Borissiak, 1928; †Aphanobelodon Wang et al., 2016 * Family †Gomphotheriidae Hay, 1922 (paraphyletic): †Gomphotherium Burmeister, 1837; †Gnathabelodon Barbour & Sternberg, 1935; †Blancotherium May, 2019; †Eubelodon Barbour, 1914; †Rhynchotherium Falconer, 1868; †Stegomastodon Pohlig, 1912; †Cuvieronius Osborn, 1923; †Notiomastodon Cabrera, 1929; †Sinomastodon Tobien et al., 1986

* Superfamily Elephantoidea Gray, 1821: * Tetralophodont gomphothere grade: †Tetralophodon Falconer, 1857; †Anancus Aymard, 1855; †Paratetralophodon Tassy, 1983; †Pediolophodon Lambert, 2007 * Family †Stegodontidae Osborn, 1918: †Stegolophodon Schlesinger, 1917; †Stegodon Falconer, 1857 * Family Elephantidae Gray, 1821: * Subfamily †Stegotetrabelodontinae Aguirre, 1969: †Stegotetrabelodon Petrocchi, 1941; †Stegodibelodon Coppens, 1972; †Selenotherium Mackaye et al., 2005 * Subfamily Elephantinae Gray, 1821: †Primelephas Maglio, 1970; Loxodonta Anonymous, 1827 (extant); †Palaeoloxodon Matsumoto, 1924; †Mammuthus Brookes, 1828; Elephas Linnaeus, 1758 (extant)

Extant Species and Conservation

Only three species of the order survive today, all belonging to the family Elephantidae: the Asian elephant (Elephas maximus), the African bush elephant (Loxodonta africana), and the African forest elephant (Loxodonta cyclotis).^[1] The African forest elephant was traditionally considered a subspecies of Loxodonta africana, but genomic analyses confirmed profound genetic divergence, establishing it as an independent species.^[228]

All three living species are threatened with extinction and are categorized on the IUCN Red List.^{[159][160][161][229]} Asian elephants have declined by approximately 50 percent over the last three generations, and their population is now fragmented across an area of 486,800 square kilometres.^[230] Imbalances between the numbers of males and females caused by hunting for ivory have been recorded, such as in India's Periyar Tiger Reserve where the adult male-to-female ratio skewed from 1:6 to 1:122 over a twenty-year span.^[231] In Africa, populations face continuing habitat fragmentation across 37 range states, with nearly one-third of surviving African bush elephants concentrated in Botswana.^[163][229] Escalating human-elephant conflict resulting from agricultural conversion poses a major threat across both continents, prompting mitigation strategies such as beehive acoustic fences, early-warning crop monitoring, and habitat connectivity corridors.^[162][163]

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