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Elephantiformes
Elephantiformes
Elephantiformes
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Elephantiformes
Temporal range: Lutetian–Recent
Skull of Phiomia
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Proboscidea
Suborder: Elephantiformes
Tassy, 1988
Subgroups[1]

Elephantiformes is a suborder within the order Proboscidea.[1] Members of this group are primitively characterised by the possession of upper tusks, an elongated mandibular symphysis (the frontmost part of the lower jaw) and lower tusks, and the retraction of the facial region of the skull indicative of the development of a trunk.[2] The earliest known member of the group, Dagbatitherium is known from the Eocene (Lutetian) of Togo, which is only known from isolated teeth, while other primitive elephantiforms like Phiomia and Palaeomastodon are known from the Early Oligocene onwards.[3] Phiomia and Palaeomastodon are often collectively referred to as "palaeomastodonts" and assigned to the family Palaeomastodontidae.[4] Most diversity of the group is placed in the subclade Elephantimorpha, which includes mastodons (family Mammutidae), as well as modern elephants and gomphotheres (Elephantida),[3] which are distinguished from more primitive elephantiforms by the development of horizontal tooth replacement.[5] It is disputed as to whether Phiomia is closely related to both Mammutidae and Elephantida with Palaeomastodon being more basal, or if Palaeomastodon is closely related to Mammutidae and Phiomia more closely related to Elephantida.[4]

Phylogeny of Proboscidea including Elephantiformes, following Hautier et al. 2021[3]

Proboscidea

References

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Elephantiformes

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Elephantiformes is a of proboscidean mammals that encompasses the early elephant-like forms and their descendants, including the lineage leading to modern elephants, primitively characterized by the development of upper tusks and specialized dental adaptations such as trilophodont molars with lamellar enamel structures and a three-layered enamel schmelzmuster. Originating in during the Middle Eocene around 46–44 million years ago, this group represents a pivotal radiation in proboscidean evolution, bridging primitive bunodont ancestors and the more derived Elephantimorpha. The clade Elephantiformes includes several key families and genera, such as Phiomiidae (e.g., Phiomia), Palaeomastodontidae (e.g., ), and the basal members leading to Elephantimorpha, with early representatives like Dagbatitherium tassyi from exhibiting diagnostic features including transversely enlarged buccal cusps, a mesoconid on lower molars, and a three-layered enamel schmelzmuster. These animals were generally smaller than later proboscideans, with body sizes ranging from subadult forms estimated at around 484 kg and shoulder heights of 122–132 cm, as seen in late fossils like Eritreum melakeghebrekristosi from . Dentally, they displayed intermediate morphologies, including four-lophid m3 molars, reduced anterolingual cingulids, and early signs of horizontal tooth displacement, which became more pronounced in descendant groups. Elephantiformes flourished primarily in from the late Eocene through the , with some archaic taxa persisting into the early , before being largely replaced by the more specialized Elephantimorpha (encompassing mammutids, gomphotheres, and elephantids) around 26–27 million years ago. This transition highlights as a cradle for proboscidean diversification, where archaic elephantiforms like Phiomia and coexisted with emerging elephantimorphs, facilitating biogeographic expansions into . Ecologically, these proboscideans were likely browsers or mixed feeders in forested or woodland environments, utilizing their developing tusks for , display, or defense, though some derived forms within related lineages later evolved unique adaptations like shovel-shaped lower jaws in amebelodontids. This replacement of archaic forms underscores broader patterns in proboscidean evolution, driven by climatic shifts and competitive dynamics that ultimately led to the modern elephants.

Taxonomy and Classification

Definition and Characteristics

Elephantiformes is a suborder of the mammalian order , encompassing elephant-like proboscideans that share derived cranial and dental features distinguishing them from more primitive relatives. This suborder was formally established by paleontologist Pascal Tassy in to group taxa exhibiting advanced adaptations toward the modern body plan, including the development of a and specialized incisors. Within , Elephantiformes represents a major evolutionary lineage that diverged from earlier forms, with Elephantimorpha serving as an advanced subclade containing mammutids and elephantids. Primitive characteristics of Elephantiformes include the possession of upper tusks formed by elongated upper incisors, which are often paired with lower tusks in early members, providing both defensive and foraging functions. These taxa also feature an elongated , the fused forward portion of the lower jaw that supports lower incisors and contributes to a more specialized feeding apparatus. Additionally, the exhibits retraction of the nasal region, evidenced by shortened and repositioned external nares, signaling the early evolutionary development of a trunk-like for manipulation and grasping. The temporal range of Elephantiformes extends from the Lutetian stage of the Eocene epoch, approximately 48 to 41 million years ago, to the present day, marking one of the longest durations among proboscidean suborders. The earliest known member, Dagbatitherium, from , represents a stem form with incipient elephantiform dental traits, pushing the group's origin back significantly from previous estimates. Elephantiformes is distinguished from other proboscidean suborders by its combination of upper development and trunk indicators; for instance, Moeritherioidea lacks prominent tusks and shows no clear evidence of retraction for a trunk. In contrast, Deinotherioidea features only downward-curving lower tusks without upper counterparts or associated adaptations. These traits collectively define Elephantiformes as a cohesive group adapted for terrestrial herbivory with enhanced manipulative capabilities.

Hierarchical Position

Elephantiformes is recognized as one of the three primary suborders within the order , alongside Moeritherioidea and Deinotherioidea, comprising the more derived proboscideans that exhibit advanced morphological traits leading to extant . This suborder was formally introduced by Tassy in 1988 to unite proboscideans characterized by the primitive presence of upper tusks, distinguishing them from earlier basal groups like moeritheres and deinotheres. Within Elephantiformes, subgroups are divided into primitive elephantiforms, such as those assigned to the family Palaeomastodontidae (including genera like Phiomia and Palaeomastodon), and the more derived clade Elephantimorpha. Elephantimorpha, defined by Tassy and Shoshani in 1997, encompasses Mammutidae (mastodons), Gomphotheriidae (gomphotheres), and Elephantidae (true elephants and their close relatives), representing a monophyletic group nested within Elephantiformes based on shared dental and cranial synapomorphies. Classification history includes ongoing debates regarding the status of early forms like Phiomia and Palaeomastodon, with some analyses proposing they constitute a monophyletic family (Palaeomastodontidae) while others view them as a paraphyletic assemblage of basal elephantiforms transitional to Elephantimorpha. Shoshani and Tassy (1996) provided foundational definitions for these subgroups, emphasizing synapomorphies such as increased lophodonty in molars. Recent morphological and total-evidence phylogenetic studies, including ancient DNA analyses, have confirmed the nested position of Elephantimorpha within Elephantiformes, resolving much of the paraphyly debate around gomphotheres and supporting early Miocene divergences among major lineages.

Evolutionary History

Origins and Early Evolution

The clade, encompassing elephant-like proboscideans closely related to modern , likely originated in from proto-proboscidean ancestors during the late to early Eocene, with the group's earliest definitive members appearing in the Middle Eocene. These proto-proboscideans, such as Eritherium from the late Paleocene of (ca. 60 million years ago), represent small, basal forms with primitive dental features that set the stage for later diversification within the order . The transition to Elephantiformes involved the development of more specialized and cranial structures adapted for herbivory in aquatic and semi-terrestrial environments. The earliest known stem Elephantiformes is Dagbatitherium, discovered in Middle Eocene deposits (approximately 46.5–44.0 million years ago) from the Dagbati quarry in , . This pig-sized proboscidean is represented primarily by isolated dental fossils, including brachyodont, bunolophodont molars characterized by transversely enlarged buccal cusps, a large mesoconid, and a third lophid (tritolophid) with a hypoconulid and postentoconulid, indicating a primitive elephantiform-like dental pattern. As a stem Elephantiformes, Dagbatitherium pushes back the known origin of the by about 10 million years compared to previous late Eocene records, bridging the gap between early bunodont proboscideans and more derived forms. Its molars, measuring around 37.5 mm in length and 29.2 mm in width, suggest an adaptation for soft vegetation, reflecting the clade's initial radiation in Africa's tropical environments. A basal proboscidean predating Elephantiformes, from northern (approximately 37 million years ago), exemplifies the semi-aquatic ancestry of the group. Moeritherium, a tapir-sized , inhabited freshwater swamp and riverine settings, as evidenced by stable isotope analysis of showing depleted δ¹⁸O values and low variability, alongside cranial features like a long tubular skull and anteriorly positioned orbits indicative of an amphibious lifestyle. This genus represents a key stage in the broader shift toward advanced proboscideans, with its molars suited for grinding aquatic plants and early signs of development. By the late Eocene, upper tusks began emerging as or display tools, marking a pivotal . This early diversification culminated in Oligocene genera like Phiomia from the Fayum Depression in (approximately 33–28 million years ago), which exhibited the first clear evidence of upper tusks and trunk indicators, such as a retracted nasal and backward-shifted orbits. Phiomia, comparable in size to a modern or small , possessed downward-curving upper tusks (up to 450 mm long in some species) with lateral enamel bands and short, procumbent lower tusks forming a shovel-like structure for gathering vegetation. As browsers in well-watered forested and woodland habitats, these animals displayed lophodont cheek teeth with chisel-like crests for vertical shearing and a graviportal suited to terrestrial movement. The Eocene-Oligocene transition, characterized by and drying around 34 million years ago, likely drove these adaptations by favoring forms capable of exploiting persistent forested niches amid changing climates.

Major Lineages and Extinctions

Elephantiformes encompasses primitive forms and the derived clade Elephantimorpha. During the and early , Elephantiformes underwent significant diversification, marked by the emergence of more advanced forms from African origins. , known from fossils in dated to approximately 36–28 million years ago (Ma), represented an early diversification with a larger body size—reaching up to 2.5 meters at the shoulder—and a morphology approaching that of later , featuring elongated tusks and a more robust . This genus, part of the primitive Palaeomastodontidae family, exemplified the transition toward more specialized elephantiform traits. Around the late , approximately 26 Ma, the clade Elephantimorpha arose, as evidenced by transitional fossils like Behemotops from (dated to 26.8 ± 1.5 Ma), which displayed innovative horizontal tooth replacement patterns bridging early elephantiforms and more derived proboscideans. The major lineages within Elephantimorpha included three primary subclades: (mastodons), Gomphotheriidae (gomphotheres), and (true elephants). , characterized by cone-shaped teeth adapted for in forested environments, diverged early within Elephantimorpha and persisted until their extinction around 10,000 years ago during the . Gomphotheriidae, with shovel-like tusks and grinding molars suited to mixed feeding, achieved greater diversity and geographic range, but became extinct approximately 12,000 years ago. In contrast, , encompassing modern elephants and their immediate precursors like mammoths, originated in the and remains the only extant lineage, with three living species today. Primitive elephantiforms, such as Palaeomastodontidae, largely declined and went extinct by the early , likely due to competitive pressures from these more efficient Elephantimorpha descendants. Elephantiformes achieved a global distribution through successive migrations out of Africa, facilitated by land bridges. By the early (around 19–16 Ma), lineages like crossed into via the Gomphotherium Landbridge, an episodic connection between and , leading to further radiations. Subsequent dispersals reached by the mid- and via the around 2.5 Ma, with and Gomphotheriidae establishing populations across these continents. The Pleistocene megafaunal extinctions, which eliminated and Gomphotheriidae, were driven by a combination of climate fluctuations, , and human hunting pressures, leaving as the sole survivors.

Physical Description

General Morphology

Elephantiformes display significant variation in body size, reflecting their evolutionary progression from modest primitive forms to gigantic advanced species within . The earliest known elephantiform, Dagbatitherium tassyi, represents one of the smallest members of the , with diminutive molar dimensions indicating a small body size. Primitive elephantiforms such as Palaeomastodon beadnelli achieved larger dimensions, reaching shoulder heights of 1.8–2 m and body masses of 2–2.5 tons, supported by robust postcranial skeletons. In contrast, some advanced species within Elephantimorpha, such as certain extinct mammoths () and large mastodons (), reached body masses exceeding 10 tons, establishing them as among the largest terrestrial mammals ever. Key skeletal features of Elephantiformes emphasize adaptations for supporting increasing body mass and transitioning to terrestrial lifestyles. Limbs are characteristically robust and pillar-like, with elongated long bones that provide vertical support and minimize bending stress under immense weight; this is evident even in primitive forms like , where femora measured up to 875 mm in length. Skulls in advanced forms, particularly , feature a high-domed cranium for accommodating enlarged nasal cavities, while the neck is relatively short and the tail reduced compared to more basal paenungulates. Early evidence of development appears in the retracted nasal opening, a defining synapomorphy of the that facilitated trunk evolution. Postcranial adaptations further highlight the clade's terrestrial specialization. Primitive elephantiforms retained five-toed manus and pes, with short, stout metapodials and phalanges suited to subunguligrade locomotion; in advanced forms, the hind foot reduced to four functional toes, enhancing stability for heavy loading. The and exhibit modifications such as broadened glenoid fossae and expanded ilia, respectively, which improve and facilitate efficient terrestrial movement without the flexibility seen in smaller mammals. is pronounced, with males typically larger in body size and exhibiting more prominent tusks, a observed across the from primitive to derived taxa. Compared to their semi-aquatic ancestors in Moeritheriidae, Elephantiformes underwent a marked shift toward fully terrestrial habits, evidenced by more columnar limb postures and increased shoulder heights reaching up to 4 m in modern elephants. This transition is underscored by stable isotope analyses confirming amphibious lifestyles in moeritheres, contrasting with the derived weight-bearing morphology of elephantiforms. The overall integrates skeletal robustness with cranial features, including adapted for grinding .

Dentition and Tusks

The dentition of Elephantiformes exhibits a progressive evolution from primitive bilophodont molars, characterized by two transverse ridges, in early forms such as , to more advanced multilophodont molars with multiple ridges in later Elephantimorpha. This transition is evident in Middle Eocene taxa like Dagbatitherium, which displays a tritolophodont with three lophids, bridging bunodont ancestors to the derived elephantiform dental morphology seen in Phiomia and . A key synapomorphy of Elephantimorpha within Elephantiformes is horizontal tooth replacement, where molars migrate forward in a "conveyor-belt" fashion, allowing sequential eruption and wear over the animal's lifespan without vertical succession. In modern elephants, this results in a reduced of three molars per quadrant, with no permanent premolars, supporting lifelong replacement of up to six molariform teeth. Tusks in Elephantiformes derive primarily from the elongation of upper incisors (I2), forming continuously growing structures composed mainly of dentine covered by a thin enamel cap that wears away early in life. In primitive forms like and Phiomia, both upper and lower incisors develop into tusks, with lower ones often flattened and pyriform in shape, reaching lengths of about 460 mm in Phiomia. Advanced gomphotheres, such as those in the Amebelodontinae (e.g., ), feature distinctive shovel-like lower tusks with extreme dorsoventral compression, adapted for specific foraging functions, while upper tusks remain conical and curved. In late Elephantimorpha, including mammoths and modern elephants, lower tusks are absent, with only upper tusks retained, exhibiting pronounced curvature and lengths up to 3 meters in woolly mammoths. Jaw structures in Elephantiformes show an elongated in early taxa, such as Phiomia (up to 382 mm long), providing robust support for developing tusks and facilitating their emergence. This is nearly straight or downturned in palaeomastodonts and early gomphotheres like Choerolophodon, but undergoes reduction in advanced lineages, becoming shorter and more spout-shaped in to accommodate trunk evolution. Enamel thickness in molars increases over time, from thinner layers in primitive bilophodont forms to 5–11 mm in advanced multilophodont molars of gomphotheres and , enhancing durability against abrasive . These adaptations reflect the clade's specialization for processing tough material, with and lophodont molars becoming increasingly complex.

Paleobiology and Ecology

Diet and Foraging

Elephantiformes, from the Middle Eocene onward, exhibited primitive diets centered on soft vegetation, fruits, and leaves, as inferred from their low-crowned, bunolophodont teeth in early forms like Phiomia, which facilitated shearing of leafy materials. These adaptations allowed for efficient processing of nutritious but low-volume browse, with dental morphology emphasizing vertical shearing over grinding. Phiomia and similar genera were likely , consuming mainly leaves, soft , and shrubs in forested environments. As Elephantiformes persisted through the , their dietary strategies remained focused on mixed in settings, with no of significant shifts to , unlike in descendant Elephantimorpha. , including dental microwear, supports a diet dominated by C3 such as woody vegetation and fruits. in Elephantiformes utilized developing tusks for uprooting , stripping bark, or defense, as observed in forms like . The trunk, evolving by the , served as a manipulator for plucking foliage, enhancing precision in diverse habitats. The digestive system featured a large caecum and , allowing intake of fibrous browse despite inefficient nutrient extraction compared to ruminants. This high-throughput strategy enabled consumption of voluminous low-nutrient , complemented by structures for pre-processing tough materials.

Habitats and Biogeography

Elephantiformes originated in during the Middle Eocene, inhabiting tropical to subtropical wetlands, forests, and woodlands. Early forms such as Dagbatitherium, known from the phosphate basins of , occupied wetland-influenced terrestrial ecosystems. Phiomia, from the Fayum Depression in , lived in lowland coastal plains with damp soils and evergreen tropical forests during the late Eocene to . Similarly, Palaeomastodon from deposits in Egypt's Fayum region thrived in woodland-bushland habitats. Transitional forms from late sites in , such as Eritreum melakeghebrekristosi, indicate persistence in Afro-Arabian forested lowlands around 27 million years ago. Elephantiformes remained largely confined to throughout their existence from the Eocene to early , coexisting with emerging Elephantimorpha in East African environments before being replaced. These habitats supported diverse faunas adapted to forested and marshy settings, reflecting the clade's role in early proboscidean diversification on the continent. Key biogeographic patterns highlight as the cradle, with no significant dispersals outside the continent documented for this group.

References

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