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Table 1 Diversity of craniofacial morphology in mammals and recent studies evaluating this diversity using landmark-based geometric morphometrics

From: Creating diversity in mammalian facial morphology: a review of potential developmental mechanisms

Clade Remarks on diversity of craniofacial morphology Landmark-based geometric morphometric studies
Monotremata All extant monotremes have a toothless bill covered by electro- and mechano-receptors. The platypus has a flat, widened, duck-like bill. Echidna bills are more pointed, slender compared to platypus bills None
Marsupialia The viscerocranium, which includes the early-ossifying bones of the oral region, is morphologically less diverse than in placentals. The level of disparity of late-ossifying neurocranium is equivalent with that in placentals. This suggests that the ossification of marsupial oral bones is more constrained compared to placentals [102,103,104,105]
Xenarthra
Cingulata Armadillo skulls are elongated anteroposteriorly and flattened dorsoventrally. The zygomatic arch is complete, differing from those of another xenarthran lineage, Pilosa. The dentary bone is thin and long. Variation in skull shape is only described in the family Pampatheriidae which is an extinct group of Cingulata. Skull shape is highly conserved among extant members [106]
Pilosa The suborder Forivora (sloths), which consists of Bradypodidae (three-toed sloth) and Megalonychidae (two-toed sloths), has a short, high skull with a strongly reduced rostrum. The zygomatic arch is robust but incomplete. The skulls of three-toed and two-toed sloths are distinct to one another according to morphometric analyses. Three-toed sloth skulls have a relatively shortened rostrum and no diastema. The suborder Vermilingua (anteaters) has a specialized skull for eating small insects; the skull is highly elongated and has no tooth. Its pointed rostrum encases a long tongue. The zygomatic arch is incomplete [107, 108]
Afrotheria
Tubulidentata Aardvark skulls are elongated anteroposteriorly, accompanied by long and slender dentary bones. The nasal bone is triangular in shape. The frontal bones expand dorsally in front of the orbit as a result of a highly developed nasal chamber. The zygomatic arch is complete but slender. There is no postorbital bar None
Macroscelidea Macroscelidea species have a tall, dome-shaped cranium. The zygomatic arch is complete. The rostrum is long. Macroscelidae consists of two subfamilies: Rhynchocyoninae and Macroscelidinae. Rhynchocyoninae species have a relatively large skull with nasal bones having partially ossified tips. The bony palate is not perforated. Macroscelidinae species have a relatively smaller skull and wholly cartilaginous nasal bone tips. The bony palate has some holes [109, 110]
Afrosoricida Afrosoricida consists of two families: Tenrecidae (tenrecs) and Chrysochloridae (golden moles). Tenrec skulls have a long, slender rostrum. The jugal bone is absent and the orbital bone is usually small. The skull of golden moles is abruptly conical, its anterior portion is pointed, and its posterior portion widened. The zygomatic arch is formed by an elongated process of the maxilla, and the occipital area contains the tabular bones, which are not typical in mammals. Tenrec skulls are less morphologically diverse than those of golden moles. It is suggested that the similarities in skull morphology among the speciose genus Microgale masks morphological diversity among the rest of the family [111]
Hyracoidea All four extant hyrax species have short skulls and deep dentary bones. The skull has a postorbital bar, which is sometime complete (Dendrohyrax) and sometime incomplete (Heterohyrax and Procavia) None
Proboscidea All extant elephant species (Loxodonta and Elephas) have short, tall skulls which are pneumatized particularly in the cranial roof, thereby reducing cranium weight. Skulls bears two tusks derived from the second incisors of the upper jaw [112]
Sirenia The skulls of Sirenia species are highly specialized for aquatic life, including adaptations such as deep dentary bones. Sirenia consists of two families: Dugongidae and Trichechidae. In Dugongidae skulls, the premaxilla bones are relatively larger, the nasal bones are absent, and the nasal cavity is shortened. In Trichechidae skulls, the premaxilla bones are small, the nasal bones are present, and the nasal cavity is elongated. Within Trichechidae, Trichechus inunguis is distinct in skull shape. The skull shape of T. senegalensis, T. manatus manatus, and T. m. latirostris are more similar to each other. Within T. manatus, geographic variations in skull morphology, perhaps caused by geographic isolation, are reported [113]
Laurasiatheria
Eulipotyphla Disparity in skull morphology among eulipotyphylans may be explained by phylogeny rather than ecology. In the genus Sorex, similarities and differences in skull shape between species are proportional to the phylogenic distance between them. Similarly, the degree of morphological variation in the dentary bone between talpid species corresponds to the phylogenetic distance between the species [114,115,116,117,118,119]
Perissodactyla Perissodactyla skulls are adapted to an herbivorous diet. Extant Perissodactyla consists of three families: Equidae, Tapiridae, and Rhinocerotidae, and all have a long skull with an elongated face and large cheek teeth adapted for grinding coarse vegetation. Equid skulls are generally flat in a mediolateral direction, with long, deep rostrums. The skulls of the Tapiridae have a well-developed sagittal crest, rostrally positioned orbital bones, and a small cranium with a reduced posterior region. Rhinocerotidae have a thickened, enlarged nasal bone which extends anteriorly beyond the anterior margin of the premaxilla bone. The occipital bone is unusually high where the neck muscles attach to sustain the heavy head None
Chiroptera Bat skulls are morphologically highly diverse. However, the degree of morphological disparity in skull shape is not the same among taxa. The family Pteropodidae, which lost the ability to echolocate, have large orbits accompanied with a well-developed postorbital bar. The rostrum is morphologically uniform despite variation in diet between species. The family Phyllostomidae shows a high level of variation in skull morphology explained by a diversity of diets. Nectarivorous species possess an elongated face while fruigivorous species have a shortened face. Skull morphology of the family Vespertilionidae is highly conserved, although it is the most speciose group in the order [82, 85, 86, 94, 120, 121]
Carnivora Carnivoran skulls are characterized by an expanded braincase in which the frontal-parietal suture is located posteriorly relative to the postorbital constriction, as well as fully or partially ossified ectotympanic bones that are firmly fused to the skull. Carnivoran skulls are highly varied corresponding to different diets. In general, felid species have a shorter rostrum for production of higher bite force, while canid species typically have a longer rostrum with a large nasal chamber associated with a well-developed olfactory sense. The pinnipeds, semiaquatic marine mammals, usually have a short rostrum, and enlarged orbits [53, 122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146]
Pholidota All extant pangolin species have a long, narrow, toothless skull. The dentary bone is narrow and slender as well. The surface of the cranium is smooth without any ridges or crests. The zygomatic arch is present but incomplete. The postorbital bar is absent None
Cetartiodactyla The skulls of Cetartiodactyla usually have a long rostral portion. The postorbital bar is always present. When horns are present, they are most often formed on the frontal bones. The extant Cetartiodactyla consists of the suborder Suina (pigs and peccaries), the infraorder Cetacea (whales), the infraorder Ancodonta (hippos), and the suborder Ruminantia (cows, goats giraffes, deers etc.). Suiforme skulls are distinct from those of other cetartiodactyls, having a posteriorly extended squamosal bone that contacts the exoccipital bone. Ancodontids have a tall skull with high-positioned orbits, enlarged as well as tusk-like canines and incisors. Ruminantids bear antlers or horns that are often large and complex in shape. The mastoid bone is exposed between the squamosal and occipital bones. Cetaceans have a highly modified skull caused by posterior migration of the nostrils. The premaxilla and maxilla bones form the roof of the rostrum. Enlarged occipital bones occupy the posterior part of the skull. The nasal and parietal bones are highly reduced in size [147,148,149,150,151,152,153,154,155,156,157]
Euarchontoglires
Scandentia Treeshrews have a unique, prominent hole in the zygomatic arch. The postorbital bar is well developed and contacts the zygomatic arch. There is variation in skull morphology within Tupaia glis that might be due to the geographic barriers between populations. For example, island populations have a smaller skull than continental ones [158,159,160]
Rodentia Rodent skulls are unique, bearing a single pair of persistently growing incisors in the upper and lower jaws. The orbital cavity is located dorsal to the cheek teeth. The zygomatic arch fuses to the maxilla in line with the first cheek teeth. The vertical ramus of the dentary bone is enlarged and provides the area for insertion of the masseter muscle. Rodentia consists of three suborders: Myomorpha, Sciuromorpha, Hystricomorpha. Myomorpha have enlarged temporal bones where a large temporal muscle attaches. The muscle produces high mastication power using cheek teeth. Sciuromorpha have a large vertical ramus of dentary bone where the masseter muscle attaches. This produces a high power in biting using incisors. Hystricomorpha have a large infraorbital foramen in their skull. Both phylogenetic and ecological factors influence the determination of skull morphology in rodents. In Hystricomorphids (e.g., guinea pigs, porcupines, and spiny rats), phylogenetic constraints are more important than ecological factors in generating morphological variation of the dentary bone. On the other hand, morphological variation of skulls is mainly brought about by ecological factors. Hystricomorphids living in open habitats, such as guinea pigs, have upward-facing orbits and a wide basicranium. Hystricomorphids living in woody areas, such as spiny rats, have more laterally facing orbits and a narrow basicranium [161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194]
Lagomorpha Rabbits have a fenestrated skull which is unique among mammals. The fenestration (lattice-like bone) is seen in the proximolateral part of the rostrum. Morphological disparity of skull morphology in the family Leporidae is mainly explained by differences in the degree of facial tilt among species [195,196,197,198]
Primates Skull morphology is very different between haplorhines and strepsirrhines, mainly in relative skull length and width and facial depth. Haplorhines tend to have a mediolaterally wide as well as dorsoventrally tall skull. Strepsirrhines have a narrower, shallower skull, an elongated face, and a narrower snout. Intraspecific variation in skull shape has been studied in several groups of primates, including Cercopithecoidea and Hominidae [199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214]
Dermoptera The two extant colugo species have skulls with large front-facing orbits that improve binocular vision. The position of three pairs of upper incisors is shifted laterally, and the second upper incisors are transformed into a canine-like shape. The first two lower incisors are broad and form a comb-like shape None