Phiomia (after the Ancient Greek phiom "lake", an ancient name for the Fayum),
Phiomia was fairly small in terms of body size, with an estimated shoulder height in the case of P. serridens. In some regards it resembled Palaeomastodon, though was less basal and bore similarities to Gomphothere, to the point where it was briefly considered their ancestor. A retracted Nostril (nasal cavity) with strong muscle attachment sites, long snout and protruding mandible all suggest that Phiomia was among the first proboscideans to possess a true Elephant. Both the upper jaw and mandibles were tusked, with those of the upper jaw being thin, recurved and blade-like, while those of the mandibles were flat, straight and broad.
Taxonomy
Early history
The type specimen of
Phiomia, a partial left
mandible (lower jaw), was recovered from strata belonging to the Jebel Qatrani Formation, part of the Eocene-age Fayum fossil deposits of Egypt.
In 1902, the mandible was described by Charles William Andrews and Hugh John Llewellyn Beadnell, as part of a paper naming several mammal genera from the Fayum. They assigned to it the binomial name of
Phiomia serridens,
after the locality of its discovery (the name "Fayum" derives from the Greek
phiom, meaning "lake")
and its serrated lower
Incisor.
Andrews and Beadnell were uncertain of
Phiomia's relationships, though, believing it to have been carnivorous, tentatively suggested that it was a highly specialised member of the now-disused order
Creodonta.
Following a review by Max Schlosser,
in 1906, Andrews re-examined
P. serridens, and suggested a closer relationship to
Palaeomastodon, an early
Proboscidea. He tentatively suggested that it may have represented a smaller species of that genus, or that it belonged, at the very least, to the same family,
Palaeomastodon.
In 1908, Andrews wrote a paper on
Palaeomastodon. By that point, he had become unconvinced that
Phiomia could be reliably separated from it, and suggested due to its small body size that its holotype belonged to the same taxon as
Palaeomastodon minor. Thus, he formally synonymised the two.
Validity and internal systematics
The proposed synonymy between
Phiomia and
Palaeomastodon lasted only fourteen years. In 1922, Matsumoto Hikoshichirō once again separated the two, based on various characteristics of the skull and teeth. Notably, the morphology of the two genera's
cheek teeth (the
Premolar and molars) were different, with those of
Palaeomastodon being (having inner cusps that are blunt cones, and outer cusps modified into ridges) and those of
Phiomia being conventionally bunodont. Furthermore, based on a second mandible from the Jebel Qatrani, Matsumoto described a second
Phiomia species,
P. osborni, and reassigned two species of
Palaeomastodon, thus adding
P. minor and
P. wintoni to
Phiomia.
An additional
Phiomia species,
P. major, was described in 2004 by William J. Sanders, John Kappelman, and D. Tab Rasmussen, based on teeth from the
Chilga district of Ethiopia. In the paper describing it, the authors synonymised all of Matsumoto's taxa with
P. serridens, thus reducing
Phiomia to just two species.
Subsequent authors have maintained this synonymy.
Classification
Since its reclassification as a proboscidean,
Phiomia has always been regarded as a fairly basal member of the order. Henry Fairfield Osborn believed that it was an early member of a long-jawed
Mammutidae lineage, and that it was directly ancestral to
Trilophodon (now a synonym of
Gomphotherium).
In Pascal Tassy's 1988 paper on the phylogeny of proboscideans, wherein he erected the suborder
Elephantiformes, he classified
Phiomia as a basal member of the group, intermediate between
Palaeomastodon and the later
Hemimastodon.
In 1992, the family Phiomiidae was erected to encompass
Phiomia.
A phylogenetic analysis of basal proboscideans was performed by Lionel Hautier et al. (2021), which recovered
Phiomia in a similar position.
Below is a cladogram depicting the results of Hautier et al. (2021):
Description
Phiomia serridens is estimated to have had a shoulder height of .
seems to have been larger, though by how much is uncertain due to its nature as a taxon known solely from teeth.
Skull and dentition
The skull of
Phiomia is similar to that of
Palaeomastodon.
It was fairly long in comparison to its width, more so than in that genus, though not to the extent of gomphotheres like
Gomphotherium and
Megabelodon.
The
Nostril (nasal cavity) was retracted, to the extent where its topped just before the orbits.
That it is surrounded by strong muscle attachment sites suggests the presence of a small
Elephant.
's
Palatine bone was fairly narrow, while that of
Palaeomastodon was wider.
The symphysis of the mandible was very elongated, though again, not to the extent of the aforementioned gomphotheres;
both genera were among the first proboscideans to meaningfully elongate the mandibular symphysis and tusks.
Like
Palaeomastodon,
Phiomia had a high
Occipital bone with a large
Nuchal lines, possibly to counterbalance the weight of the tusks, elongated mandible, and trunk.
Phiomia had a dental formula of .
Palaeoenvironment
The environment of the Jebel Qatrani Formation, from which
Phiomia serridens originates, has been described as a
subtropical to
tropical lowland plain by Bown, who further suggests the presence of streams and ponds.
Based on the occurrence of birds that are associated with water (such as
Osprey, early
Flamingo, jacanas,
Heron,
Stork,
Cormorant and
Shoebill), Rasmussen and colleagues inferred that the environment featured slow-moving freshwater with a substantial amount of aquatic vegetation, which matches the prior hypothesis. Although
lithology suggests that most fossils were deposited on sandbanks after being transported by currents, the authors argue that
swamps could have easily formed along the banks of the river that was present during the Oligocene and may account for the mudstone found in certain quarries. They furthermore suggest that the fossil birds of Fayum, due to their affinities with modern groups, should be considered a more valuable indicator of the environment when compared with the fossil mammals, many of which belonged to families lacking modern examples. The absence of other birds typical for such an environment may be explained either through sampling bias or due to the fact that said groups had simply not yet been present in Oligocene Africa. Generally, Rasmussen and colleagues compare the environment of Jebel Qatrani to freshwater habitats in modern Central Africa.
The discovery of snakehead fossils seem to support Rasmussen's interpretation, as the genus
Parachanna today prefers slow-moving backwaters with plenty of vegetation. Other fish present meanwhile, notably
Tylochromis, suggest that deep, open water was likewise present. The river channels may have been overgrown with reeds,
Cyperus papyrus and featured floating vegetation like
Nymphaeaceae and
Salvinia.
In a 2001 paper Rasmussen et al
. argued that the sandstone and mudstone of the formation likely formed as sediments were
Aggradation by a system of river channels that emptied towards the west into the
Tethys Ocean. Here they reconstructed the environment as a tropical lowland swamp forest intermingled with marshes. They furthermore suggest that the environment would have experienced
Monsoon.
Overall this indicates that this region was a part of an extensive belt of tropical forest that stretched across what is now northern Africa, which would gradually give rise to open woodland and even steppe the further one was to travel inland.
Notes
