Palaeomastodon ("ancient mastodon") is an extinct genus of basal from the Oligocene of North Africa. The first specimen discovered was recovered from strata belonging to the Fayum fossil deposits of Egypt. It was described and named in 1901 by Charles Williams Andrews, who named its type species, P. beadnelli, after a colleague. Multiple species have been named since, though have either been reassigned to Phiomia or synonymised with P. beadnelli. Three (possible) unnamed taxa are known from Ethiopia and Libya. All remains are from strata that date to 33–27 million years ago.
Palaeomastodon was fairly large for an early proboscidean. It had an estimated shoulder height of , and a body mass of around . Similar to Phiomia, its nasal cavity was retracted and surrounded by strong muscle attachment sites, indicating that it was among the first proboscideans to possess a trunk. Like in modern elephants, the orbits (eye sockets) were positioned further back on the skull, and sat over the molars. The (lower jaw) mandible was very long, with a symphysis whose morphology suggests a long tongue was present. Like many extinct proboscideans, Palaeomastodon had two sets of tusks, one on the upper jaw and one on the lower jaws, formed from the second of the maxilla and mandible respectively.
Taxonomy
Early history
The first specimens of
Palaeomastodon were recovered from lower
Oligocene strata, part of the Fayum fossil deposits of
Egypt.
The first specimen to be discovered, consisting of a partial
mandible (lower jaw) with two
and three molars, was recovered from the Jebel Qatrani Formation,
formerly referred to as the "fluvio-marine formation".
It was described in 1901 by British palaeontologist Charles William Andrews, who named its
type species,
P. beadnelli, after his colleague, Hugh John Llewellyn Beadnell.
[Andrews, C.W. 1901. Über das Vorkommen von Proboscidiern in untertertiären Ablagerungen Aegyptens. Tageblatt des V Internationalen Zoologischen Kongresses, Berlin 6: 4–5.]
Other species
Four years after describing
P. beadnelli, Andrews named two additional
Palaeomastodon species:
P. parvus, based on a partial right mandible, bearing premolars and molars; and
P. wintoni, based on two near-complete, articulated mandibles, lacking only the
Angular bone and the anterior (front) right cheek teeth. Andrews noted a second mandible housed in Cairo, which he considered a "co-type".
In a 1922 revision of the genus' taxonomy, Japanese palaeontologist Matsumoto Hikoshichirō reassigned
P. minor and
P. wintoni to
Phiomia,
a genus which had been briefly synonymised with
Palaeomastodon.
In addition, he described a fourth species,
P. intermedius, based on a partial left
Mandible that bore all of the molars and parts of the last premolars. Three more specimens were also known, including a large skull fragment consisting mostly of the
palate.
In 2010, William J. Sanders et al. regarded all of the named species (except for
P. minor, regarded as a synonym of
Phiomia serridens) as junior synonyms of
P. beadnelli. They did, however, note the possible presence of three species, two from
Chilga, and one from Zella, Libya.
Classification
Palaeomastodon is the
type genus and namesake of the family Palaeomastodontidae.
Though
Phiomia has been occasionally assigned to the family, generally it is recovered in a different phylogenetic position, and likely belongs to a family of its own.
Writing in 1926, Henry Fairfield Osborn suggested that
Palaeomastodon was a direct, if remote, ancestor of
and
Mammutidae (which he referred to under the umbrella of "mastodonts").
In a 1988 paper discussing the taxonomy of proboscideans, Pascal Tassy suggested that
Palaeomastodon fell under the suborder
Elephantiformes, being phylogenetically closer to modern elephants than to taxa like
Deinotherium and
Moeritherium, though was still more basal than
Phiomia.
A 2021
Phylogenetics of basal proboscideans performed by Lionel Hautier et al. recovered similar results:
Palaeomastodon was within the suborder
Elephantiformes, though was basal to
Phiomia and
Elephantimorpha.
Below is a cladogram depicting the results of Hautier et al. (2021):
Description
Few postcranial remains from
Palaeomastodon are known. However, based on the reported length of one femur, a 2016 study estimated an adult shoulder height of , and a body mass of over .
A 2004 study estimated a weight of based on a long femur, while another, long femur was estimated at and a long ulna was estimated at .
Skull and dentition
Palaeomastodon's skull was similar in many regards to that of
Phiomia.
The
Nostril (nasal cavity) was retracted, and sat just in front of the orbits (eye sockets).
Around the naris were attachment sites for strong muscles, and together, these attributes suggest the presence of a small
Elephant, a precursor to that seen in later proboscideans.
This is contrary to Henry Fairfield Osborn's suggestion that
Palaeomastodon was trunkless, and instead had a large upper lip.
The orbits, too, had shited backwards, and sat above the molars as in modern elephants.
The
Occipital bone was high and the
Nuchal lines was very large, possibly to counterbalance the elongated mandible, tusks, and trunk.
Unlike later proboscideans,
Palaeomastodon had a
sagittal crest, and the
Neurocranium was small and fairly low.
The mandibular symphysis is extremely elongated,
with a dorsal surface that bears a deep and wide supra-symphyseal groove, indicating that
Palaeomastodon possessed a long tongue.
Palaeomastodon had a dental formula of .
Palaeoenvironment
The environment of the Jebel Qatrani Formation, from which
Palaeomastodon is known, 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
, early
, jacanas,
,
,
and
), 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
.
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.
Chronology
All specimens of
Palaeomastodon are known from the Oligocene, between 33-27
Year. The strata from which
P. beadnelli is known have been dated to ca. 33–30 Ma. The two Ethiopian taxa, which both come from the Chilga Formation, have both been dated to 28–27 Ma. The taxon from Libya, whose stratigraphic unit is unclear, has been tentatively dated to the early Oligocene, though no specific time frame has been given.
Notes
