Sespia: an oreodont with an antorbital fenestra

Sespia (Schultz and Falkenbach 1968, Late Oligocene, 25 mya, Fig 1) is a genus of four species (Fig 2) of basal oreodont in the large reptile tree (LRT, 2159 taxa). Genus members were cat-sized to goat-sized and known from thousands of specimens from California to Nebraska. Apparently few to none are close to complete.

Figure 1. Sespia ultima in several views from Schultz and Falkenbach 1968. Note the large antorbital fenestra (aof) here and in other species (Fig 2). Arrow points to the very loose jaws joint apparently not so loose in two other species (Fig 2). Note the very large tympanic bulla (dark yellow) and raised auditory meatus, as in extant horses (Fig 4) by convergence.

Sespia was allied with Leptauchenia
(Fig 3, Schultz and Falkenbach 1968). Leptauchenia has not yet been tested in the LRT.

From a recent master’s thesis:
“Consensus is, that the oreodonts were an early group of artiodactyls. Most evidence puts the oreodonts in the suborder Tylopoda (Wikipedia, 2012). The cranium proper of the oreodonts is similar to that of the llama and camel.”

Why rely on consensus? That indicates laziness, the inability or lack of interest in finding out for oneself by examining data, creating a matrix and recovering a cladogram. Relying n consensus indicates trust in what others are saying. That quickly turns into hearsay, then rumor then mythology. Try not to rely on consensus. Build your own LRT.

Figure 2. Four species of Sespia from Schultz and Falkenbac 1968. S californica is the smallest. Note the large canines in S ultima, indicating male. Compare to the female in figure 1.

In the LRT
oreodonts are not close to camels, nor do they nest within Artiodactyla. Rather they nest with Phenacodus, basal to mesonychids and hippos on one branch, Homalodotherium + Protypotherum and then artiodactyls on the other branch.

An antorbital fenestra is present.
That’s really odd for a mammal. Schultz and Falkenbach called it a nasal-facial vacuity. From their footnote: “The term “nasal-facial vacuity” is used to distinguish the area from a facial vacuity which does not invade the nasal bone and little, if any, of the frontal. The nasal-facial vacuity invades the maxilla, nasal, frontal, and lacrimal bones and also opens into the orbit through the usually solid anterior orbital wall.” And later in their text: “Also noteworthy is the large nasal-facial vacuity which extends posteriorly through the anterior wall of the orbit. This character is not present in other oreodonts, nor in mammals in general.”

So far, I haven’t found any hypotheses regarding the presence of this vacuity, but my first guess is a relationship with the raised orbit and raised external auditory meatus (= eye hole and ear hole). Because the eyes and nostrils were placed high on the head.

Figure 3. Leptauchenia decora in water and sand up to their sensory organs. Neither is appropriate. Likely it was just the larger jaw muscles needed for grazing teeth that migrated the sensory organs dorsally, as in convergent horses (figure 4).

According to Wikipedia,
“it was long assumed that Leptauchenia was an aquatic, or semi-aquatic animal.” In counterpoint, Prothero and Sanchez 2008 suggested leptauchenids were desert-dwellers. The high-placed eyes, nostrils and ears served to filter out sand while burrowing, or while digging themselves free of sand dunes since their fossils are found in sand dunes.” See figure 3 to see an illustration of three niche hypotheses. The third is proposed below.

Figure 1. Equus the extant horse.
Figure 4. Equus the extant horse with high-placed nose, eyes and ears.

In counter-counterpoint,
another animal with deep jaws below high-placed eyes and ears is the extant horse. Equus is another running herbivore with deeply rooted molars (compare to Sespia in figure 1). Perhaps it was the convegent need for larger jaw muscles in grazing leptauchenid oreodonts that was responsible for moving the sensory organs dorsally. Neither an aquatic nor a sand burrowing niche seems appropriate (Fig 3). Instead, like little horses, oreodonts appear to be running, grazing herbivores adapted to tough land plants filled with grit.

Figure 1. Pika skull (genus: Ochotona) in three views.
Figure 5. Pika skull (genus: Ochotona) in three views. Note a similar antorbital fenestra.

Why did an antorbital fenestra develop
in Sespia (Fig 1) and Leptauchenia, distinct from almost all other mammals? The extant pika (Ochotona, Fig 5) has something similar by convergence. All ideas are welcome.

References
Prothero DR and Sanchez F 2008. Systematics of the Leptaucheniine oreodonts (Mammalia: Artiodactylia) from the Oligocene and earliest Mioceneof North America. In: Lucas et al., eds., 2008, Neogene Mammals. New Mexico Museum of Natural History and Science Bulletin 44.
Schultz CB and Falkenbach CH 1968. The phylogeny of the oreodonts, Parts 1 and 2. Bulletin of the American Museum of Natural History 139:498pp.

wiki/Sespia
wiki/Leptauchenia
wiki/Merycoidodontoidea = Oreodonta

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