Protosiren smithae

Overview

Protosiren smithae was an early sea cow from Eocene Egypt. It had small hind limbs as well as forelimbs. Its body, retaining traces of four legs, reveals part of the transition of sea cows toward aquatic life.^[1][2]

Key facts

Having hind limbs did not necessarily mean walking well on land. Protosiren’s limbs are interpreted as having been poorly suited to supporting its full weight on land. This animal shows that the presence of limbs and their use are different questions.^[2]

Time and place

It is associated with the end of the middle Eocene to the beginning of the late Eocene, approximately 40–36 million years ago. Fossils came from marine sediments at Wadi Al-Hitan in present-day Egypt. The same region also preserves records of early whales and other sea cows.^[1][4]

The time range describes source context, not the complete duration of this species.

Size, form and skeleton

CGM42292 preserves the skull and lower jaw, vertebrae, shoulder and pelvis, and forelimb and hind limb bones. The proportions of the forelimbs can be compared with those of the hind limbs, which are small relative to the trunk. This skeleton helps explore how limbs changed with aquatic life.^[1][2]

The type specimen of Protosiren smithae has a lower jaw 22.2 cm long. Forelimbs, hind limbs, and vertebrae were found alongside the jaw, allowing several regions of this early marine sirenian’s body to be examined. This measurement is the lower jaw length of one individual.^[6]

Habitat and reproduction

It is interpreted as a sea cow that lived mainly in water. Research comparing its humerus with those of other Eocene sea cows examined similarities in forelimbs used for feeding and movement. It invites consideration of limbs serving roles different from weight support on land.^[5][2]

Fossil discovery

Domning and Gingerich introduced it as a new sea cow species from Wadi Al-Hitan in 1994. In 2012, its forelimb bones were compared again with other sea cows from the same region. A 2018 study of early sea cows continued discussion of shoulder and limb features.^[1][5][3]

References

1. Domning & Gingerich1994 — Discovery of Protosiren smithae
2. Sirenia — Sea cow skeletons and evolution
3. Díaz-Berenguer et al.2018 — Comparing skeletons of early sea cows
4. International Commission on Stratigraphy — Geological timescale
5. Zalmout & Gingerich2012 — Late Eocene sirenians from Egypt
6. Zalmout·Gingerich (2012) — Eocene sirenians from Wadi al-Hitan, Egypt
7. Díaz-Berenguer et al.2022 — The skull of Sobrarbesiren and sea cow evolution

Research questions

When and in what sequence did sea cow hind limbs become smaller? Pelves and limbs of several early sea cows are compared together. Protosiren shows considerable adaptation to aquatic life even at a stage when legs remained.^[2][3]

Observation

Find the small hind limbs beneath the trunk and compare their size with the forelimbs. Go beyond counting legs to consider what roles they may have played underwater.^[1][2]