The sea cows (Order Sirenia, infraclass Eutheria) are represented at the present day by four species, three of which are known as manatees and live along the coastal Atlantic and in associated rivers, and one of which is known as the dugong, which lives along the coastlines of the Indian and Pacific oceans (Encyclopaedia Britannica).

The phylogenetic tree presented in Figure 4 of Heritage and Seiffert (2022) is used as the basis for the following time tree:
​Figure 1. Time tree of the stem-Sirenia
Two fossils, both of Middle Eocene (Early Lutetian) age, represent the oldest known members of the stem-Sirenia (Heritage and Seiffert, 2022):
  1. An un-named sirenian, represented by a petrosal (the dense part of the temporal bone at the base of the skull, surrounding the inner ear), from Chambi, Tunisia (specimen number CBI-1-542 of Benoit et al, 2013).
  2. Libysiren sickenbergi, described from the locality of Bu el Haderait in in north-central Libya (Domning et al, 2017).

​The Chambi specimen is illustrated below, together with other stem-group fossils for which images are available in the public domain (click on image for a larger view):
Names in   red indicate   that the fossil is younger  than the oldest known crown-group fossil.
​​​Figure 2. Images of stem-Sirenia
​The above images are ordered from most basal to most crownward, and a general trend can be seen from four-legged animals (e.g. Pezosiren portelli, a member of the extinct family Prorastomidae) that are thought to have been amphibious (Díaz-Berenguer et al, 2020) to a body form with no hind legs and a whale-like tail (e.g. Halitherium schinzii). This trend illustrates the transition from a terrestrial to an aquatic lifestyle. The Sirenia are one of several mammalian lineages (other examples are whales, seals and sea otters) that have independently made that adaptation (Díaz-Berenguer et al, 2020).

The oldest known crown-group sirenian is Priscosiren atlantica, a member of the stem-Dugongidae described from the Early Oligocene San Sebastian Formation at the LACM Location 8060 on the Río Guatemala in Puerto Rico (Vélez-Juarbe and Domning, 2014). No public-domain image of this species is available.

The sirenian stem line includes a ghost lineage (shown as a blue bar in Figure 1), because the oldest known total-group elephant (of Early Eocene age; see page on the elephants) is at least 8.2 million years older than the stem-group Sirenia, which are of Middle Eocene and younger age. Given that the two stem groups must have appeared at the same time, the sea cow stem group transition must also have begun in the Early Eocene. This implies a stem-to-crown transition of between 22 and 31 million years, from Early Eocene to Oligocene time.

References

Benoit, J., Adnet, S., El Mabrouk, E., Khayati, H., Ben Haj Ali, M., Marivaux, L., ... & Tabuce, R. (2013). Cranial remain from Tunisia provides new clues for the origin and evolution of Sirenia (Mammalia, Afrotheria) in Africa. PLoS One, 8(1), e54307.

Díaz-Berenguer, E., Houssaye, A., Badiola, A., & Canudo, J. I. (2020). The hind limbs of Sobrarbesiren cardieli (Eocene, northeastern Spain) and new insights into the locomotion capabilities of the quadrupedal sirenians. Journal of Mammalian Evolution, 27, 649-675.

Domning, D. P., Heal, G. J., & Sorbi, S. (2017). Libysiren sickenbergi, gen. et sp. nov.: a new sirenian (Mammalia, Protosirenidae) from the middle Eocene of Libya. Journal of Vertebrate Paleontology, 37(2), e1299158.

Heritage, S., & Seiffert, E. R. (2022). Total evidence time-scaled phylogenetic and biogeographic models for the evolution of sea cows (Sirenia, Afrotheria). PeerJ, 10, e13886.

Vélez-Juarbe, J., & Domning, D. P. (2014). Fossil Sirenia of the West Atlantic and Caribbean region: X. Priscosiren atlantica, gen. et sp. nov. Journal of Vertebrate Paleontology, 34(4), 951-964.
Image credits – stem-Sirenia
  • Header (Dugong - Dugong dugon - in Toba Aquarium, Mie prefecture, Japan): pelican from Tokyo, Japan, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
  • Figure 2 (Chambi sirenian CBI-1-542 and Prorastomus): From Open Access article Benoit, J., Adnet, S., El Mabrouk, E., Khayati, H., Ben Haj Ali, M., Marivaux, L., ... & Tabuce, R. (2013). Cranial remain from Tunisia provides new clues for the origin and evolution of Sirenia (Mammalia, Afrotheria) in Africa. PLoS One, 8(1), e54307.
  • Figure 2 (Prorastomus sirenoides): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 2 (Pezosiren portelli, fossil): Photographed by Bob James (owner of website) at National Museum of Nature and Science, Tokyo, Japan, April 2023.
  • Figure 2 (Pezosiren portelli, life restoration): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
  • Figure 2 (Prototherium veronense, fossil): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Prototherium veronense, life restoration): Pinzid, Public domain, via Wikimedia Commons
  • Figure 2 (Sobrarbesiren cardieli, skeletal reconstruction): Díaz-Berenguer, Ester; Badiola, Ainara; Moreno-Azanza, Miguel; Canudo, José Ignacio, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
  • Figure 2 (Sobrarbesiren cardieli, life restoration): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
  • Figure 2 (Protosiren sp.): Conty, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Eotheroides sp.): Eduard Solà, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
  • Figure 2 (Eosiren sp., skeletal reconstruction): Conty, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Eosiren libyca): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Eotheroides sandersi): Conty, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Halitherium sp.): Ra'ike (see also: de:Benutzer:Ra'ike), CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Halitherium schinzii): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 2 (Kaupitherium gruelli): Szilas, Own work, Public domain, via Wikimedia Commons
  • Figure 2 (Miosiren kocki): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Lentiarenium cristolii): Manja Voss, Björn Berning & Erich Reiter, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons