''Tutcetus'' is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of diminutive

basilosaurid Basilosauridae is a family of extinct cetaceans that lived during the middle to late Eocene. Basilosaurids are known from all continents including Antarctica, and are probably the first fully aquatic cetaceans.Buono M, Fordyce R.E., Marx F.G., F ...

cetacean Cetacea (; , ) is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively c ...

from the

Bartonian The Bartonian is, in the International Commission on Stratigraphy's (ICS) geologic time scale, a stage or age in the middle of the Eocene Epoch or Series. The Bartonian Age spans the time between . It is preceded by the Lutetian and is follow ...

of Egypt. ''Tutcetus'', named after the child

pharaoh Pharaoh (, ; Egyptian language, Egyptian: ''wikt:pr ꜥꜣ, pr ꜥꜣ''; Meroitic language, Meroitic: 𐦲𐦤𐦧, ; Biblical Hebrew: ''Parʿō'') was the title of the monarch of ancient Egypt from the First Dynasty of Egypt, First Dynasty ( ...

Tutankhamun Tutankhamun or Tutankhamen, (; ), was an Egyptian pharaoh who ruled during the late Eighteenth Dynasty of Egypt, Eighteenth Dynasty of ancient Egypt. Born Tutankhaten, he instituted the restoration of the traditional polytheistic form of an ...

, is both one of the oldest known basilosaurids from

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

and the smallest member of the family. It is suggested that the type specimen, a subadult close to maturity, only measured approximately long. The genus is

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...

, only including the species ''T. rayanensis''.

History and naming

''Tutcetus'' was first described in August 2023 on the basis of a single specimen, MUVP 501. The specimen consists of an incomplete skull found alongside both

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

s, the

hyoid The hyoid-bone (lingual-bone or tongue-bone) () is a horseshoe-shaped bone situated in the anterior midline of the neck between the chin and the thyroid-cartilage. At rest, it lies between the base of the mandible and the third cervical verte ...

, and some of the first

vertebrae Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

of the neck. The remains stem from the Fayum Depression of

Egypt Egypt ( , ), officially the Arab Republic of Egypt, is a country spanning the Northeast Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via the Sinai Peninsula. It is bordered by the Mediterranean Sea to northe ...

, which is well known for its rich record of early whales. More specifically, the bones of ''Tutcetus'' stem from the

Sath El-Hadid Formation The Sath El-Hadid Formation, translating to "Iron Surface" in Arabic "سطح الحديد", is a geological formation in Egypt characterized by a nummulitic limestone bank containing large and small ''Nummulites''. Introduced into the stratigraphy ...

, which dates to the early

(ca. 41 mya). The name ''Tutcetus'' is a reference to the pharaoh

, also known as King Tut, a child king who died when he was only 18 years old. The name was chosen to reflect the small size and young age of ''Tutcetus''. The second part of the generic name is the

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

word for whale, "cetus". The species name meanwhile means "from the Wadi El-Rayan Area" in reference to the type locality.

Description

Most of the sides of the

nasal bone The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose. Eac ...

s is in contact with the

maxillae In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxillar ...

, with only a small section towards the front of the nasals contacting the

premaxillae The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals has ...

. The upper and lower

premolar The premolars, also called premolar Tooth (human), teeth, or bicuspids, are transitional teeth located between the Canine tooth, canine and Molar (tooth), molar teeth. In humans, there are two premolars per dental terminology#Quadrant, quadrant in ...

s of ''Tutcetus'' differ from those of all other basilosaurids in the number of accessory tooth cusps. ''Tutcetus'' features two mesial accessory cusps (directed towards the front of the jaw) and three distal accessory cusps (directed towards the back of the jaw). The premolars also differ in some other aspects. They are notably more gracile than those of other basilosaurids and the

tooth enamel Tooth enamel is one of the four major Tissue (biology), tissues that make up the tooth in humans and many animals, including some species of fish. It makes up the normally visible part of the tooth, covering the Crown (tooth), crown. The other ...

is much smoother. The fourth premolar appears to have been the largest tooth in both the upper and lower jaw. The first premolar does not appear to have had a replacement, which suggests one of two things. Either the

deciduous In the fields of horticulture and botany, the term deciduous () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed Leaf, leaves, usually in the autumn; to the shedding of petals, aft ...

first premolar is retained even into adulthood, or the first premolar is developed without having a temporary precursor.

Size

''Tutcetus'' may have been the smallest known basilosaurid with an estimated length of and an estimated weight of . Although maturity is difficult to determine in fossil, Antar and colleagues note several factors observed in ''Tutcetus'' that give clues to the animal's age at the time of its death . These factors include both the stage of the tooth eruption and the fusion of the bones, which are all more advanced than those of the oldest known ''

Dorudon ''Dorudon'' ("spear-tooth") is a genus of extinct basilosaurid ancient whales that lived alongside ''Basilosaurus'' 41.03 to 33.9 million years ago in the Eocene. It was a small whale, with ''D. atrox'' measuring long and weighing . ''Dorudon'' ...

'' juvenile and suggest that the holotype of ''Tutcetus'' was an advanced subadult at the verge of adulthood.

Classification

According to the Bayesian tip-dating analysis conducted for ''Tutcetus'', Basilosauridae was a

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

clade that included both traditional basilosaurids as well as all modern whale groups (

Mysticeti Baleen whales (), also known as whalebone whales, are marine mammals of the parvorder Mysticeti in the infraorder Cetacea (whales, dolphins and porpoises), which use baleen plates (or "whalebone") in their mouths to sieve plankton from the wate ...

and

Odontoceti The toothed whales (also called odontocetes, systematic name Odontoceti) are a parvorder of cetaceans that includes dolphins, porpoises, and all other whales with teeth, such as beaked whales and the sperm whales. 73 species of toothed whales ar ...

). Among core basilosaurids, ''Tutcetus'' was recovered within a weakly-supported group of early diverging, middle Eocene whales that also included ''

Chrysocetus ''Chrysocetus'' is a genus of extinct Archaeoceti, archaeocete ceteceans belonging to the Basilosauridae that is known from the Late Eocene of the eastern United States and western Africa. Across its range two species are known, the American '' ...

'' from North America and Africa as well as ''

Ocucajea ''Ocucajea'' is an extinct genus of basilosaurid cetacean from Middle Eocene (Bartonian stage) deposits of southern Peru. ''Ocucajea'' is known from the holotype MUSM 1442, a partial skeleton. It was collected in the Archaeocete Valley site ...

'' from South America. Within this group, termed the ''Tutcetus''-clade by Antar ''et al.'', ''Tutcetus'' was found to be most closely related to ''Ocucajea'', with ''Chrysocetus'' having diverged before the split between the other two.

Paleobiology

Mohamed Antar, Abdullah Gohar, Hesham Sallam and colleagues note that, beyond determining the age at which the animal died, the pattern of tooth replacement seen in ''Tutcetus'' may also give additional clues to how the animal lived. ''Tutcetus'' had molar teeth that erupted rather early, suggesting that the switch from deciduous to permanent teeth occurred rapidly in members of this species. According to Antar ''et al.'', rapid onset of tooth replacement is associated with animals that mature quickly and don't grow exceptionally old. They argue that the inferred low lifespan of ''Tutcetus'', combined with its diminutive size, indicate that it was a

precocial Precocial species in birds and mammals are those in which the young are relatively mature and mobile from the moment of birth or hatching. They are normally nidifugous, meaning that they leave the nest shortly after birth or hatching. Altricial ...

animal that grew quicker and died younger than the larger basilosaurids of its time. The deposits in which ''Tutcetus'' was found in match the type of warm, shallow, tropical waters that whales will seek out as calving grounds, with authors suggesting that the locality represents just that. Antar and colleagues note that the stage of tooth replacement during which the holotype specimen of ''Tutcetus'' died might suggest that the species had a relatively younger infant mortality rate compared to the much better sampled ''Dorudon''. They further highlight how such patterns would be expected in species that give birth every year to a single young. However, they also point out how this hypothesis cannot be tested until more material of ''Tutcetus'' is found and described. Another aspect of ''Tutcetus biology briefly explored in the type description is the relation between its environment and its small body size. While the small size of ''Tutcetus'' could be simply a left over of its smaller protocetid ancestry, it is also possible that the small size of it could be tied directly during the period of global warming known as the Lutetian thermal maximum. As aging and mortality are more prominent in warmer conditions, the small size and early maturing of ''Tutcetus'' could have been adaptations to these warmer conditions, allowing for the animal to reproduce more quickly. On the flipside, the large sizes of later basilosaurids such as ''Basilosaurus'' itself may have been driven by the middle Eocene climatic optimum or the brief cooling period between the Lutetian thermal maximum and the middle Eocene climatic optimum. Antar and colleagues conclude that body size in these early whales may have been primarily driven by climate, whereas body shape would have been mostly changed in response to competition.

References