Pachycetinae
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Pachycetinae is an extinct subfamily of
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 ...
s that lived during the middle
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
. The best-dated remains stem from
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 ...
strata, but some finds suggest that they could have first appeared during the
Lutetian The Lutetian is, in the geologic timescale, a stage (stratigraphy), stage or age (geology), age in the Eocene. It spans the time between . The Lutetian is preceded by the Ypresian and is followed by the Bartonian. Together with the Bartonian it ...
and may have survived until the
Priabonian The Priabonian is, in the ICS's geologic timescale, the latest age or the upper stage of the Eocene Epoch or Series. It spans the time between . The Priabonian is preceded by the Bartonian and is followed by the Rupelian, the lowest stage ...
. Fossils of pachycetines are chiefly known from the southern
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
,
Ukraine Ukraine is a country in Eastern Europe. It is the List of European countries by area, second-largest country in Europe after Russia, which Russia–Ukraine border, borders it to the east and northeast. Ukraine also borders Belarus to the nor ...
,
Morocco Morocco, officially the Kingdom of Morocco, is a country in the Maghreb region of North Africa. It has coastlines on the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to Algeria–Morocc ...
and
Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...
, among others. They differ from other basilosaurids in having pachyostotic and osteosclerotic
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 ...
and ribs, making them denser and heavier by comparison. Based on this it has been suggested that these whales lived in shallow waters and that these thickened bones act as a buoyancy control as seen in
sirenians The Sirenia (), commonly referred to as sea cows or sirenians, are an order (biology), order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The extant Sirenia comprise tw ...
. Analysis of the teeth suggests that pachycetines had a varying diet, with the robust teeth of the larger ''Pachycetus'' indicating that it possibly fed on sharks, whereas the more gracile teeth of ''
Antaecetus ''Antaecetus'' is an extinct genus of pachycetine basilosaurid from the middle Eocene Aridal Formation of Morocco as well as the Fayum, Egypt. ''Antaecetus'', although known from fewer remains in total, is markedly more complete than the closely ...
'' suggest a diet of smaller prey items. The clade currently only includes two genera, ''Antaecetus'' and ''
Pachycetus ''Pachycetus'' (meaning "thick whale") is an extinct genus of Pachycetinae, pachycetine basilosaurid from Middle Eocene of the eastern United States (North Carolina & Virginia) and Europe (chiefly Germany and Ukraine). The best known remains ge ...
'', but a 2023 study suggests that the Peruvian ''
Supayacetus ''Supayacetus'' is an extinct genus of basilosaurid cetacean from the Middle Eocene (Bartonian stage) Paracas Formation of Peru. It has been noted for its relatively small size and basal morphology, with the sternum bearing close resemblance to t ...
'' may at least be a close relative.


History of discovery

Pachycetines have a long history of research on a genus level, even though the family itself was only named in the early 2020s. This is primarily due to the taxonomic confusion and nomenclatural changes surrounding the genera ''Platyosphys'' and ''Pachycetus'', both of which trace their origins to fossils discovered during the late 19th century. The earliest finds were recovered from Ukraine and named ''Zeuglodon paulsonii'' by
Johann Friedrich Brandt Johann Friedrich von Brandt (25 May 1802 – 15 July 1879) was a German-Russian natural history, naturalist, who worked mostly in Russia. Brandt was born in Jüterbog and educated at a Gymnasium (school), gymnasium in Wittenberg and the Humboldt ...
. Around the same time,
Harry Govier Seeley Harry Govier Seeley (18 February 1839 – 8 January 1909) was a British paleontologist. Early life Seeley was born in London on 18 February 1839, the second son of Richard Hovill Seeley, a goldsmith, and his second wife Mary Govier. When his fa ...
named poorly preserved remains from the Barton Clay of
Great Britain Great Britain is an island in the North Atlantic Ocean off the north-west coast of continental Europe, consisting of the countries England, Scotland, and Wales. With an area of , it is the largest of the British Isles, the List of European ...
''Zeuglodon wanklyni'' while
Pierre-Joseph Van Beneden Pierre-Joseph Van Beneden FRS FRSE FGS FZS (19 December 1809 – 8 January 1894) was a Belgian zoologist and paleontologist. He has been credited with introducing the terms " mutualism" and "commensalism" into biology in 1875 and 1876 respectively ...
coined the name ''Pachycetus'' for fossil remains from
Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...
which he thought belonged to a
mysticete 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 water ...
. Recognizing at least two morphotypes, van Beneden suggested that they belonged to two distinct but related species which he named ''Pachycetus robustus'' and ''Pachycetus humilis''. The German remains were eventually recognized as having belonged to archaeocetes. However,
Oskar Kuhn Oskar Kuhn (7 March 1908, Munich – 1 May 1990) was a German palaeontologist. Life and career Kuhn was educated in Dinkelsbühl and Bamberg and then studied natural science, specialising in geology and paleontology, at the University of Mun ...
rejected the idea that they represented a distinct genus, instead drawing comparisons to ''Zeuglodon isis'' (''Basilosaurus'') and ''Zeuglodon osiris'' (''Saghacetus''). Only a year later, in 1936,
Remington Kellogg Arthur Remington Kellogg (5 October 1892 – 8 May 1969) was an American naturalist and a director of the United States National Museum. His work focused on marine mammals. Early life and career Kellogg was born in Davenport, Iowa, and quickly ...
published an extensive
monograph A monograph is generally a long-form work on one (usually scholarly) subject, or one aspect of a subject, typically created by a single author or artist (or, sometimes, by two or more authors). Traditionally it is in written form and published a ...
on fossil archaeocetes, including the ''"Zeuglodon"'' remains from Ukraine. In this publication, he argued for the fossil material to be distinct on a genus level, coining the name ''Platyosphys''. Things largely went quiet around both the German and the Ukrainian remains in the following 70 years, with ''Pachycetus'' slipping into obscurity and ''Platyosphys'' likewise receiving little attention. Research somewhat picked up again towards the end of the 20th century and the early 2000s. In 1999
Mark D. Uhen Mark may refer to: In the Bible * Mark the Evangelist (5–68), traditionally ascribed author of the Gospel of Mark * Gospel of Mark, one of the four canonical gospels and one of the three synoptic gospels Currencies * Mark (currency), a currenc ...
described ''Eocetus wardii'' based on cranial and postcranial remains from the eastern
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
, believing that the material represented a type of
protocetid Protocetidae, the protocetids, form a paraphyletic, diverse and heterogeneous group of extinct cetaceans known from Asia, Europe, Africa, South America, and North America. Description There were many genera, and some of these are very well kno ...
, archaeocetes even more basal than basilosaurids, although subsequent research repeatedly disagreed on this matter, arguing that the fossils could not be assigned to ''Eocetus''. In 2001 more material from Europe was described, this time under the name ''Platyosphys einori''. With more material coming out of Ukraine, the status of ''Platyosphys'' was reexamined in a more critical light. With the holotype presumably lost sometime during
WW2 World War II or the Second World War (1 September 1939 – 2 September 1945) was a global conflict between two coalitions: the Allies and the Axis powers. Nearly all of the world's countries participated, with many nations mobilising ...
, Pavel Gol'din and Evgenij Zvonok argued that ''Platyosphys'' should be considered a
nomen dubium In binomial nomenclature, a ''nomen dubium'' (Latin for "doubtful name", plural ''nomina dubia'') is a scientific name that is of unknown or doubtful application. Zoology In case of a ''nomen dubium,'' it may be impossible to determine whether a ...
given the lack of material to compare more recent finds with. To compensate for this, they erected the genus ''Basilotritus'' to include much of the ''Platyosphys'' material previously discovered. At the same time, they noted that Uhen's ''"Eocetus" wardii'' was remarkably similar, referring the species to ''Basilotritus'' as well. ''Platyosphys einori'' on the other hand was dismissed as being too fragmentary and undiagnostic to be referred to anything more specific than the family Basilosauridae. While this would have provided a more accessible basis for future referrals, the proposition was not unanimously accepted by other researchers, with some later publications highlighting the fact that a missing holotype alone is not enough reason to disregard the use of ''Platyosphys''. Supporters of this line of thinking argue that the illustrations provided by Brandt and later authors are sufficient in diagnosing and comparing the material to more recent finds, therefore maintaining the validity of ''Platyosphys''. Two of the researchers remarking on the matter, Phillip D. Gingerich and Samir Zhouri, subsequently question the validity of ''P. uheni'' (alongside that of the poorly preserved ''P. einori'') while naming a species of their own: ''Platyosphys aithai''. The name ''Pachycetus'' eventually returned to prominence in 2020, when Henk Jan van Vliet and his colleagues noticed the similarities between the German remains and those of ''Platyosphys'', lumping the two genera into a single taxon. This effectively left them with five species: ''P. paulsonii'', ''P. robustus'', ''P. wardii'', ''P. uheni'' and ''P. aithai''. ''Pachycetus paulsonii'' became a new combination and was recognized as the
type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...
, combining the older species' name with the older genus name following their respective priority. While the idea that both ''P. paulsonii'' and ''P. robustus'' could have been entirely synonymous was entertained, the limited material available for the latter effectively rendered a thorough comparison impossible. For this reason, ''P. robustus'' was retained, if only for a single vertebra and rib fragment. ''P. uheni'', ''P. aithai'' and ''P. wardii'' were carried over as per the works of Gol'din and Zvonok as well as Gingerich and Zhouri. ''P. einori'', whose validity had already come to be questioned by Gol'din and Zvonok as well as Gingerich and Zhouri, was not mentioned in their work. The most recent changes made to the taxonomy of pachycetines came in 2022 with another paper authored by Gingerich. In it, both ''Pachycetus robustus'' and ''Pachycetus uheni'' were fully integrated into ''Pachycetus paulsonii'', meaning that all material from continental Europe was now placed within a single species. The North African remains, previously named ''Pachycetus aithai'', were split off and assigned to their own genus based on the discovery of additional remains. With this new genus, dubbed ''Antaecetus'', clearly distinct from yet also similar to ''Pachycetus'' proper, the team further went on to create a new
subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end botanical subfamily names with "-oideae", and zo ...
to house the two genera. This led to the establishment of Pachycetinae. Finally, Seeley's remains of ''"Zeuglodon" wanklyni'' were hypothesized to either represent an additional third species of ''Pachycetus'' or simply another instance of ''P. paulsonii''.


Species

Across the long research history, a variety of other species have been named, but few are still regarded as valid species. ''Pachycetus humilis'', the second of the two original ''Pachycetus'' species, is now thought to be a distinct archaeocete. ''Platyosphys einori'' was based on highly fragmentary remains, which were later determined to have been too poorly preserved to allow assignment beyond a family level. Subsequently, ''Platyosphys einori'' is now simply regarded as Basilosauridae ''
incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...
''. ''Pachycetus uheni'' (the type species of ''Basilotritus'') is regarded as a potential synonym of ''Pachycetus paulsonii''. Things are slightly different for ''"Zeuglodon" wanklyni'', which has never been formally included under ''Pachycetus'' but may represent an additional species according to Gingerich, Amane and Zhouri. Alternatively, it may not be distinct and instead could simply expand the range of ''Pachycetus paulsonii'' to also include the UK. Smaller specimens of ''Pachycetus'' are also known from isolated remains collected in
Spain Spain, or the Kingdom of Spain, is a country in Southern Europe, Southern and Western Europe with territories in North Africa. Featuring the Punta de Tarifa, southernmost point of continental Europe, it is the largest country in Southern Eur ...
and the
North Sea The North Sea lies between Great Britain, Denmark, Norway, Germany, the Netherlands, Belgium, and France. A sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian Se ...
off the coast of
Belgium Belgium, officially the Kingdom of Belgium, is a country in Northwestern Europe. Situated in a coastal lowland region known as the Low Countries, it is bordered by the Netherlands to the north, Germany to the east, Luxembourg to the southeas ...
and the
Netherlands , Terminology of the Low Countries, informally Holland, is a country in Northwestern Europe, with Caribbean Netherlands, overseas territories in the Caribbean. It is the largest of the four constituent countries of the Kingdom of the Nether ...
, though their fragmentary nature means that it is uncertain whether or not they represent distinct species.


Description

The most distinct feature of pachycetines, and the one they derive their name from, is the density of their skeleton. The vertebrae and ribs of pachycetines are both pachyostotic and osteosclerotic. Pachoystosis describes the condition of the
cortical bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...
forming thickened layers, which are prominent in the vertebrae and ribs of pachycetines. In the ribs in particular, the pachyostosis occurs around the middle of the bone, with
cancellous bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...
being restricted to two cones, with the narrowest points facing each other. The fact that the cancellous bone took on a cone shape was originally noted to be distinct to pachycetines, but later studies argued that this condition is much more widespread across basilosaurids and not diagnostic. While pachyostosis describes a thickening of the cortical bone,
osteosclerosis Osteosclerosis is a disorder characterized by abnormal hardening of bone and an elevation in bone density. It may predominantly affect the medullary portion and/or cortex of bone. Plain radiographs are a valuable tool for detecting and classif ...
means that the cortical bone is much denser compared to the same structure in other whales, and contains fewer porosities. Overall these skeletal features have been compared to members of
Sirenia The Sirenia (), commonly referred to as sea cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The extant Sirenia comprise two distinct famili ...
, a group that includes the modern
dugong The dugong (; ''Dugong dugon'') is a marine mammal. It is one of four living species of the order Sirenia, which also includes three species of manatees. It is the only living representative of the once-diverse family Dugongidae; its closest ...
and
manatees Manatees (, family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living spe ...
. The density of the skeleton is at least somewhat increased by the articulation between the spine and the ribs as well. In pachycetines, the ribs are thought to connect to the vertebrae not through synovial articulation but through ligaments or cartilage. Another notable feature of the vertebrae is how the
thoracic vertebrae In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebra (anatomy), vertebrae of intermediate size between the ce ...
rapidly increase in size, with individual bones appearing trapezoid in shape due to how much larger the posterior surface is relative to the beginning of the element. The
lumbar vertebrae The lumbar vertebrae are located between the thoracic vertebrae and pelvis. They form the lower part of the back in humans, and the tail end of the back in quadrupeds. In humans, there are five lumbar vertebrae. The term is used to describe t ...
, which follow those of the thorax, are highly elongated, a trait shared by basilosaurines. Unlike basilosaurines however, the transverse processes that emerge from the sides of the vertebrae are nearly as elongated as the vertebral centra themselves. The surface of the bones is littered with numerous vascular canals, which give them a distinct pattern described as "pitted" or "pockmarked" by researchers. The only
innominate bone The hip bone (os coxae, innominate bone, pelvic bone or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three parts: the ilium, isch ...
known from a pachycetine is that of ''Pachycetus wardii'', which appears to show a much more basal condition than that of other basilosaurids and features a much larger
obturator foramen The obturator foramen is the large, Bilateral symmetry, bilaterally paired opening of the bony pelvis. It is formed by the pubis and ischium. It is mostly closed by the obturator membrane except for a small opening, the obturator canal, through wh ...
. No clear size estimates for the various pachycetines have been published, but several papers mention their size relative to other basilosaurids. Both species of ''Pachycetus'' are described as larger than ''Antaecetus'', with ''P. paulsonii'' being notably larger than ''P. wardii''. ''Antaecetus'' has been described as "medium-sized" and slightly smaller than ''P. wardii''. It's said to be similar in size to the large protocetid ''
Pappocetus } ''Pappocetus'' is an extinct protocetid cetacean known from the Eocene of southern Nigeria's Ameki Formation and Togo. More recently, fossil teeth and femurs have also been discovered in the Aridal Formation of the Sahara Desert in southwester ...
'' with a skull that is the same size as that of ''
Saghacetus ''Saghacetus'' is an extinct genus of basilosaurid early whale, fossils of which have been found in the Upper Eocene (middle Priabonian, ) Qasr el Sagha Formation, Egypt (, paleocoordinates ). Discovery In 1879, German botanist Georg August ...
''.


Phylogeny

Historically, pachycetines were typically regarded as basilosaurids, with some older publications using the antiquated term zeuglodonts. Though this broader relationship is maintained across studies, few make any more definitive statements of how pachycetines may relate to other contemporary archaeocetes within this family, which is commonly found to be
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 ...
. In their 2013 paper, Gol'din and Zvonok recover ''Pachycetus'' (by them referred to as ''Basilotritus'') as a comparably basal basilosaurid. In their study Basilosauridae, though broadly paraphyletic, still contained a monophyletic branch they referred to as "crown basilosaurids" containing taxa such as ''Dorudon'' and ''Basilosaurus'' itself. Within this grouping, the two species of ''Basilotritus'' they erected formed a clade of their own alongside specimen MUSM 1443, which was found to be more derived than ''Supayacetus'' yet more basal than ''Zygorhiza''. A somewhat similar conclusion was reached by van Vliet in 2020 based in part on the well-developed innominate bone of ''P. wardii''. However, no full
phylogenetic analysis In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...
was performed in this study, nor was one conducted in the description of ''Antaecetus''. Following the establishment of Pachycetinae as a recognized clade, the group was recovered in a notably different position in Antar ''et al.'' 2023. Similar to Gol'din and Zvonok's prior work basilosaurids were recovered as a paraphyletic group leading up to Neoceti with many of the core basilosaurids forming their own monophyletic group within this grade. The key difference lies in the position of pachycetines relative to other basilosaurids. Unlike previous publications, Antar and colleagues recovered pachycetines as being more derived than other basilosaurids, being placed as the immediate
sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...
to Neoceti. A further difference to the prior studies is that ''Supayacetus'' was found to not merely be a basilosaurid more basal than pachycetines but to clade with them, possibly being the basalmost pachycetine. This study also suggests that ''Pachycetus'' itself may be paraphyletic, as the American ''Pachycetus wardii'' claded closer with ''Antaecetus'' than with the European ''Pachycetus paulsonii''.


Evolutionary history

Most pachycetines with well-dated fossil remains suggest that the family was most prominent during the Bartonian, however, some localities that have yielded pachycetines might extend their range. For instance, the fossils of ''Pachycetus wardii'' stem from formations that span the Lutetian-Bartonian boundary, whereas some ''Pachycetus paulsonii'' remains may be as young as the Priabonian. Regardless of the first confirmed appearance, it is likely that the origin of Pachycetinae is tied to the global warming that occurred during the Lutetian thermal maximum and the middle Eocene climatic optimum. The brief period of cooler temperatures between these may be another point of origin for pachycetines, as it was during this time that the sea levels began to rise, leading to the conditions seen during the Bartonian. This interpretation is supported by the divergence time calculated by Antar and colleagues, who suggest that pachycetines split from other archaeocetes some 43 to 42 million years ago.


Paleobiology


Locomotion and lifestyle

The combination of a dense skeletal structure coupled with elongated vertebrae has been used to infer various aspects of the locomotion and lifestyle of pachycetines. For instance, the elongation of the lumbar vertebrae highly resembles what is seen in basilosaurines and suggests that these whales swam by undulating their entire bodies. However, since the transverse processes of pachycetines are nearly as elongated as the vertebral centra themselves, they would be a lot more limited in the movements they could perform. Specifically, the smaller space between the adjacent processes meant there was less space for muscles to contract, which effectively hindered the ability of pachycetines to move their bodies side to side. Because of this, the mobility of these whales would be much lower than that of other basilosaurids and likely restricted to undulating their spine dorsoventrally (up-and-down), leading to researchers comparing pachycetines to manatees. At the same time, research also suggests that pachycetines would have been powerful swimmers, despite their low mobility. The pachyostosis, osteosclerosis and rib articulations also inform the lifestyle suggested for these animals. Gingerich and colleagues highlight how these traits may be advantageous for a multitude of purposes which may go hand in hand with each other. The increased density could serve as a form of
ballast Ballast is dense material used as a weight to provide stability to a vehicle or structure. Ballast, other than cargo, may be placed in a vehicle, often a ship or the gondola of a balloon or airship, to provide stability. A compartment within ...
, which is commonly associated with a greater lung capacity, a hypothesis further supported by the rib articulation that would allow the animal to expand and collapse its
ribcage The rib cage or thoracic cage is an endoskeletal enclosure in the thorax of most vertebrates that comprises the ribs, vertebral column and sternum, which protect the vital organs of the thoracic cavity, such as the heart, lungs and great vessel ...
, allowing for a greater intake of air at the surface and the option to expel air to reduce buoyancy after diving. In combination, these could indicate that pachycetines spent a lot of time feeding on the
seafloor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
in relatively shallow water, that it was a slow swimmer and/or that it required large oxygen stores.


Diet

Owing to their dense skeleton and the inferred slow speed, it is thought that pachycetines were
ambush predator Ambush predators or sit-and-wait predators are carnivorous animals that capture their prey via stealth, luring or by (typically instinctive) strategies utilizing an element of surprise. Unlike pursuit predators, who chase to capture prey u ...
s rather than
pursuit predator Pursuit predation is a form of predation in which predators actively give chase to their prey, either solitarily or as a group. It is an alternate predation strategy to ambush predation — pursuit predators rely on superior speed, endurance a ...
s. However, the specific diet likely differed significantly between species. ''Antaecetus'' had much more gracile teeth that would have been more prone to abrasion and wear, which Gingerich and colleagues argue rules out the possibilities that it fed on hard-shelled molluscs or obtained food directly from the seafloor (as the ingested sand would contribute to tooth wear). Herbivory as displayed by sirenians is also ruled out. Instead, a more generalized diet of fish and marine invertebrates has been proposed for ''Antaecetus''. The diet of ''Pachycetus'' has been studied in greater detail. Several types of abrasion have been found on the teeth of ''Pachycetus'', which were notably more robust than those of ''Antaecetus''. These patterns of tooth wear have been compared to those seen in modern whales and seemingly indicate that ''Pachycetus'' at least occasionally fed on sharks and may have even been specialised in them. Following this hypothesis, the teeth were worn down by coming in contact with the
placoid scales A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scale (zoology), scales, which can also provide effective Underwater camouflage, camouflage through the us ...
of sharks and other cartilaginous fish. This notion finds support in the diverse
chondrichthyan Chondrichthyes (; ) is a class (biology), class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fish'', which ...
fauna of the localities that yield ''Pachycetus'' fossils. Some Ukrainian localities for instance yield up to 35 species of shark including '' Scyliorhinus sp.'', '' Hemiscyllium bruxelliensis'', '' Carcharias acutissimu'' and '' Clerolamna umovae''. The American localities that were once home to ''Pachycetus wardii'' were similar, with the Piney Point Formation preserving 19 species of sharks and an additional 9 species of rays.


Paleoenvironment

The idea that pachycetines were shallow-water animals that has been derived from the skeletal anatomy finds additional support in the types of environments that fossils of these animals were found in. For instance, the region around
Helmstedt Helmstedt (; Eastphalian: ''Helmstidde'') is a town on the eastern edge of the German state of Lower Saxony. It is the capital of the District of Helmstedt. The historic university and Hanseatic city conserves an important monumental heritage o ...
that has yielded much of the German remains once formed the southern edge of the Paleogene North Sea, with the environment featuring
estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environm ...
fed by the rivers that ran into the sea from the continent. The
Annenberg Formation Annenberg may refer to: * Annenberg (surname) * The Annenberg Foundation, formerly Annenberg/CPB, known for funding educational television and the Annenberg Channel * Annenberg School for Communication (disambiguation) ** The USC Annenberg School f ...
seems to suggest a depth of , while the overlying
Gehlberg Formation The Gehlberg Formation is a geologic formation in Germany. It preserves fossils dating back to the Paleogene period. See also * List of fossiliferous stratigraphic units in Germany See also * Lists of fossiliferous stratigraphic units i ...
was deposited at a depth of only around . The same applies to the Piney Point Formation of Virginia, which represents shallow marine sediments deposited at a depth of approximately . Molluscs from this formation further suggest mild to warm water temperatures whereas the terrestrial climate is interpreted as having been tropical with dry winters. There may however be some evidence that pachycetines were not restricted to shallow waters at all times. Ukrainian remains have been recovered from sediments that contain
glauconite Glauconite is an iron potassium phyllosilicate ( mica group) mineral of characteristic green color which is very friable and has very low weathering resistance. It crystallizes with a monoclinic geometry. Its name is derived from the Greek ...
, a mineral formed between depths of and most common between . Gol'din and Zvonok have suggested that this could hint at potential migrating behavior, with the animals entering deeper waters while travelling. Such behaviour would profit from the increase in ballast and the powerful swimming musculature that is suggested by the anatomy of the vertebrae. The
Aridal Formation The Aridal Formation in the Sahara, Sahara Desert of southwestern Morocco is a fossil formation with rocks dating to the Lutetian - Priabonian (47.8 - 33.9 million years ago), preserving a middle Eocene shallow coastal environment. The formatio ...
where ''Antaecetus'' has been discovered stands out primarily due to the large amount of cetaceans recovered from its sediments. In addition to ''Antaecetus'', a minimum of five other species are known from there, together representing a broad range in size. The smallest forms are two protocetids and the basilosaurid '' Chrysocetus fouadassaii''. The largest protocetid from the Aridal Formation is ''
Pappocetus } ''Pappocetus'' is an extinct protocetid cetacean known from the Eocene of southern Nigeria's Ameki Formation and Togo. More recently, fossil teeth and femurs have also been discovered in the Aridal Formation of the Sahara Desert in southwester ...
'' which is thought to be within the same size range as ''Antaecetus''. The largest cetacean of the formation is ''
Eocetus ''Eocetus'' is an extinct Protocetidae, protocetid Archaeoceti, early whale known from the early-late Eocene (Bartonian, ) Giushi Formation in Mokattam, Gebel Mokattam, (, paleocoordinates ) outside Cairo, Egypt. Fossil remains have also been di ...
''.


References

{{Taxonbar, from=Q121338969 Eocene first appearances Eocene extinctions