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Protocetidae
View on Wikipediafrom Wikipedia
| Protocetidae | |
|---|---|
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| Maiacetus | |
Scientific classification 
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| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Infraorder: | Cetacea |
| Family: | †Protocetidae Stromer 1908 |
| Subfamilies | |
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3, See text | |
Protocetidae, the protocetids, form a paraphyletic, diverse and heterogeneous group of extinct cetaceans known from Asia, Europe, Africa, South America, and North America.
Description
[edit]
There were many genera, and some of these are very well known (e.g., Rodhocetus). Known protocetids had large fore- and hindlimbs that could support the body on land, and it is likely that they lived amphibiously: in the sea and on land. It is unclear at present whether protocetids had flukes (the horizontal tail fin of modern cetaceans). However, what is clear is that they are adapted even further to an aquatic life-style. In Rodhocetus, for example, the sacrum – a bone that in land-mammals is a fusion of five vertebrae that connects the pelvis with the rest of the vertebral column – was divided into loose vertebrae. However, the pelvis retain a sacroiliac joint. Furthermore, the nasal openings are now halfway up the snout; a first step towards the telescoped condition in modern whales. Their supposed amphibious nature is supported by the discovery of a pregnant Maiacetus,[2] in which the fossilised fetus was positioned for a head-first delivery, suggesting that Maiacetus gave birth on land. The ungulate ancestry of these early whales is still underlined by characteristics like the presence of hooves at the ends of toes in Rodhocetus.[citation needed]
Taxonomy
[edit]
The protocetid subfamilies were proposed by Gingerich et al. 2005. They placed Makaracetus in its own subfamily (Makaracetinae) based on its unique adaptations for feeding (including only two incisors in each premaxilla). They then erected two subfamilies for the rest of the protocetids based on their degree of aquatic adaptation:[3]
Protocetinae- Protocetines are Lutetian protocetids with generalized skulls retaining three incisors in the premaxilla and three molars in the maxilla. To the extent postcrania are known (primarily from Rodhocetus, and more recently Peregocetus),[4] they possess a pelvis similar to those in land-living mammals, a sacrum articulated to the ilia and innominates and large hindlimbs used for foot-powered propulsion.[3] Genera:
Georgiacetinae- Georgiacetines are Bartonian protocetids, considered transitional to the basilosaurids. Their skulls and dentition are similar to those of protocetines, but the pelvis in Georgiacetus and Aegicetus indicate a reduced sacroiliac joint, with no substantial articulation between backbone and innominates. Until recently, no hindlimb material was known for this subfamily, although the pelvic morphology seemed to hint at a tail-based rather than foot-based mode of aquatic locomotion. The discovery of Aegicetus supports this notion.[3][5] Genera:
Notes
[edit]- ^ "Protocetidae". paleobiodb.org. Retrieved 2021-09-03.
- ^ Gingerich et al. 2009
- ^ a b c Gingerich et al. 2005, Table 1, pp. 207–8
- ^ Olivier L, Bianucci G, Salas-Gismondi R, Di Celma C, Steurbaut E, Urbina M & de Muizon C (2019). "An amphibious whale from the Middle Eocene of Peru reveals early South Pacific dispersal of quadrupedal cetaceans". Current Biology 29(8): p. 1352–1359.e3.
- ^ Gingerich P.D., Antar M.S.M., Zalmout I.S. (2019). "Aegicetus gehennae, a new late Eocene protocetid (Cetacea, Archaeoceti) from Wadi Al Hitan, Egypt, and the transition to tail-powered swimming in whales". PLoS ONE 14(12):e0225391
References
[edit]- Gingerich, Philip D; ul-Haq, Munir; von Koenigswald, Wighart; Sanders, William J; Smith, B Holly; Zalmout, Iyad S (2009). "New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and Sexual Dimorphism". PLOS ONE. 4 (2) e4366. Bibcode:2009PLoSO...4.4366G. doi:10.1371/journal.pone.0004366. PMC 2629576. PMID 19194487.
- Gingerich, Philip D.; Zalmout, Iyad S.; Ul-Haq, Munir; Bhatti, M. Akram (2005). "Makaracetus bidens, a new protocetid archaeocete (Mammalia, Cetacea) from the early middle Eocene of Balochistan (Pakistan)" (PDF). Contributions from the Museum of Paleontology. 31 (9): 197–210. OCLC 742723177. Retrieved 1 March 2013.
- Stromer, Ernst (1908). Die Archaeoceti des ägyptischen Eozäns: Beiträge zur paläontologie und geologie Österreich-Ungarns und des Orients (PDF). W. Braumüller. OCLC 21174007. Retrieved 1 July 2013.
Wikispecies has information related to Protocetidae.
Archaeocete genera by family | |||||||
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| Pakicetidae | |||||||
| Ambulocetidae | |||||||
| Remingtonocetidae | |||||||
| Protocetidae |
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| Basilosauridae |
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Protocetidae
View on Grokipediafrom Grokipedia
Description
Physical Characteristics
Protocetids were medium-sized cetaceans, typically measuring 2 to 3 meters in length, with body masses estimated between 280 and 390 kilograms based on complete skeletons such as that of Maiacetus inuus from the middle Eocene of Pakistan.[5] Their body plan featured an elongated snout and a robust axial skeleton, including dense bones that enhanced bone compactness for buoyancy control in aquatic environments, as evidenced by high compactness indices (CI) in long bones and ribs ranging from 69% to 92% in genera like Rodhocetus and Qaisracetus.[6] This density, particularly in the femur (CI 83.6–87.4%) and humerus (CI ~69–92%), reflects adaptations intermediate between terrestrial and fully aquatic lifestyles.[6] Cranially, protocetids exhibited a moderately elongated rostrum with nasal openings positioned approximately halfway along the snout, marking an early stage of telescoping toward the dorsal position seen in later cetaceans. In Maiacetus inuus, the skull measured about 56 cm in length, with anteriorly placed nares and large tympanic bullae indicative of enhanced underwater hearing.[5] The dentition was heterodont, featuring a dental formula of 3.1.4.3 in both upper and lower jaws, with robust anterior teeth suited for grasping prey and molars displaying trigonids and talonids for processing.[5] The postcranial skeleton included large forelimbs and hindlimbs capable of supporting body weight on land, with preserved phalanges in the manus and pes indicating weight-bearing functionality. Limb proportions showed robust but relatively short humeri and femora compared to fully terrestrial artiodactyl ancestors, as in Peregocetus pacificus where the femur was 18% shorter than the humerus; elbow and knee joints permitted flexion for both terrestrial walking and aquatic paddling. Some genera, such as Rodhocetus kasranii, retained hooves on the distal phalanges of the toes, linking them to their ungulate heritage and suggesting occasional terrestrial locomotion.Adaptations for Amphibious Life
Protocetids exhibited specialized skeletal modifications that facilitated a semi-aquatic lifestyle, particularly through increased bone density in their axial skeleton. Their ribs and vertebrae displayed pachyosteosclerosis, characterized by thickened cortical bone with reduced or absent marrow cavities, resulting in high overall density that served as ballast to counteract buoyancy in shallow waters.[8] For instance, in Georgiacetus vogtlensis, thoracic ribs showed a bone volume fraction (BV/TV) of approximately 0.54 and cortical thicknesses up to 6.67 mm, composed largely of dense woven bone with endosteal lamellae, while Gaviacetus sahnii ribs reached BV/TV values of 0.76, indicating inhibited bone resorption for enhanced stability during submersion.[8] This adaptation, distinct from the lighter bones of terrestrial artiodactyls, allowed protocetids to remain submerged for foraging without constant active propulsion, though it limited terrestrial agility.[9] Tail and vertebral structures in protocetids reflected transitional flexibility for both terrestrial support and incipient aquatic propulsion. In Georgiacetus vogtlensis, caudal vertebrae lacked the compression and simplification seen in later cetaceans, indicating the absence of a fully developed tail fluke and reliance on hindlimb paddling or spinal undulation for swimming, rather than oscillatory tail movement. Conversely, Rodhocetus kasrani possessed a divided sacrum comprising four unfused vertebrae with a retained sacroiliac joint connecting to the ilium, enabling greater spinal mobility for lateral undulation in water while still providing partial weight-bearing on land.[10] This configuration marked an evolutionary shift toward enhanced caudal flexibility, though without the full decoupling from the pelvis observed in more derived forms. Auditory adaptations in protocetids supported dual terrestrial and aquatic environments, with modifications to the middle ear enhancing underwater sound reception. The tympanic bullae were thickened, featuring a robust medial involucrum and a relatively thin lateral tympanic plate, which transmitted vibrations from water via the mandible and associated fat pads to the ossicles and inner ear.[11] In species like Indocetus ramani, the tympanic plate area measured about 1225 mm², larger and thicker than in modern odontocetes, facilitating passive sound conduction but with lower sensitivity and no evidence of echolocation capabilities.[11] Unlike the fully isolated tympanoperiotic complexes of later cetaceans, protocetid ears retained contact with the skull, limiting directional acuity in water but preserving functionality for both media.[11] Recent analyses of forelimb evolution highlight how relaxed selective pressures drove morphological changes in protocetid humeri toward aquatic forms. A 2025 study using geometric morphometrics on protocetid fossils revealed that humerus shape underwent random drift under diminished negative selection, with deviations from artiodactyl ancestors including humeral head torsion (up to 27° in related forms like Aegicetus gehennae) and diaphysis shortening, fostering paddle-like structures for steering and stability in water.[12] This drift, rather than strong directional selection, accounts for the increased variance in forelimb morphology observed in Protocetidae compared to terrestrial relatives, reflecting reduced locomotor demands on land.Evolutionary History
Origins and Timeline
Protocetidae, an extinct family of early cetaceans, are known from the fossil record of the Eocene epoch, spanning approximately 48 to 35 million years ago (Ma).[13][14] This temporal range encompasses the Lutetian, Bartonian, and earliest Priabonian stages, during which protocetids diversified across multiple continents.[13] The origins of Protocetidae trace back to terrestrial even-toed ungulates (artiodactyls), with key evidence provided by the morphology of the astragalus, a double-pulleyed ankle bone characteristic of artiodactyls. Fossils of genera such as Artiocetus and Rodhocetus from Pakistan exhibit an astragalus with a well-developed navicular trochlea, a convex fibular facet, and a concave astragalar facet paired with a notched, convex calcaneal facet—features diagnostic of early artiodactyls and absent in other mammalian groups like mesonychians. These traits confirm that cetaceans, including protocetids, evolved from early artiodactyl stock rather than from unrelated carnivorans. Closest relatives among earlier cetaceans are the pakicetids, more terrestrial precursors from the early Eocene that shared similar artiodactyl-like ankle structures but lacked the advanced aquatic adaptations seen in protocetids. The earliest known protocetid fossils date to around 47.5 Ma and come from marine sedimentary rocks in Pakistan and India, marking the onset of protocetid diversification following the decline of pakicetids near the end of the early Eocene. Notable early specimens include those of Maiacetus inuus from the upper Habib Rahi Formation in eastern Balochistan, Pakistan, and various partial skeletons from Eocene deposits in northern India. These finds indicate a rapid radiation of amphibious forms in nearshore environments shortly after pakicetid extinction around 48 Ma.[13][15] Protocetids emerged during a phase of Eocene global warming, associated with the Early Eocene Climatic Optimum's tail end and subsequent middle Eocene thermal maximum, which drove sea-level rise and expanded coastal and marginal marine habitats. This climatic shift likely facilitated the transition of artiodactyl-like ancestors into amphibious lifestyles, providing nutrient-rich estuaries and shallow seas conducive to the initial evolution of cetaceans.[13]Role in Cetacean Evolution
Protocetidae represents a paraphyletic assemblage of semiaquatic whales that served as a critical stem group in cetacean evolution, bridging the gap between the more terrestrial, amphibious pakicetids and the fully aquatic basilosaurids of the late Eocene.[16] These Eocene cetaceans (approximately 48–35 million years ago) exhibited a mosaic of transitional traits, retaining functional hind limbs for both terrestrial movement and aquatic propulsion while developing enhanced adaptations for marine life, such as elongated snouts and robust dentition suited for piscivory.[13][14] A notable example is Maiacetus inuus, where the fossilized fetus is positioned head-first toward the mother's pelvis, indicative of birth on land similar to modern terrestrial mammals and artiodactyls, underscoring that protocetids had not yet fully committed to an aquatic reproductive strategy.[17] This perinatal evidence highlights their role in illustrating the gradual shift from land-based to water-based lifestyles during the Eocene.[17] Within Protocetidae, evolutionary advancements in locomotion marked significant steps toward modern cetacean swimming mechanics. Early members of the subfamily Protocetinae relied primarily on foot-propelled paddling, using their powerful hind limbs to generate thrust in shallow coastal waters, much like an otter.[14] In contrast, later forms in the subfamily Georgiacetinae, such as Aegicetus gehennae, showed vertebral elongation in the posterior thoracic, lumbar, and caudal regions, along with reduced sacroiliac articulation and hind limb size, facilitating a transition to mid-body and tail undulation for propulsion.[14] These changes decreased the efficiency of pelvic paddling while enhancing the potential for oscillatory tail movements, directly bridging to the fluke-driven locomotion of basilosaurids and extant whales.[14] Such innovations within protocetids demonstrate how incremental skeletal modifications enabled progressively greater aquatic efficiency, setting the stage for the pelagic lifestyles of crown cetaceans. The biogeographic expansion of protocetids further underscores their pivotal influence on cetacean diversification. Originating in the Indo-Pakistan region during the early middle Eocene, they rapidly dispersed across low-latitude epicontinental seas via connections in the Tethys Sea, reaching Africa by around 43 million years ago and subsequently North and South America.[16] Fossils like Peregocetus pacificus from Peru (42.6 million years ago) reveal their early incursion into the Pacific via the South Atlantic, achieving a near-circumglobal tropical distribution that facilitated gene flow and adaptive radiation.[18] This swift dispersal, enabled by favorable paleocurrents and coastal habitats, positioned protocetids as precursors to the global proliferation of later cetaceans, including basilosaurids that colonized higher latitudes.[18] Protocetidae's evolutionary significance also reinforces the understanding of cetaceans' origins within even-toed ungulates (Artiodactyla), integrating fossil and molecular evidence. Shared anatomical features, such as double-pulley astragali and S-shaped ectotympanic bones, link protocetids to terrestrial artiodactyls like early hippos and raoellids, while molecular phylogenies confirm Cetacea nesting deeply within Artiodactyla as the sister group to Hippopotamidae.[19] This congruence between paleontological records—exemplified by protocetid intermediates—and genomic data (e.g., from mitochondrial and nuclear sequences) solidifies the artiodactyl ancestry of whales, with protocetids embodying a key phase in the macroevolutionary transition from terrestrial herbivores to fully marine carnivores.[20]Paleobiology
Locomotion and Diet
Protocetids exhibited an amphibious lifestyle, enabling quadrupedal locomotion on land through the use of all four limbs, as evidenced by the robust structure of their fore- and hindlimbs in genera such as Peregocetus, which retained functional feet capable of weight-bearing and terrestrial walking.[21] In aquatic environments, these early cetaceans primarily relied on hindlimb-powered paddling for propulsion, with short but muscular limbs facilitating effective swimming in shallow coastal waters, similar to modern semiaquatic mammals like otters.[13] More derived protocetids, such as Georgiacetus, showed transitional features including tail vertebrae that suggest an emerging role for caudal movement in swimming, though hindlimb paddling remained the dominant aquatic strategy.[22] The diet of protocetids was carnivorous, focused on fish and likely invertebrates, supported by anatomical evidence of conical, interlocking teeth suited for grasping and shearing prey, along with robust jaws that allowed for powerful biting forces.[23] Within the family, Makaracetus displayed specialized suctorial feeding adaptations, including a narrow, T-shaped rostrum, hypertrophied facial musculature, and enlarged palatal vacuities that created a thin midline palate, enabling suction to capture benthic prey such as mollusks in shallow marine settings.[24] Protocetids preferred coastal and estuarine habitats, where stable isotope analysis of tooth enamel reveals mixed dietary sources incorporating both freshwater and marine resources, with δ¹³C values indicating foraging in nearshore environments influenced by riverine inputs. Anatomical features such as prominent limb muscle attachment scars on the humerus, femur, and pelvic girdle imply substantial muscular investment for both terrestrial walking and aquatic paddling, supporting biomechanical models that estimate high metabolic costs associated with this dual locomotion in an amphibious lifestyle.[9]Reproduction and Sensory Capabilities
Protocetids were viviparous mammals, giving birth to live young, with evidence indicating that birthing occurred on land to facilitate head-first delivery and prevent drowning during the process. The most direct evidence comes from the fossil of Maiacetus inuus, a middle Eocene protocetid from Pakistan, where a female skeleton preserves a near-term fetus positioned head-first within the pelvic region, consistent with terrestrial mammalian birth postures that allow immediate access to air. This adaptation reflects their semi-aquatic lifestyle, where females likely returned to coastal or shoreline environments for parturition, minimizing risks associated with underwater birth seen in modern cetaceans. The single calf preserved in the Maiacetus specimen suggests singleton litters, similar to other early archaeocetes. Sensory capabilities in protocetids retained terrestrial traits while showing early aquatic adaptations, particularly in olfaction and hearing. The ethmoid bones and associated turbinates were well-developed, supporting a functional olfactory system inherited from land-dwelling ancestors, which aided in detecting chemical cues in air and possibly dilute scents in water. For instance, in Aegyptocetus tarfa from Egypt, robust ethmoidal turbinal bones indicate preserved olfaction for environmental navigation and prey detection in murky coastal waters. Recent endocranial analyses reveal that protocetid brains exhibited an earlier-than-expected increase in encephalization quotient relative to body mass, with enlarged olfactory bulbs enhancing chemosensory processing for survival in transitional habitats. Hearing adaptations featured intermediate middle ear structures, including pachyosteosclerotic bullae and ossicles geared for sound transmission in both air and water, allowing efficient propagation of low-frequency underwater sounds while maintaining aerial sensitivity. Inferences of social behavior in protocetids draw from neuroanatomical evidence, suggesting possible herd-like structures for protection during vulnerable land-water transitions. The expanded brain regions observed in endocranial casts further imply emerging cognitive capacities for social interactions, potentially including communication via vocalizations adapted for dual media. These traits likely supported reproductive synchronization influenced by Eocene coastal ecosystems, where seasonal environmental shifts prompted communal behaviors.Taxonomy and Classification
Historical Development
The family Protocetidae was established by Ernst Stromer in 1908 to classify the genus Protocetus, initially based on Eocene fossils from Egypt that exhibited primitive cetacean features such as double-rooted cheek teeth and a transitional skeletal morphology.[15] Early interpretations viewed Protocetidae as a monophyletic group bridging terrestrial artiodactyls and fully aquatic cetaceans, emphasizing their role in the Eocene radiation of archaeocetes.[25] However, by the late 1990s and into the 2000s, cladistic analyses incorporating expanded morphological datasets began to challenge this, demonstrating that Protocetidae formed a paraphyletic assemblage rather than a cohesive clade, with various genera representing successive grades toward more derived cetaceans like basilosaurids.[26] Significant revisions to protocetid classification emerged in the 1990s through discoveries such as Rodhocetus kasrani, unearthed in Pakistan in 1993 and formally described in 1994, which revealed robust hind limbs and pelvic adaptations indicative of amphibious locomotion rather than fully aquatic lifestyles previously assumed for the family. This finding, supported by subsequent postcranial analyses, shifted perceptions of protocetids from obligate swimmers to versatile, land-capable predators, prompting reevaluations of their ecological niche and locomotor evolution.[27] More recently, the 2019 description of Aegicetus gehennae from late Eocene deposits in Egypt's Wadi Al-Hitan further refined subfamily boundaries by assigning it to Georgiacetinae, highlighting regional endemism and reinforcing the family's paraphyletic nature through phylogenetic placement near basilosaurids. The integration of molecular data in the 1990s provided crucial corroboration for protocetids' transitional status, as studies using mitochondrial DNA and retroposon insertions confirmed cetaceans' nested position within Artiodactyla, aligning fossil evidence of protocetid ankle bones and auditory bullae with genetic signals of a close artiodactyl affinity dating to approximately 50 million years ago. This molecular-fossil synthesis bolstered arguments for protocetids as key intermediates in the land-to-sea transition, influencing taxonomic frameworks by emphasizing shared synapomorphies like astragalar morphology with hippopotamids.[28] Ongoing debates center on the degree of paraphyly within Protocetidae, with recent phylogenies consistently positioning Protocetus—the family's type genus—as more closely related to basilosaurids than to other protocetids like Georgiacetus or Maiacetus, based on shared endocranial and postcranial traits such as expanded cerebral hemispheres and reduced hind limb functionality.[29] These analyses, drawing from comprehensive character matrices, underscore how new fossils continue to reshape understandings of protocetid interrelationships, portraying the family as a series of evolutionary grades rather than a unified lineage.[26]Subfamilies and Genera
Protocetidae comprises approximately 10–12 genera, classified into three subfamilies that reflect a graded progression in aquatic adaptations toward basilosaurids, though the family as a whole is paraphyletic.[29] The subfamilies were formalized by Gingerich et al. (2005), based on locomotor and cranial features distinguishing degrees of terrestriality and aquatic specialization.[30] The subfamily Protocetinae, known from the Lutetian stage (early middle Eocene, approximately 48–41 Ma), includes genera with relatively more terrestrial hindlimbs suited for foot-propelled swimming in shallow waters. Key examples are Protocetus atavus from Egypt, characterized by robust limbs and a skull adapted for grasping prey, and Babiacetus indicus from Pakistan and India, which retained strong pelvic connections for weight support on land.[31] Diagnostic traits include elongated femora and unfused sacral vertebrae, indicating amphibious capabilities similar to modern pinnipeds. Georgiacetinae, from the Bartonian stage (late middle Eocene, approximately 41–38 Ma), features genera with reduced hindlimbs and evidence of tail-propelled locomotion, marking a shift toward greater aquatic reliance. Representative genera include Georgiacetus vogtlensis from the southeastern United States, with a detached pelvis and elongated tail vertebrae suggesting improved swimming efficiency, and Aegicetus gehennae from Egypt, known for its partial skeleton showing shortened hindlimbs and a more streamlined body.[14] These taxa exhibit diagnostic reductions in limb robusticity and the presence of a pterygoideus sinus fossa in the basicranium, facilitating underwater maneuvering.[32] The subfamily Makaracetinae is monotypic, containing only Makaracetus bidens from Pakistan (early middle Eocene), distinguished by specialized feeding adaptations such as procumbent anterior teeth for suction-feeding on soft-bodied prey, unlike the raptorial dentition of other protocetids.[33] Its unique cranial morphology, including a shortened rostrum and enlarged temporal fossae, sets it apart, potentially representing an early experiment in cetacean trophic diversity.[30] Notable genera outside these strict subfamily assignments, but integral to protocetid diversity, include Maiacetus inuus from Pakistan (Lutetian, ~47 Ma), a 2.6 m-long species known from a pregnant female specimen indicating land-based birth and precocial development; Rodhocetus kasrani from Pakistan, with sacral ribs and possible tail fluke precursors hinting at enhanced propulsion; and Phiomicetus anubis from Egypt (Lutetian, ~43 Ma), a medium-sized (3 m) form with a robust skull for piscivory.[17][34][35] Phylogenetic analyses position Protocetinae as basal, with Georgiacetinae forming a grade closer to basilosaurids, as reinforced by recent endocranial studies emphasizing olfactory retention in early forms.[29] No new subfamilies have been proposed in 2025 analyses, which continue to support this framework amid ongoing discoveries.[29]| Subfamily | Age (Stage) | Key Genera | Diagnostic Traits |
|---|---|---|---|
| Protocetinae | Lutetian | Protocetus, Babiacetus | Robust limbs, foot-propelled swimming, strong pelvic girdle |
| Georgiacetinae | Bartonian | Georgiacetus, Aegicetus | Reduced hindlimbs, tail propulsion, detached pelvis |
| Makaracetinae | Lutetian | Makaracetus | Suction-feeding teeth, shortened rostrum, enlarged temporal fossae |