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Mystacodon selenensis

Mammalia - Ungulata - Mystacodontidae

Taxonomy
Mystacodon selenensis was named by Lambert et al. (2017). Its type specimen is MUSM 1917, a partial skeleton ( cranium, mandibles, teeth, cervical, thoracic, lumbar and caudal vertebrae, ribs, partial right and left forelimbs, and left innominate), and it is a 3D body fossil. Its type locality is Playa Media Luna, Priabonian, which is in a Priabonian deep-water siltstone in the Yumaque Formation of Peru. It is the type species of Mystacodon.

Synonymy list
YearName and author
2017Mystacodon selenensis Lambert et al. p. 1 figs. Figs. 1-2
2018Mystacodon selenensis Fordyce and Marx p. 5 figs. Fig. 4
2019Mystacodon selenensis de Muizon et al. p. 406

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RankNameAuthor
kingdomAnimalia()
Triploblastica
Nephrozoa
Deuterostomia
phylumChordataHaeckel 1847
subphylumVertebrata
superclassGnathostomata
Osteichthyes()
Sarcopterygii
subclassDipnotetrapodomorpha(Nelson 2006)
subclassTetrapodomorpha()
Tetrapoda()
Reptiliomorpha
Anthracosauria
Batrachosauria()
Cotylosauria()
Amniota
Synapsida()
Therapsida()
RankNameAuthor
infraorderCynodontia()
Epicynodontia
infraorderEucynodontia
Probainognathia
Mammaliamorpha
Mammaliaformes
classMammalia
subclassTribosphenida()
infraclassEutheria()
orderUngulata(Linnaeus 1766)
orderArtiodactyla()
Cetacea()
Pelagiceti
Neoceti
suborderMysticeti
familyMystacodontidae
genusMystacodon
speciesselenensis

If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.

Diagnosis
ReferenceDiagnosis
O. Lambert et al. 2017MUSM 1917 is identified as a Neoceti based on the following derived characters, absent in basilosaurid archaeocetes: partly open mesorostral groove; anteroposteriorly elongated rostral portion of maxilla; loss of sagittal crest; supraoccipital shield an- terodorsally inclined; apex of zygomatic process of squamosal nearly contacting postorbital process of frontal; and distal epiphysis of the humerus divided in two angled radial and ulnar facets. It can be referred to the Mysticeti due to the following combination of derived characters: dorsoventrally thin lateral edge of maxilla on rostrum; presence of an antorbital process of the maxilla; presence of a maxillary infraorbital plate; and triangular supraoccipital shield. It is further diagnosed by two possibly au- tapomorphic features: nasal anteroposteriorly longer than frontal plus parietal and strong tuberosity on anterior edge of radius; two additional derived characters: posteriormost upper tooth anterior to level of antorbital process of maxilla and broad-based rostrum (ratio between width of skull at rostrum base and width at postorbital process > 0.8); and a series of plesiomophic features: supraoccipital shield not extending anterior to anterior level of squamosal fossa, only two dorsal infraorbital foramina, a basilosaurid dental formula 3.1.4.2/3.1.4.3, no wide diastemata between posterior cheek teeth, sutured mandibular symphysis, and well-defined acetabulum on innominate. Finally, MUSM 1917 lacks cranial synapomorphies of Odontoceti: facial con- cavity, presence of premaxillary foramen and premaxillary sac fossa, and posterior expansion of maxilla over the supraorbital region (see [1, 5, 11–13]) (Figures 1, 2, 3, and S2).
C. de Muizon et al. 2019Mystacodon selenensis is identified as a Neoceti based on the following derived characters, absent in basilosaurid archaeocetes: partly open mesorostral groove; anteroposteriorly elongated rostral portion of maxilla; presence of an antorbital process of the maxilla; supraoccipital shield anterodorsally inclined; and distal epiphysis of the humerus divided in two angled radial and ulnar facets.
Mystacodon selenensis is referred to the Mysticeti due to the fol- lowing combination of derived characters: dorsoventrally thin posterolateral region of maxilla on rostrum; antorbital process of maxilla separated from lacrimal; presence of a maxillary infraorbital plate; apex of zygomatic process of squamosal closely apposed to postorbital process of frontal or situated ventral to the latter; external occipital crest restricted to anterodorsal half of supraoc- cipital shield; and triangular supraoccipital shield.
Mystacodon selenensis differs from all other toothed mysticetes in bearing the following combination of features: 1) elongated nasal, the nasal being longer than the length of the frontal + parietal, at midline; 2) anteriorly located antorbital foramina, at about the level of the diastema between P2 and P3; 3) absence of real antorbital notch; 4) long jugo-squamosal suture, with an anterior prolongation of the zygomatic process of the squamosal, extending as far as the level of the postorbital process of the frontal anteriorly; 5) short anterior process of the frontal separating the posterior apices of the nasals; 6) sutured mandibular symphysis, the posterior edge of which is at the level of the i3-p1 diastema; and 7) ventral border of the mandibular ramus being slightly concave ventrally.
Mystacodon selenensis also bears four additional derived characters: upturned (i.e., dorsally concave) anterior region of the rostrum; posteriormost upper tooth anterior to level of antorbital process of maxilla; broad-based rostrum (ratio between width of skull at rostrum base and width at postorbital process > 0.8); and strong tuberosity on anterior edge of radius (the latter condition is un- known in the other toothed mysticetes).
Furthermore, Mystacodon selenensis bears a series of plesiomophic features: supraoccipital shield not extending anterior to anterior level of squamosal fossa; only two dorsal infraorbital foramina; a basilosaurid-like dental formula 3.1.4.2/3.1.4.3; no wide diastemata between posterior cheek teeth; sutured mandibular symphysis; and well-defined acetabulum on innominate.
Mystacodon selenensis differs from Llanocetus denticrenatus in the following characters: 1) much smaller size (less than half the estimated body size); 2) rostrum narrower with concave edges while the preserved posterior part of the rostrum of Llanocetus is much wider and laterally expanded; 3) alveolar border of the maxilla on posterior region of the rostrum rounded dorsolaterally in cross-section with a subvertical lateral wall, while it is flat and crest-like in Llanocetus; 4) preserved parts of the palate lack deep lateral sulci, while they are distinctly present and abundant in Llanocetus; 5) premaxillae abruptly depressed just anterior to the nasals, while in Llanocetus the transition to the post narial part of the premaxillae is smooth; 6) upper molars closely approximated, while they are separated by large diastemata in Llanocetus; 7) upper molars much larger; 8) short diastema between P3 and P4 (less than one tooth mesiodistal length), while in Llanocetus the diastema is two to three times a tooth length; 9) much longer orbit; 10) nearly straight lateral edge of the supraorbital process, while it is deeply concave in Llanocetus; 11) posterior edge of the supraorbital process roughly straight, while it is concave in Llanocetus; 12) orbitotemporal crests extend posteriorly until anteroposterior mid-point of the parietals, while in Llanocetus they end anteriorly, at the posteromedial angle of the supraorbital process of the frontal; 13) dorsal edge of the parietals is subhorizontal in lateral view, while it ascends steeply toward the vertex in Llanocetus; 14) intertemporal bridge much narrower than long transversely, while it is as wide as long in Llanocetus; 15) anterior angle of nuchal crest posterior to anterior margin of squamosal fossa, while it approximately reaches the level of the middle of the fossae in Llanocetus; 16) lateral edges of the supraoccipital in dorsal view straight, while they are sigmoid in Llanocetus; 17) vertex and dorsal aspect of the braincase almost in the same plane as the posterior region of the rostrum, while in Llanocetus they slope anteriorly and distinctly overhang the rostrum; 18) distal extremity of anterior ribs expanded and pestle-like, while this condition is absent in Llanocetus.
Mystacodon selenensis differs from Coronodon havensteini in having a longer rostrum and longer diastemata between the premolars; much longer nasals associated to more anterior bony nares, located in the anterior half of the rostrum (in the posterior third in Coronodon); temporal fossae more elongated anteroposteriorly; a long and thin (dorsoventrally) zygomatic process of the squamosal (short and stout in Coronodon); a sharp external occipital crest; a tightly articulated mandibular symphysis; and an sub-horizontal or gently sloping tooth wear on all preserved teeth with strong edges at the apical surface.
In addition to its autapomorphies, Mystacodon selenensis differs from mammalodontids in having: a rostrum at least proportionally twice longer; much more anteriorly placed bony nares; a sharply triangular occipital shield; a well-developed external occipital crest (low, when present, in mammalodontids); a strong palatine sulcus on the medial edge of the premaxilla and maxilla anteriorly; much longer diastemata between the premolars; and absence of the markedly V-shaped fronto-parietal suture (as observed in mammalodontids).
As compared with aetiocetids, Mystacodon selenensis has a basilosaurid-like dental formula; an almost horizontal apical tooth wear; much larger postcanine teeth; and a long and slender zygomatic process of the squamosal, which is lowest at its anteriormost portion. M. selenensis lacks all the aetiocetid characteristic features, such as the extreme dorsoventral compression of the rostrum; the deeply concave lateral edge of the supraorbital process of the frontal in dorsal view; the anteroposteriorly short zygomatic process of the squamosal (as compared to that of basilosaurids); and the very thin jugal.
Finally, Mystacodon selenensis lacks cranial synapomorphies of Odontoceti: facial concavity; presence of premaxillary foramen and premaxillary sac fossa; and wide posterior expansion of maxilla overlapping the supraorbital region.