Aondelphis talen, Viglino & Buono & Gutstein & Cozzuol & Cuitiño, 2018

Viglino, Mariana, Buono, Mónica R., Gutstein, Carolina S., Cozzuol, Mario A. & Cuitiño, José I., 2018, A new dolphin from the early Miocene of Patagonia, Argentina: Insights into the evolution of Platanistoidea in the Southern Hemisphere, Acta Palaeontologica Polonica 63 (2), pp. 261-277 : 264-271

Aondelphis talen sp. nov.

Figs. 2–6 , Table 1.

Etymology: From the Tehuelche language t’alen, small; in reference to small size.

Holotype: MPEF-PV 517 ; incomplete skull including basioccipital, exoccipitals, squamosals, a portion of the sphenoids, almost complete left tympanic bulla, and complete left periotic.

Type locality: Bryn Gwyn (= Loma Blanca), southern cliff of Chubut River Valley , 8 km southeast of the town of Gaiman, in Chubut Province, Patagonia, Argentina ( Fig. 1 ) .

Type horizon: Gaiman Formation, early Miocene.

Diagnosis.— Aondelphis talen gen. et sp. nov. differs from all other Platanistoidea (sensu lato; see Phylogenetic analysis section) in the unique autapomorphy: convex dorsal surface of periotic (character 182).

Aondelphis talen differs from cf. Papahu ZMT- 73 in the absence of a ridge on the inside of the tympanic bulla (character 221). Aondelphis talen differs from Squalodon calvertensis in the concave ventral surface of the posterior process of the periotic (character 201). Aondelphis talen differs from cf. Papahu ZMT-73 and S. calvertensis in the development of

Tympanic bulla

Distance from anterior tip to posterior end of outer posterior prominence +40

Distance from anterior tip to posterior end of inner posterior prominence +38

Width across inner and outer posterior prominence 19.5

Greatest depth of interprominential notch 5?

Maximum length of posterior process 19

Maximum width 23

Maximum width of involucrum 13

Periotic

Standard length of periotic, from tip of anterior process to posterior end of posterior process, measured on a straight

32

line parallel with cerebral border

Width of periotic across cochlear portion and superior process, at the level of upper tympanic aperture 19

Least distance between the margins of fundus of internal auditory meatus and of aperture of ductus endolymphaticus 2

Least distance between the margins of fundus of internal auditory meatus and of aperture of aqueduct cochleae 1.7

Length of articular facet of the posterior process of the periotic for the posterior process of tympanic bulla 13

Antero-posterior diameter of cochlear portion 14

Dorsoventral depth at fovea epitubiaria 4

Anteroposterior diameter of facial canal 7

Maximum width of anterior process at base 11

Transverse width of pars cochlearis from internal edge to fenestra ovalis 11

Length of posterior process 14

Length of anterior process from anterior apex to level of posterior of mallear fossa 13

Maximum width of posterior process 12

Anteroposterior length of aperture for cochlear aqueduct 2

Width of aperture for cochlear aqueduct 2

Anteroposterior length of aperture for vestibular aqueduct 1

Width of aperture for vestibular aqueduct 2

an anteroposterior ridge on the dorsal side of the anterior process and body of periotic (character 174). Aondelphis talen differs from Platanistoidea sensu stricto (see Phylogenetic analysis section) in the anteroposteriorly wide and squared-off postglenoid process of the squamosal (character 118); no obvious lateral groove on profile of the periotic (character 172); from the Platanistoidea sensu stricto (except Platanista gangetica ) in the tubular fundus of the internal acoustic meatus of the periotic (character 189); from Platanistoidea sensu stricto (except Awamokoa tokarahi and Notocetus vanbenedeni ) in the weakly-curved parabullary sulcus (character 176). Aondelphis talen differs from S. calvertensis and Platanistoidea sensu stricto in the excavated dorsal margin of the involucrum of the tympanic bulla at mid-length (character 220); in the absence of an articular rim in the periotic (character 196); from S. calvertensis and Platanistoidea sensu stricto (except A. tokarahi and P. gangetica ) in the slit-like external auditory meatus (character 157);from S. calvertensis and Platanistoidea sensu stricto (except P. gangetica ) in the wide angle between the posterior process and pars cochlearis of the periotic (character 199); from the S. calvertensis and Platanistoidea sensu stricto (except Pomatodelphis inaequalis ) in the high lateral wall of the internal acoustic meatus (character 191). Aondelphis talen further differs from S. calvertensis , and Platanistoidea sensu stricto (except Otekaikea marplesi ; unknown in Phocageneus venustus ) in the deep emargination of the neck muscle fossa in the zygomatic process of the squamosal (character 114).

Aondelphis talen differs from cf. Papahu ZMT-73 and Platanistoidea sensu stricto in the rounded profile of the cochlear aqueduct (character 194); and in the absence of a subcircular fossa (character 152); from cf. Papahu ZMT-73, Squalodelphinidae and Platanistidae in the poorly defined ventromedial keel of the tympanic bulla (character 222).

Aondelphis talen differs from Squalodon calvertensis , Awamokoa tokarahi , Otekaikea , Notocetus vanbenedeni , and Platanistidae in the absence of a fossa for articular rim of the periotic (character 287); from S. calvertensis , Waipatia maerewhenua , A. tokarahi , Squalodelphis fabianii , and N. vanbenedeni in the distinctly shorter inner posterior prominence of the tympanic bulla with respect to the outer posterior prominence (character 219); from S. calvertensis and N. vanbenedeni in the smoothly deflected anterior process of the periotic (character 173). Aondelphis talen differs from S. calvertensis , N. vanbenedeni and Platanistidae in the shallow posterior portion of the periotic fossa (character 155). Aondelphis talen differs from S. calvertensis , Phocageneus venustus , N. vanbenedeni , and Platanistidae in the presence of a posterodorsal edge of the stapedial muscle fossa in the periotic (character 184); and a long posterior process of the periotic (character 203). MPEF-PV 517 differs from N. vanbenedeni and Zarhachis flagellator in the open jugular notch (character 164). Aondelphis talen differs from Platanistidae in the triangular tympanosquamosal recess medial to the postglenoid process (character 148); and in the absence of an excavation of the tegmen tympani at the base of the anterior process (character 188).

Description.— Physical maturity and body size: Due to the fragmentary condition of this specimen, it is difficult to determine the age of MPEF-PV 517. The lack of punctate texture on the occipital condyles suggests that at least it is not a juvenile specimen ( Aguirre-Fernandez and Fordyce 2014). Even though the zygomatic processes are incomplete, an estimated bizygomatic width was measured in order to infer the total length of this species. We applied the formula proposed by Pyenson and Sponberg (2011) for stem Platanistoidea : Log(L) = 0.92 × (log(BIZYG) – 1.51) + 2.49. The BIZYG for Aondelphis talen gen. et sp. nov. is 21.55 cm, giving a reconstructed body length of 2.13 m. It is of similar size to Waipatia maerwhenua (BIZYG: 24.4 cm, total length 2.4 m; Fordyce 1994), Huaridelphis raimondii (BIZYG: 20.7 cm, total length 2.05 m; Lambert et al. 2014) and adult males of Platanista gangetica (2.2 m; Jefferson et al. 2008). However, Aondelphis talen gen. et sp. nov. is smaller than the remaining platanistoids, such as Zarhachis flagellator (BIZYG: 26.8 cm, total length 2.6 m; Kellogg 1924), Squalodelphis fabianii (BIZYG: 25.6 cm, total length 2.5 m; Dal Piaz 1917) and Otekaikea marplesi (BIZYG: 25.7 cm, total length 2.5 m; Tanaka and Fordyce 2014).

Exoccipital ( Fig. 2 ): The occipital condyles are gently convex, with a smooth articular face and a short but clearly delimited pedicle that projects them posteriorly (greatest occipital breadth 91.98 mm). The ventral condyloid fossa is not clearly delimited. Ventrally, the paroccipital process is wide but eroded. There is a shallow fossa on the anterior face of this process, dorsal to the paroccipital process. In odontocetes, there are at least two bony correlates of the pterygoid sinus system in the exoccipital: one corresponds to the posterior sinus (in the anteroventral surface of the paroccipital process) and the other one to the posterolateral extension of the peribullary sinus (ventral surface of paroccipital process; Mead and Fordyce 2009). The identification of these bony correlates is confusing in the literature, as the posterior sinus fossa is variably developed ( Fordyce 1994; Fraser and Purves 1960). Based on the anatomical location of the fossa observed in Aondelphis talen gen. et sp. nov. on the anterior surface of the paroccipital process, it is tentatively identified as a posterolateral sinus fossa. Another interpretation is that the concavity on the ventral surface of the paroccipital process corresponds to the point of articulation with the stylohyal ( Fraser and Purves 1960; Mead and Fordyce 2009; Marx et al. 2016). There is a large fissure, similar to what was described for Otekaikea ( Tanaka and Fordyce 2014) and Awamokoa ( Tanaka and Fordyce 2017) on the exoccipital-squamosal suture anterior to the latter fossa ( Fig. 2B ).

Basioccipital ( Fig. 3 ): The basioccipital is ventrally trapezoideal (greatest length 85.3 mm), with no clear sutures with the basisphenoid. Dorsally, there is a distinct rounded pontine impression on the anterior portion of the basioccipital. Just posterolaterally, there are two dorsolateral projections followed by a deep elliptical fossa that is longer mediolaterally than dorsoventrally deep. They mark the anterior limit of the cerebellar lobe (sensu Anderson 1878). In ventral view, the basioccipital crest is transversely thick with a strong laterally projection on its posterior portion. Medially, the posterior portion of the basioccipital has a well-developed muscular tubercle for the insertion of the m. rectus capitis ventralis. The posteroventral margin of the basioccipital crest has a distinct and narrow depression, oriented dorsolaterally. A shallow depression on the lateral surface of the crest ( Fig. 3F ) indicates the probable medial extension of the peribullary sinus.

Parietal ( Fig. 2B ): There appears to be a small exposure of the parietal in ventral view, medial to the squamosal and posterior to the alisphenoid, at the basicranium, similar to what was described in Waipatia ( Fordyce 1994; Tanaka and Fordyce 2015b), Otekaikea ( Tanaka and Fordyce 2014, 2015a), cf. Papahu ZMT-73 ( Tanaka and Fordyce 2016) and Papahu taitapu ( Aguirre-Fernández and Fordyce 2014) . No distinctive features could be recognized.

Squamosal ( Figs. 2 , 4 ): Laterally, the short postglenoid process is robust and blunt, ventrally oriented. On the lateral surface of the zygomatic process (bizygomatic width: 215.5+ cm), dorsal to the external auditory meatus, there is a long circular rugose-surfaced neck muscle fossa (sensu Fordyce 1981); the posterior margin is formed by the exoccipital, like in cf. Papahu ZMT-73 ( Tanaka and Fordyce 2016). Like this latter species, there is a small fossa (probably for the digastric muscle) on the lateral surface of the post-tympanic process. A sigmoideal notch dorsal to the external auditory meatus is more visible on the left side. The same condition occurs in cf. Papahu ZMT-73 ( Tanaka and Fordyce 2016). The posteroventral portion of the temporal fossa (floor of temporal fossa or squamosal fossa sensu Lambert et al. 2015) is preserved in dorsal view and of small size.

In ventral view, the glenoid fossa is shallow. The tympano-squamosal recess is wide and very deep, delimited laterally by a distinct crest and medially by the broken base of the falciform process. It is longer anteroposteriorly and narrow lateromedially, and extends posteriorly at about the postglenoid process. The surface presents multiple striae anterolaterally to posteromedially oriented, and there is a low but distinct anteroposteriorly oriented crest that divides the tympano-squamosal recess in two portions; the medial portion is the smallest. Mesoplodon europaeus , M. mirus , and M. stejnegeri show a similar condition. Posteriorly, there is a shallow but distinct oval-shaped sigmoid fossa (sensu Geisler et al. 2005). The spiny process is broken, but the base is oval-shaped and descends to the start of the falciform process. The falciform process is better preserved on the right side; it is very thin and presents a sigmoidal shape, with a distinct notch just anterior to the spiny process. The falciform process is ventrally oriented, skewing slightly medially. The squamosal-alisphenoid suture is not very clear.

The periotic, when in situ, lies posterior to the falciform process, medial to the external auditory meatus, anterior to the posterior sinus fossa and lateral to the foramen ovale. The periotic fossa is triangular-shaped, apparently formed only by the squamosal. The low supratubercular ridge, more distinct on the medial area of the fossa, divides it in shallow anterior and posterior portions. A circular foramen spinosum opens anteriorly, on the medial margin of the anterior portion of the periotic fossa, just lateral to the most posterior portion of the alisphenoid-squamosal suture. It resembles the condition found in Waipatia maerewhenua ( Fordyce 1994) . A distinct and wide path for the mandibular nerve (V3) is observed on the alisphenoid on the right side, which runs mediolaterally at an oblique angle. Unfortunately, the foramen ovale was not preserved. Posteromedial to the periotic fossa, there is a concave surface ( Fig. 2B 2 : fossa?) of unknown homology or function.

The external auditory meatus is long, slit-like and deep (similar to Platanista gangetica ; Anderson 1878), slightly wider laterally and delimited by distinct anterior and posterior meatal crests. Posterior to the external auditory meatus is the post-tympanic process (better preserved on the right side), which provides an area of contact with the posterior process of the tympanic bulla. There is also a small fossa just medial to this process, here interpreted as the area of contact with the posterior process of the periotic. The post-tympanic process is shorter lateromedially than anteroposteriorly.

Basisphenoid ( Fig. 3 ): Only a portion of this bone was preserved, including the large oval ventral carotid foramen on the lateral surface of the basiooccipital crest. The foramen appears to have been covered by the peribullary sinus. There is no visible suture with the basioccipital and thus, its extension cannot be inferred.

Alisphenoid ( Fig. 2B ): There is a small portion preserved in the basicranial region, lateral to the squamosal and anterior to the parietal. On the left side of the skull the groove for the mandibular nerve is preserved. The exposure of this bone is longer anteroposteriorly than lateromedially wide.

Periotic ( Figs. 4–5 ): For description purposes, the isolated periotic was placed sitting in stable position on a flat surface with the internal acoustic meatus facing dorsally, to produce a dorsal view. The periotic has a short and wide anterior process, wider posterior process, a dorsoventrally inflated pars cochlearis and in dorsal view, the periotic has a crescentic outline. When in place on the skull, the anterior process is roughly parallel with the anteroposterior axis, whilst the posterior process is posterolaterally oriented.

The anterior process of the periotic is anteroposteriorly short, with a narrow apex (similar to Waipatia maerewhenua and cf. Papahu ZMT-73) but wider at its base ( Table 1). It is anteriorly oriented with a strongly concave anterodorsal angle and an oval-shaped anterior bullar facet. In ventral view, the sigmoidal parabullary sulcus (sensu Tanaka and Fordyce 2014) is deep and more elongated on its posterior portion. When articulated ( Fig. 4 ), the parabullary sulcus is medial to the falciform process. Posterior to this is the shallow anteroexternal sulcus, which does not reach the dorsal crest. Between the anteroexternal sulcus and the lateral tuberosity is a small circular fossa ( Fig. 5B 2 : fossa?) that might represent an area of contact of the sigmoid process of the tympanic bulla. The small and shallow anterior bullar facet has an elliptical outline with low but distinct margins. The fovea epitubaria is a rounded and deep depression placed between the mallear fossa and the anterior bullar facet. The mallear fossa is rounded and deep, medial to the rounded lateral tuberosity. The latter, though not markedly inflated, projects outside the outline of the periotic in dorsal view, like W. maerewhenua ( Fordyce 1994) , Otekaikea ( Tanaka and Fordyce 2014, 2015a), Papahu taitapu ( Aguirre-Fernández and Fordyce 2014) and Awamokoa tokarahi ( Tanaka and Fordyce 2017) . When the periotic is articulated, the lateral tuberosity lies medial to the notch on the base of the falciform process of the squamosal.

In dorsal view, the body of the periotic has a vestige of a dorsal crest, that extends anteriorly up to the base of the anterior process; a concave surface ( Fig. 5A 2 : concave surface) is medial to this crest and anterior to the anterior incisure. In Otekaikea and Waipatia maerewhenua the dorsal crest is more conspicuous ( Fordyce 1994; Tanaka and Fordyce 2014, 2015a). The anterior incisure (groove for the tensor tympani muscle) is a shallow and narrow sulcus between the anterior process and the pars cochlearis, extending posteriorly on the ventral surface towards the fenestra ovalis like in W. maerewhenu a and Otekaikea ( Fordyce 1994; Tanaka and Fordyce 2014, 2015a). It also extends dorsally towards the dorsal crest. Posteriorly, there is a small hiatus Fallopii for the exit of the major petrosal nerve. The apex of the anterior process presents a marked groove, of unknown homology or function ( Fig. 5A, C, E : groove on anterior process).

The pars cochlearis is dorsoventrally thin, longer anteroposteriorly than mediolaterally, with a rounded anterior margin and a straight posterior margin ( Table 1). The internal acoustic meatus is piriform and wider posteriorly, and opens slightly anteriorly in dorsal view ( Fig. 5C ). It comprises four foramina, which open deeply: (i) the area cribosa media, (ii) the spiral cribiform tract, (iii) the foramen singulare and (iv) the proximal opening of the facial canal. The latter is separated from the elliptical foramen singulare by a short but distinct transverse crest. The spiral cribiform tract is the largest of the four foramina and is separated from the foramen singulare by a thin and high crest ( Fig. 5C 2 : crest), as also observed in Otekaikea ( Tanaka and Fordyce 2014, 2015a). The large and subcircular aperture for the cochlear aqueduct (anteroposterior length 1.93 mm, width 2.36 mm) opens dorsomedially on the posterior portion of the pars cochlearis. The small and oval-shaped aperture for the vestibular aqueduct (anteroposterior length 1.37 mm, width 1.76 mm) is widely separated from the former. A shallow median promontorial groove is observed on the medial margin of the pars cochlearis. In posterior view, the fenestra rotunda is small and has a trapezoid-like outline, with a short fissure on its medial edge towards the aperture of the cochlear aqueduct. Ventrally, and medial to the prolonged stapedial muscle fossa, there is a concave surface ( Fig. 5F 2 : fossa? 3) of unknown homology or function. There is no caudal tympanic process.

In ventral view, the round fenestra ovalis has some portion of the stapes preserved. Laterally, the rounded distal opening of the facial canal has a facial sulcus extending posteriorly on the medial margin of the posterior bullar facet. This sulcus is delimited laterally by the facial crest, followed by the parafacial sulcus (sensu Tanaka and Fordyce 2016), a deep groove that widens posteriorly. The fossa incudis is shallow and elliptical. The deep fossa for the stapedial muscle has a rhomboidal outline, extending posteriorly on the dorsomedial surface of the posterior process. The wide epitympanic hiatus has a small and deep fossa ( Fig. 5B 2 : fossa? 2) posteriorly, along the anterior margin of the posterior process. When in situ, this fossa housed the spiny process of the squamosal.

The posterior process has a rectangular outline ( Table 1) and it is posterolaterally oriented. In ventral view, the posterior bullar facet has a smooth and wide surface, deeper anteromedially and with a few shallow grooves. On the lateral surface of this process, there are two deep grooves separated by a sharp crest; we infer that this area presumably articulated with the post-tympanic process of the squamosal (see above). Similar structures are present in one specimen of Pomatodelphis cf. inequalis (USNM 13768), referred to Zarhachis cf. flagellator by Muizon (1987: fig. 6, 14d), and in Otekaikea huata ( Tanaka and Fordyce 2015a) . Anteromedially, there is a minute posteroexternal foramen, more anteriorly located than in Waipatia ( Fordyce 1994) and Otekaikea ( Tanaka and Fordyce 2014, 2015a). No articular rim (sensu Muizon 1987) could be recognized and the transverse groove is almost indistinct.

Tympanic bulla ( Fig. 6 ): For the purpose of description, the dorsal view is defined as the position of the tympanic bulla when the ventral face is sitting on a flat surface. In general, the tympanic bulla is wider posteriorly and narrows anteriorly in dorsal view, with a heart-shaped outline and a thin involucrum. It is incomplete, with two additional detached fragments.

In dorsal view, the anterior portion of the tympanic tapers slightly anteriorly, which could suggest the presence of an anterolateral convexity and notch but is not possible to ascertain its condition because this area was not completely preserved. The medial profile is straight and flattened in posterior view. The involucrum is nearly straight, narrower anteriorly and dorsoventrally short, with a marked depression approximately 6mm anterior to the base of the inner posterior prominence. The involucrum presents smooth transverse ridges on its dorsal surface (like in Waipatia maerewhenua and Otekaikea marplesi ; Fordyce 1994; Tanaka and Fordyce 2014). Medially, a low but distinct crest marks the dorsal limit of the peribullary sinus ( Fraser and Purves 1960; Mead and Fordyce 2009). In dorsal view, a small tubercle projects medially from the involucrum, but the expected ridge that would divide the tympanic cavity is absent. Anteriorly, the involucrum gently tapers into the tympanic cavity, which is relatively large.

In posterior view, the tympanic bulla is bilobed, with a deep and medially oriented interprominential notch. The lateral margin of the tympanic bulla is slightly convex. Because the area is not properly preserved, no inferences can be made regarding the size and shape of the elliptical foramen. In ventral view, the inner posterior prominence is narrow mediolaterally and slightly longer dorsoventrally, whilst the outer posterior prominence is wider and projects further posteriorly ( Table 1). There is no horizontal ridge between the two prominences. Ventrally, from the interprominential notch, the median furrow extends anteriorly up to the center of the ventral surface of the tympanic bulla ( Fig. 6B ), with a sigmoid profile as seen in Waipatia maerewhenua ( Fordyce 1994) and Platanista gangetica ( Anderson 1878) . Both the furrow and ventral surface are rugose. The ventral keel is almost indistinct.

In dorsal view, the posterior process is posterolaterally oriented ( Table 1) and three articular surfaces may be recognized. Dorsally, the smooth surface for articulation with the posterior process of the periotic is the largest, delimited by two thin crests. Anterolaterally, there is a rugose and deep surface, possibly for articulation with the posterior meatal crest of the squamosal. The most lateral portion of this process has a surface of articulation with the post-tympanic process of the squamosal. This contact is inferred due to erosion of the region.

There are two loose fragments of tympanic bulla ( Fig. 6G, H ). One fragment contains the region from the sigmoid process to the accessory ossicle (greatest length 27.08 mm; greatest width 9.45 mm). The sigmoid process, though broken, has a squared profile (greatest width 8.42 mm; greatest height 7.11 mm) and thick edges. Just anteriorly, is a short mallear ridge (greatest length 3.45 mm), with the apex broken. The accessory ossicle is anteroposteriorly elongated and small (greatest length 5.98 mm; greatest width 4.43 mm). It has a minute oval-shaped foramen on its dorsal surface, and another one on its anterior surface. A small concave surface anterolateral to the accessory ossicle could correspond to the most-dorsal part of the lateral furrow. The other fragment of tympanic bulla preserved appears to correspond to the outer lip, although no contact surface was found.

A

Stem Odontoceti calvertensis talen maerewhenua Waipatiidae hectori tokarahi huata marplesi

Squalodelphis fabianii

Phocageneus venustus “ Squalodelphinidae ” Platanista vanbenedeni gangetica

Pomatodelphis inaequalis Platanistidae Zarhachis flagellator emlongi bossi

Physeteroidea

Ziphioidea

76 Delphinida

B

Stem Odontoceti talen calvertensis tokarahi

huata marplesi maerewhenua hectori Waipatiidae gangetica Pomatodelphis inaequalis Platanistidae flagellator

22670

Squalodelphis fabianii

Phocageneus venustus “ Squalodelphinidae ” vanbenedeni taitapu bossi emlongi

76 Delphinida

Figures

Fig. 2. Partial skull of the holotype of the platanistoid dolphin Aondelphis talen gen. et sp. nov. (MPEF-PV 517) from the early Miocene Gaiman Formation, Bryn Gwyn, Patagonia, Argentina; in dorsal (A), ventral (B), posterior (C), and lateral (D) views. Continuous lines indicate sutures, whilst dashed lines indicate outlines of a particular structure. Hatched outlines show broken areas of the specimen. Photographs (A1–D1), photographs with explanations (A2–D2).

Fig. 3. The portion of basioccipital of the holotype of the platanistoid dolphin Aondelphis talen gen. et sp. nov. (MPEF-PV 517) from the early Miocene Gaiman Formation, Bryn Gwyn, Patagonia, Argentina; in dorsal (A), ventral (B), and lateral (C) views. Dashed lines indicate specific structures. Photographs (A1–C1), photographs with explanations (A2–C2).

Fig. 4. Partial skull and periotic in situ of the holotype of the platanistoid dolphin Aondelphis talen gen. et sp. nov. (MPEF-PV 517) from the early Miocene Gaiman Formation, Bryn Gwyn, Patagonia, Argentina; left side in ventral view. Dashed lines indicate specific structures (black, skull; white, periotic), whilst continuous lines indicate sutures. It should be noted that the position of the periotic does not take into account the volume that may have occupied the peribullary sinus and other soft tissue structures. Photograph (A), photograph with explanations (B).

Fig. 5. Left periotic of the holotype of the platanistoid dolphin Aondelphis talen gen. et sp. nov. (MPEF-PV 517) from the early Miocene Gaiman Formation, Bryn Gwyn, Patagonia, Argentina; in ventral (A), medial (B), dorsal (C), lateral (D), anterior (E), and posterior (F) views. Dashed lines indicate specific structures. Photographs (A1–F1), photographs with explanations (A2–F2).

Fig. 6. Left tympanic bulla of the holotype of the platanistoid dolphin Aondelphis talen gen. et sp. nov. (MPEF-PV 517) from the early Miocene Gaiman Formation, Bryn Gwyn, Patagonia, Argentina; in dorsal (A), medial (B), ventral (C), lateral (D), anterior (E), and posterior (F) views. Isolated fragments sigmoid process (G) and a portion of outer lip (H); in posterior (G1, H1) and anterior (G2, H2) views. Dashed lines indicate specific structures. Hatched outlines show broken areas of the specimen. Photographs (A1–F1), photographs with explanations (A2–F2, G, H).

Fig. 1. Map and stratigraphic section of Bryn Gwyn, the type locality of Aondelphis talen gen. et sp. nov. A. Regional map of the study area. B. Detailed map with stratigraphic information of the type locality (dolphin outline). C. Simplified stratigraphic section of Bryn Gwyn, modified from Scasso and Bellosi (2004). Dolphin outline indicates type horizon of Aondelphis talen gen. et sp. nov. Trelew Mb., Trelew Member of Sarmiento Formation.