Basilosauridae Cope 1868

Abstract

Family BASILOSAURIDAE Cope, 1868 Basilosauridae Cope, 1868: 144. Zeuglodontidae Bonaparte, 1849: 618 (type genus junior synonym of Basilosaurus). Hydrarchidae Bonaparte, 1850: 1 (type genus junior synonym of Basilosaurus). Stegorhinidae Brandt, 1873: 7. Prozeuglodontidae Moustafa, 1954: 87. TYPE GENUS. — Basilosaurus Harlan, 1834 by original designation. DIAGNOSIS. — Middle to Late Eocene cetaceans that differ from Neoceti in having a closed mesorostral groove (Muizon et al. 2019), a less anteriorly inclined occipital shield (Muizon et al. 2019), the absence of an antorbital process on the maxilla (Muizon et al. 2019), and presence of mobile elbows (Martínez-Cáceres et al. 2017; Muizon et al. 2019). Basilosauridae differ from Kekenodontidae by the lack of a separate third root on all cheek teeth (Corrie & Fordyce 2022), retention of a mobile elbow (Corrie & Fordyce 2024: suppl. mat.) and the lack of a tapering region on the anterior tympanic bulla (Corrie & Fordyce 2022). An undescribed but figured kekenodontid skull shows the bones nares being further retracted to the posterior quarter of the rostrum, a reduced sagittal crest, and an occipital shield which is more anteriorly inclined such that the apex is located at the posterior margin of the temporal fossa (Clementz et al. 2014). Basilosaurids possess the following autapomorphies: prominent pterygoid sinuses, a palate that narrows at the level of P4 (Martínez-Cáceres et al. 2017), and the presence of reentrant grooves on the lower molars (Corrie & Fordyce 2022). DISCUSSION The absence of the upper third molar is often cited as an autapomorphy of Basilosauridae (e.g., Uhen 2013a; Martínez-Cáceres et al. 2017), but this feature has also been observed in the basal mysticete Mystacodon Lambert, Martínez-Cáceres, Bianucci, Di Celma, Salas-Gismondi, Steurbaut, Urbina & Muizon, 2017, and basal odontocetes from the family Simocetidae (Vélez-Juarbe 2017). A taxonomic history of the type genus Basilosaurus is given in Kellogg (1936). Corrie & Fordyce (2022) note many features of the periotic to separate basilosaurids from kekenodontids, which are not considered here as the periotic is not discussed in depth. CRANIAL VARIATION IN BASILOSAURIDAE Ontogenetic variation Ontogenetic trends could be studied only in Dorudon atrox. Three out of six specimens available for D. atrox were mature, and three were immature.Maturity was assessed mostly through size and dental eruption sequence.The dental eruption of basilosaurids is outlined in Uhen (2000). CGM 9317 (Figs 3A; 4A) represents the youngest individual, shedding deciduous incisors and being smallest in size. RGM.74261 (Figs 3B; 4B) and BSPG.1904.XII.134e (Figs 3C; 4C) represent older juveniles. BPSG.1904.XII.134e possesses permanent incisors but has yet to shed deciduous premolars. RGM.74261 still possesses deciduous P3-4. Based on the development of ontogenetically variable features, it appears that RGM.74261 represents a younger individual than BPSG.1904.XII.134e. Juvenile D. atrox show a stouter rostrum lacking several diastemata present in adult specimens. There are no diastemata between the premolars of CGM 9317 (Fig. 4A), and the diastemata between the incisors are smaller than in the adult specimens. In the intermediary specimens (Fig. 4B, C) the posteriormost diastema is located between P2 and P3, but the diastemata between more anterior teeth are still smaller than in adults. Interestingly, the overall proportions of the rostrum show no change. The rostrum of CGM 9317, the youngest specimen in the sample, measures 56% of total skull length, that of adult UMMP VP 118139 measures 57%. The anterior rostrum (posterior rim of bony nares to rostral tip) elongates from 28% of skull length in CGM 9317, to c. 35% in adults (UMMP VP 1183139, NHMUK M.10228). The supraorbital process of juvenile specimens is also less wide mediolaterally than in adults. The nuchal and sagittal crest grow more prominent with age. The temporal openings (as seen in dorsal view) likewise increase in size from 18% in CGM 9317 to c. 25% in adult specimens. When observed in dorsal view, the nuchal crest appears to become more convex with increasing age. Interestingly, while all these landmarks show marked changes in morphology during ontogeny, the relative placement of the narial fossa, and relative length of the rostrum and temporal regions remains relatively constant. Little ontogenetic variation was detected in the layout of the posterior narial region. A small exception is that the anterior processes on the frontals are wider in mature than in immature individuals.

Authors 3

1. Jonas W. P. Hakkens

   Utrecht University

   Affiliation as printed

   Utrecht University, Faculty of Geosciences, Department of Earth Sciences, Princetonlaan 8 a, 3584 CB Utrecht (the Netherlands) j. w. p. hakkens @ students. uu. nl and jonashakkens @ gmail. com (corresponding author)

2. Jelle W. F. Reumer Aachen Faculty of Geosciences Department of Earth Sciences

   Utrecht University · Naturalis Biodiversity Center · Natural History Museum Rotterdam

   Affiliation as printed

   Utrecht University, Faculty of Geosciences, Department of Earth Sciences, Princetonlaan 8 a, 3584 CB Utrecht (the Netherlands) and Naturalis Biodiversity Center, Darwinweg 2, 2333 CR, Leiden (the Netherlands) and Natural History Museum Rotterdam, Westzeedijk 345, 3015 AA, Rotterdam (the Netherlands) j. w. f. reumer @ uu. nl

3. Anne S. Schulp Aachen Faculty of Geosciences Department of Earth Sciences

   Utrecht University · Naturalis Biodiversity Center · Teylers Museum

   Affiliation as printed

   Utrecht University, Faculty of Geosciences, Department of Earth Sciences, Princetonlaan 8 a, 3584 CB Utrecht (the Netherlands) and Naturalis Biodiversity Center, Darwinweg 2, 2333 CR, Leiden (the Netherlands) and Teylers Museum, Spaarne 16, 2011 CH, Haarlem (the Netherlands) a. s. schulp @ uu. nl