The hyoid apparatus is essential for underwater feeding in marine tetrapods, but it is unclear whether this complex has evolved as convergently as other traits, such as dentition or locomotion. Here we compare the ossified hyoid elements in ophthalmosaurid ichthyosaurs and odontocete cetaceans, two groups with an overall similar body shape, to understand whether the hyoid elements show any signs of convergence in the context of feeding. We examined three types of data (size, morphology, and internal bone microstructure) in ophthalmosaurid and odontocete taxa in which these elements are preserved. Our data show that ichthyosaurs never experienced a shift in feeding mode, which might indicate that their hyoid apparatus never adapted to suction feeding. Also, the internal microstructure of the two animal groups differs, where the odontocetes have an overall less compact structure, ophthalmosaurid ichthyosaurs have cancellous inner cones in an outer, more compact sheath. These differences are likely explained as biomechanical adaptations to different feeding modes. Thus, the hyoid changed less, and acted more as a constraint for feeding innovation in ichthyosaurs compared to cetaceans, and through a much longer time span (over 150 million years).

Thirteen ophthalmosaurid specimens with one or both hyoid rods preserved were studied (Table 1), out of which 11 preserved at least one complete hyoid so that measurements could be used for calculations. For eight out of the thirteen specimens, anterior-posterior orientation was known. To compare the ophthalmosaurids to the published data on non-ophthalmosaurids (Triassic-Middle Jurassic; Motani et al. 2013), the same measurements were collected from publications, colleagues and museum visits; anteroposterior length in a straight line (HL), and width taken half-way (HW). Both were log10-transformed for analysis. Hyobranchial robustness was calculated as in Motani et al. 2013 log10 HL: log10 HW, similar to the “stylohyal robustness index” in Bloodworth and Marshall (Bloodworth and Marshall 2007) and in Johnston and Berta (2010). Among the ophthalmosaurid specimens with hyoids (Table 1), none were preserved in articulation in such a way that mandibular width (MW of Motani et al. 2013) nor width at the end of the toothrow (TW of Motani et al. 2013) could be measured. This limitation also meant that the metric Mandibular Pressure Concentration Index could not be calculated for the ophthalmosaurids.

To compare hyoid robustness in ichthyosaurs to those in tooth whales, we assembled a dataset with measurements taken from osteological museum collections in Bergen, Oslo, and Copenhagen (66 specimens) and measurements previously used in Johnston and Berta (2010, 102 specimens). For comparison to the ichthyosaur hyoid elements, the stylohyals were used, because they represent the suspensory portion of the hyoid apparatus.

For discussing the shape of the ophthalmosaurid hyoid, the following morphological data were collected from personal observations and the literature: cross-section in anterior end, midway and in the posterior end; amount of curvature and the position of the ventralmost point of the curvature; whether the element had the same width throughout, or the ends were expanded. Four specimens from the Late Jurassic- Early Cretaceous Slottsmøya Member at Spitsbergen were studied in detail: PMO 222.654 (Janusaurus lundi), PMO 222.669 (Palvennia hoybergeti), PMO 222.667 (Keilhauia sp.) and one Ophthalmosauridae indet. (Delsett et al. 2018, Delsett et al. 2019, Roberts et al. 2014). Note that even if some taxonomic issues are not fully resolved, all are nested within Ophthalmosauridae (Delsett et al. 2019, Zverkov and Prilepskaya 2019). Morphological data and the close-up study of four specimens are given in Supplementary material (Table and Text).

In this Dryad submission:

- supplementary text files with data on morphology of ichthyosaur hyoids (two word files)

- supplementary table with measurement data on cetaceans (excel file)

- the ichthyosaur measurement data also including non-ophthalmosaurids, which are the underlying data for these calculations (excel file)