DOI: 10.5061/dryad.n02v6wx20
title: Data for: Is the hyoid a constraint on innovation? A study in convergence driving feeding in fish-shaped marine tetrapods

description: 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).
description: 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 (<em>Janusaurus lundi</em>), PMO
 222.669 (<em>Palvennia hoybergeti</em>), PMO 222.667
 (<em>Keilhauia</em> 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).
description: 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)