The macroevolutionary landscape of short-necked plesiosaurians

Throughout their evolution, tetrapods have repeatedly colonised a series of ecological niches in marine ecosystems, producing textbook examples of convergent evolution. However, this evolutionary phenomenon has typically been assessed qualitatively and in broad-brush frameworks that imply simplistic macroevolutionary landscapes. We establish a protocol to visualize the density of trait space occupancy and thoroughly test for the existence of macroevolutionary landscapes. We apply this protocol to a new phenotypic dataset describing the morphology of short-necked plesiosaurians, a major component of the Mesozoic marine food webs (ca. 201 to 66 Mya). Plesiosaurians evolved this body plan multiple times during their 135-million-year history, making them an ideal test case for the existence of macroevolutionary landscapes. We find ample evidence for a bimodal craniodental macroevolutionary landscape separating latirostrines from longirostrine taxa, providing the first phylogenetically-explicit quantitative assessment of trophic diversity in extinct marine reptiles. This bimodal pattern was established as early as the Middle Jurassic and was maintained in evolutionary patterns of short-necked plesiosaurians until a Late Cretaceous (Turonian) collapse to a unimodal landscape comprising longirostrine forms with novel morphologies. This study highlights the potential of severe environmental perturbations to profoundly alter the macroevolutionary dynamics of animals occupying the top of food chains.


SUPPLEMENTARY PHYLOGENETIC INFORMATION
We modified the matrix of Fischer et al. 1 by adding six species: Rhaeticosaurus mertensi 2 , Acostasaurus pavachoquensis 3 , Pliosaurus patagonicus 4 , Pliosaurus almanzaensis 5 , Sachicasaurus vitae 6 , and Kronosaurus boyacensis 7 . We also modified the scores of Stenorhynchosaurus munozi according to 8 , except for the following four characters, for which we retained the scores of 1,9 : • #49. The parietal vault is poorly preserved and the character scores appears as ambiguous from the data provided in 8 . We scored this character as "?". • #56. We think that the bone interpreted as a jugal in 8 contains parts of the squamosal, which is especially evident in Figure 3C of 8 , where the bone interpreted as a jugal forms most of squamosal arch. If true, this would mark a clear departure from the morphology observed other derived pliosaurids [9][10][11] . Moreover, the jugal-squamosal connection appears to be broken off. For these reasons, we conservatively scored this character as "?". • #127. We kept the score "1", because the dorsal portion of the surangular still appears blade-like (Figure 2A in 8 ). • #130 We kept the score "0", because the dorsal surface of the articular does not seemingly form a concave surface (Figures 2 and 3 in 8 ). Table S1. Taxonomic sampling and completeness ratio. Manemergus anguirostris was removed prior to the analyses because of the (very) young ontogenetic stage of the type and only specimen 1,12 ; we decided to keep the data here for further use. Taxa in grey did not pass the 50% completeness threshold and were also removed prior to the analyses. Surface scanned taxa are marked with an *; see supplementary information to download the 3D models.  Table S2. Morphometric ratios and measurements used to quantify Baupläne, with description of function or characterisation justification.

Name Explanation Completeness HL_mandible
Height of the mandible (at the level of the coronoid process) ÷ length of mandible. Characterises maximum aspect ratio for jaw; proxy for the maximum dorsoventral flexural stiffness of the jaw 43-45 .

Retro_coro
Length of retroarticular process ÷ height of the mandible at the level of the coronoid process. Characterises the leverage to open the jaw against the drag of the water [46][47][48] .

Snout_width
Skull width just anterior to the orbit ÷ mandible length. Characterises skull shape and is related to (i) resistance to lateral shaking 49 , and (ii) to the volume of water needed to be expelled to close the jaws 50 .

Rel_snout
Snout length ÷ mandible length. Characterises snout shape and is related to (i) resistance to lateral shaking 49 , (ii) to the volume of water needed to be expelled to close the jaws 50 , and (iii) to the amount of drag during swimming 51 .

Rel_Symphysis
Symphyseal length ÷ mandible length. Characterises the shape of the snout and the mechanical response of the anterior jaw to dorsoventral, mediolateral and rotational loads during biting 44,45,52 .

77.5%
Symph_teeth_density Number of symphyseal teeth ÷ symphysis length. Proxy for the preferential use of the anterior jaw for prey capture 45 .

Crown_height
Maximum absolute crown height. Absolute crown size has been shown to be a major determinant of diet in modern cetaceans 53 .

Hum/Fem
Humerus proximodistal length ÷ femur proximodistal length. Characterises the appendicular body plan. Humeri and femora are known to reduce in length with increased employment for swimming 57,58 ; the length of the humerus and femur relative to one another may therefore be considered as a proxy for fore-limb driven, hind-limb driven or equally driven underwater locomotion.

Mand/fem
Mandible length ÷ femur proximodistal length. Animals with relatively long limbs and small heads will swim, accelerate, and catch food in distinct manner to animals with relatively large heads and small limbs 59,60 . This character is therefore a proxy for elongation of limbs relative to skull size.

Neck/skull
Neck length ÷ mandible length. Characterises the general body plan; proxy for potential feeding arc and reach of cranium away from the trunk 61 . Table S3. PERMANOVA p-values, testing the strength of the main clusters (cut =2) and increasingly smaller subclusters (cut = 3:10).