Identifying behavioural and functional groups using a geometric morphometric analysis of plesiosaur flipper morphology

  • Laura Austin Sydes

Student thesis: Master of Philosophy

Abstract

Plesiosaurs first evolved in the Late Triassic and diversified greatly until their demise in the end Cretaceous. During this time, they gained global distribution and could be found in many different marine habitats due to their locomotor adaptations to a fully marine lifestyle. Throughout their existence they diverged into two different morphotypes – the plesiosauromorphs with elongate necks and small heads and the pliosauromorphs with large heads upon short necks. These morphotypes are believed to reflect different predatory styles, with plesiosauromorphs interpreted to be cruising ambush predators and pliosauromorphs are believed to be fast pursuit predators. These morphotypes are not restricted to the Plesiosauroidea and Pliosauroidea clades and pliosauromorphs have been found to have evolved several times within Plesiosauria. There have been several ecological and locomotory interpretations of plesiosaurs including of their estimated swimming speeds and feeding preferences. This thesis investigates whether different ecological and locomotory behaviours are expressed as variations in flipper morphology due to the fact that plesiosaurs were limb-dominant locomotors. Additional variation in flipper morphology was predicted to occur due to taxonomic and temporal variations in diversity. The variation in flipper morphology was analysed using 2D geometric morphometrics in which specimens (n) were assessed using flipper bone proportions (n = 38), flipper shape (n = 54) and propodial shape (n = 70) using 38, 59, and 59 landmarks respectively. Ecological, taxonomic and temporal variation in flipper morphology was assessed using classifying groups. These included morphotype (plesiosauromorph, pliosauromorph); speed ('Fast', 'Medium', 'Slow'), tooth guilds ('Pierce', 'General', 'Cut'), period (Jurassic, Cretaceous) and family (Plesiosauridae, Pliosauridae, Cryptoclididae, Rhomaleosauridae, Leptocleididae, Polycotylidae and Elasmosauridae). Principal component analysis using these flipper descriptors and grouping variables revealed that assessing the flipper element proportions yielded the best separation of groups. MANOVA of these plots revealed significant differences between these groups. Flipper shape and propodial shape also produced some significant results during MANOVA, but the visual separation of groups was much weaker. This indicates that overall flipper shape varies little between plesiosaurs, however within the shape there is a greater degree of variation in bone proportions. This suggests that although the bony elements vary in shape and size they converge to form an idealised flipper shape that is shared amongst plesiosaurs. Future work may seek to assess the convergent relationship of flipper bone proportions further. The results highlighted issues in the current speed estimations for some plesiosaurs, in that the current speed estimations calculated using limited body measurements may have incorrectly classified some plesiosaur families. Meaning, some presumed fast plesiosaurs were estimated to have medium speed and some presumed slower plesiosaurs were estimated to be capable of fast locomotion. If future research is carried out into the speed estimations for plesiosaur groups then this could aid in increasing accuracy of the conclusions.
Date of Award1 Aug 2022
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorWilliam Sellers (Supervisor)

Keywords

  • Plesiosaur
  • Geometric Morphometrics
  • Functional Morphology
  • Flippers
  • Palaeontology

Cite this

'