Histological sampling of the specimen MPM-PV 1002. From D’Angelo et al., 2023.
On December 10, 1823, Mary Anning discovered the first complete Plesiosaur skeleton at Lyme Regis in Dorset. Noticed about the oddity of the specimen, George Cuvier wrote to William Conybeare suggesting that the find was a fake produced by combining fossil bones from different animals. But despite their unusual body plan Plesiosaurs were a highly successful group of Mesozoic marine reptiles. Their four limbs are enlarged and modified as propulsive flippers, the trunk is short and stiff, and proportional head size seems to vary inversely with neck length.
Elasmosaurids are a clade of plesiosaurs with extremely long necks, and represents one of the two groups of plesiosaurs to reach the end of the Cretaceous period. A new study by a team from the Argentine Natural Science Museum (MACN) describes the osteohistology of an articulated skeleton of a new and still unnamed elasmosaurid (MPM-PV 1002) from Santa Cruz province, Argentina.
Medial gastralia of MPM-PV 1002. From D’Angelo et al., 2023.
The study of bone microstructure in plesiosaurians started in the late 19th and early 20th centuries. in Europe, with the description of the bone histology of Plesiosaurus by W. Kiprijanoff in 1881. A century later, the introduction of hard plastic resins, the development of tungsten carbide microtome blades, the use of very thin diamond-edged saw blades, and the examination of bone tissue with surgically implanted orthopedic devices fostered new methods for studying the histology of fully mineralized bone.
The paleohistological study of MPM-PV 1002 include thin sections of femur, phalanx, vertebral apophyses, dorsal rib 4, and lateral and medial gastralia. Those sections are characterised by scarce primary bone, and high vascularisation with longitudinal canals forming secondary osteons, indicative of rapid growth. The most striking feature of the new specimen is the presence of an external fundamental system (EFS), a band of slow-growing tissue, deposited in the periphery of the bone cortex in the gastral elements indicative of the somatic maturity of the animal. The degree of remodelling in MPM-PV 1002 is notably complex and contradicts previous hypotheses that supported that bone mass and density decrease with age.
References:
Julia Soledad D’Angelo, Jordi Alexis Garcia Marsà, Federico Lisandro Agnolín & Fernando Emilio Novas (2023): Biological implications of the bone microstructure of a new elasmosaurid (Sauropterygia, Plesiosauroidea) from the uppermost Cretaceous of Patagonia, Historical Biology, DOI: 10.1080/08912963.2023.2180744.
De Ricqlès (1969) De Ricqlès A. L’histologie osseuse envisagée comme indicateur de la physiologie thermique chez les tétrapodes fossiles. Comptes Rendus Hebdomadaires des Séances de l’Academie des Sciences, Serie D: Sciences Naturelles. 1969;268:782–785
Letters from Gondwana. 