Unusual histology and morphology of the ribs of mosasaurs (Squamata)

Abstract

We report the presence of two previously unrecognized features in the dorsal ribs of mosasaurs: first, the presence of extremely dense, pervasive extrinsic fibres (anchoring soft tissue to bone, sometimes called Sharpey's fibres); and second, high intraspecific variation in costal bone compactness. Extensive extrinsic fibres are developed in the dorsal ribs of the mosasaurs Tylosaurus proriger and Eonatator sternbergi. The dorsal ribs of these mosasaurs are also characterized by a longitudinally ridged texture that almost completely covers the bone. Pervasive extrinsic fibres and ridged textures are absent in the mosasaur Selmasaurus russelli as well as the dorsal ribs of extant semi‐aquatic reptiles (e.g. crocodyliforms) and mosasaurs' close extant relatives and analogues (e.g. snakes and varanids). Similar ridged textures characterize the dorsal ribs of several other mosasaur taxa but are developed to a lesser extent (e.g. Mosasaurus, Clidastes, Platecarpus and Ectenosaurus), but in no other taxa have pervasive extrinsic fibres been reported. We interpret these osteohistological features in T. proriger and E. sternbergi as evidence of tendinous attachment of extensive and highly differentiated axial musculature capable of producing great stresses, most likely related to stabilization of the trunk relative to contralateral movements of the tail during carangiform locomotion. We also report the compactness indices (percentage of space occupied by bone rather than cavities) for these large mosasaur ribs, which are much higher than previously reported. This suggests high intraspecific variation in bone compactness that complicates its use in reconstructing mosasaur palaeoecology.

Microanatomical diversity of amniote ribs: an exploratory quantitative study

“…Finally, it has also been observed for some tetrapod species that rib compactness varies in a single element between the proximal head and the distal end (Waskow & Sander, ; D'Emic, Smith & Ansley, ; Houssaye et al ., ). Nonetheless, Waskow & Sander () noted that rib compactness is most uniform along the rib shaft for sauropod dinosaurs.…”

Estimating the evolutionary rates in mosasauroids and plesiosaurs: discussion of niche occupation in Late Cretaceous seas

“…A number of studies have thoroughly assessed the swimming abilities in plesiosaurs and discussed the differences in the two main plesiosaur 'body plans' -the 'long-' and 'short-necked' forms (e.g., Frey & Riess, 1982;Tarsitano & Riess, 1982;Godfrey, 1984;Halstead, 1989;Nicholls & Russell, 1991;Lingham-Soliar, 2000;O'Keefe, 2001;Carpenter et al, 2010;Liu et al, 2015;Muscutt et al, 2017;Noè, Taylor & Gómez-Pérez, 2017;Troelsen et al, 2019). The same applies to mosasauroids whose swimming abilities and especially their origin have been assessed through detailed studies of various aspects of their anatomy (see, e.g., Lindgren, Jagt & Caldwell, 2007;Lindgren et al, 2010;Lindgren, Polcyn & Young, 2011;Konishi et al, 2012;LeBlanc, Caldwell & Lindgren, 2013;Lindgren, Kaddumi & Polcyn, 2013;Houssaye & Bardet, 2013;Cuthbertson et al, 2015;D'Emic, Smith & Ansley, 2015). The mode of swimming in the two clades is known to have differed greatly.…”

Incongruent ontogenetic maturity indicators in a Late Triassic archosaur (Aetosauria: Typothorax coccinarum)

“…Histological analyses are often selective toward the shaft of long bones or osteoderms (Lee et al, 2013; Woodward et al, 2013; Griffin et al, 2020; de Buffrénil et al, 2021). However, ribs have also been found to be informative in assessing skeletal maturity in a range of tetrapods (e.g., mosasaurs, Waskow & Sander, 2014; non‐archosaur archosauromorphs, de Ricqlès et al, 2008; theropods, Erickson et al, 2004; sauropods, D'Emic et al, 2015; and pseudosuchians, Scheyer & Sues, 2017). Recently, Waskow and Mateus (2017) histologically sampled the ribs of crocodylomorph and dinosaurian archosaurs to assess ontogenetic signals.…”