Spinosaurus News ~

[DinoFan83]

It looks to me as though my suggestion of a crocodilian-like density for Spinosaurus, somewhat above that of water, may have been right on the money. Larramendi, Paul, & Hsu (2020) extensively examine the probable densities of a number of extinct animals, non-avian dinosaurs included, and have come to the same conclusion as myself: that Spinosaurus would most likely have had a density at least that of water if not somewhat denser, in contrast to the 0.833 density that DrZap and Henderson et al. 2018 have supported. Relevant parts in all quotes outlined.

Quote:A few extinct theropods appear to have been particularly dense, in particular the supposedly amphibious Spinosaurus (Ibrahim et al., 2014, 2020a; 2020b contra Henderson, 2018). Lines of evidence that appear to indicate an aquatic lifestyle for Spinosaurus, including recently discovered material attributed to S. aegyptiacus, reveals that these dinosaurs sported a transversely flattened tail with exceptionally developed caudal neural spines and chevrons apparently suitable for aquatic locomotion (Ibrahim et al., 2020b), long bones which are about 30%–40% denser than those of any other known theropod (Ibrahim et al., 2014), very poorly pneumatized or even apneumatic vertebrae (Rauhut, 2003; Ibrahim et al., 2020a, b), retracted nares (Ibrahim et al., 2014, 2020b), piscivorous adaptations of jaws and teeth (Dal Sasso et al., 2005), an elongated body, short hind limbs (Ibrahim et al., 2014, 2020a, 2020b) – although that needs to be verified by more complete remains– stable isotopes (Amiot et al., 2011) and apparently too short femur relative to its center of mass for a feasible bipedal locomotion on land (Ibrahim et al. 2020b), which is further supported by its remarkably small ilium and hindlimbs proportions relatively to its large BM (Larramendi and Paul, unpublished observations) suggesting a limited terrestrial performance. So, it is likely that the NSG of it, and perhaps other spinosaurs, was comparable to that of current crocodilians and other semi-aquatic animals at around 1.05. In the case of Spinosaurus it was probably able to regulate its FRC to achieve neutral buoyancy when diving. A recent study by Henderson (2018) that tested the hypothesis of a semi-aquatic Spinosaurus concluded that it was unsinkable. However, the study, which was based on a skeletal restoration that now appears obsolete, applied a too low SG to soft tissues and body parts, omitted that Spinosaurus vertebrae are largely apneumatic –which may suggest a restricted air-sacs system– and assumed an air-system similar to extant birds (although he tested an alternative model lacking air sacs, he committed the here mentioned errors) with the respiratory system normally fully inflated. Also excluded were possible gastroliths which despite their possibly limited mass (Cott, 1961, Henderson, 2003), could have increased the Spinosaurus NSG while lowering its center of gravity. As a result, Henderson (2018) obtained overly low SGs for all the analyzed animals in his study including a mean SG for Spinosaurus at only 0.833. 

For what it is worth (since this was brought up earlier), the paper also supports a density of over 0.9 for sauropods (that were much more pneumatic than Spinosaurus was) for the same reasons I outlined: 

Quote:The resulting NSG values in analyzed sauropods range from 0.917 to 0.984. These are significantly higher than estimated by Bates et al. (2016) which included most of the sauropod genera calculated here. They found very low SGs ranging from 0.776 to 0.929 –excluding their minimum convex hull models which further reduce the SGs–. Their low results are because the authors applied an overly low SG of just 1.0 to different apneumatic sections, and estimated extremely large respiratory systems for most of the sample which generally far exceed even what is expected in flying birds in a neutral state. Because the poorly pneumatized vertebrae of dicraeosaurids, the species in this clade were found to be the densest (Table 10), while the lowest NSG were found in the sauropods with the longest necks like mamenchisaurids, Giraffatitan, Barosaurus and Euhelopus with a NSG approaching to 0.93 in some cases. Colossal sized titanosaur carrying relatively large necks (Paul, 2019), were probably in this region (Table 10), or slightly below because their very wide torsos may have allowed them to possess relatively larger respiratory systems. If the lesser density of the neck of the neck relative to the rest of the body is not taken into account when calculating total BM, the later will be overestimated by up to ~3% among very large necked sauropods such as mamenchisaurs and brachiosaurs relative to short necked examples such as shunosaurs. The BM results herein are ~5-12% higher than for the same specimens volumetrically massed by Paul (2016, 2019) for the same specimens due to the lower NSGs used in those efforts. 

As well as 0.95 or more to be used with the majority of non-avian theropods (that were much more pneumatic than Spinosaurus as well) for the same reasons I outlined as well, since this point was brought up too:

Quote:Turning to the nonavian tridactyl footed avepod theropods (sensu Paul, 2002: theropods that either possess a foot in which metatarsal I does not contact the tarsals, or descended from such theropods, and belong to the clade to the clade that includes Neotheropoda), estimating these dinosaurs’ densities is complicated because like their close avian relations, they probably had air sacs linked with their pneumatic postcranial skeletons (Paul, 1988b, 1997, 2002, 2016; O’Connor 2004; O’Connor and Claessens 2005; Henderson, 2018). But their body form was sufficiently different to complicate matters. Being flightless with few exceptions, lacking large sterna, and having large, airless tails, it is highly improbable that any nonavian theropod had as a large a neutral respiratory space as derived flying birds. Instead, it is terrestrial birds that run but do not fly that are superior density models for such dinosaurs. It follows that a NSG comparable to big ground birds would be expected for nearly all nonavian avepods, or a very little higher in the case of large theropods as their limb-bones, contrary of what is found in extant terrestrial birds, were not comparably pneumatized. As a result, the NSG for large avepod theropods, would be about 0.95, or a bit higher, somewhere between 0.95 and 0.99 in some cases, compared to 0.85 used for derived nonavian avepods in Paul (1988, 1997, 2016).

@GuateGojira, you may be interested in this since we had this discussion earlier on the thread. Just as a comparison of the Henderson et al. 2018 model with the Ibrahim et al. 2020 model or with the mounted skeletons would suggest (as was outlined previously), the authors of the paper are in definite agreement with the suggestions of myself and SpinoInWonderland that Henderson's model was much too narrow. This is what is stated in the paper:

Quote:Finally, the thoracic region of the 3D reconstruction of Henderson (2018) appears to be too transversely narrow, nonavian theropod anterior trunks tended to be somewhat wider and more rounded (Paul, 1997, 2016), especially so in view of the strongly arced rib illustrated by Stromer (1915 Plate 1, Fig. 15). That combined with the seemingly shallow pelvis would have resulted in a high and therefore more stable metacentric height. If the Ibrahim et al. (2020b) skeletal reconstruction is correct, then the sail backed dinosaur should have had better hydrodynamic stability than indicated by Henderson (2018).

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