Perhaps new data will reveal a "strictly bipedal" Spinosaurus in the future, but the data we have now shows that Spinosaurus would have needed help walking on land. I'm going to stick with the belly-sliding posture until further notice.
Update (12/17/20): Scott Hartman has, once again, criticized the neotype specimen. He has stated that he thinks the neotype specimen might not be Spinosaurus, and, even though he admits that he might have to check his numbers again, he has resolved to using the tibia length of "Spinosaurus B" in his reconstruction of Spinosaurus, since the neotype's leg proportions don't seem to match it ("The Road to Spinosaurus III: Of Chimeras and Leg Proportions: Of Chimeras and FSAC-KK-1188" p. 1; "Leg Proportions" p. 8; "Conclusions" p. 1-2). This has produced, according to his results, a Spinosaurus with longer legs, making it a biped ("The Road to Spinosaurus IV: Not Your Father's JP3 Spinosaurus").
Scott Hartman's Reconstruction of Spinosaurus ("The Road to Spinosaurus IV: Not Your Father's JP3 Spinosaurus"):
However, "Spinosaurus B" has a tibia that is 60 cm long (Stromer, 1934, pg. 16) (Evers et al., 2015,
"'Spinosaurus B' and Sigilmassasaurus," pg. 64) This produces an animal that is 31 feet long (9.3 meters). The neotype has a tibia that is 66.8 cm long (Ibrahim et al., 2014, "Supplementary Materials" pg. 33), and is 10.3 meters long. It would be the neotype that had longer legs than "Spinosaurus B." This would still produce an animal that had small legs. Also, the sail is incorrect. Hartman thinks the structure of the sail in unknown ("The Road to Spinosaurus IV: Not Your Father's JP3 Spinosaurus: 6"), but based on the skeletal information from the holotype and neotype, it wouldn't be a perfect half-circle. Spinosaurus' sail would have been similar to a sailfish's.
Nothing against Hartman, or Witton for that matter, but I think some personal biases is being used here to produce a Spinosaurus that they think should have existed. However, the evidence is pointing in another direction. Spinosaurus is not a typical bipedal theropod dinosaur that lived on land. Once again, in the future, if evidence is presented that Spinosaurus was indeed a bipedal animal, then I will take back my belly-sliding posture. However, belly-sliding like a gharial, loon, and even a penguin, seems to be the best terrestrial form of locomotion for Spinosaurus within its swampy environment. Longer legs would get Spinosaurus stuck in the mud, exposing it to any enemies who are looking for a meal, like Carcharodontosaurus and Bahariasaurus (maybe a large crocodyliform?). Belly-sliding in the mud would help Spinosaurus to transverse easier through the swamp, while the other two theropods would have more trouble keeping up with their longer legs. Duane Nash already explained this half a decade ago.
But I digress. Aside from laying eggs, let's not forget that this animal would have spent all of its time in the water (Beevor et al., 2020), so terrestrial locomotion would have been basically nonessential.
Update (1/26-3/5/21): Hone and Holtz (2021) wrote a paper on Spinosaurus. They refuted the papers published by Ibrahim et al. in 2020, and put Spinosaurus was a wader, hunting for fish, and even swimming, but not an aquatic predator ("Abstract," "Summary" p. 6). They used a model for Spinosaurus that was bipedal (Figure 1), said that Spinosaurus' skull was closer to terrestrial animals (despite saying that Spinosaurus' skull is in between terrestrial and fully aquatic animals) ("Results: Skull Shape" p. 3) (Figure 3), questioned Spinosaurus' tail as being adapted for underwater propulsion ("Results: Tail"), questioned Spinosaurus' feet as being designed for swimming ("Results: Ungual Shape" and "Hind limbs"), said that Spinosaurus would have had trouble chasing after prey and keeping itself buoyant in the water ("Results: Aquatic Locomotion" p. 13) (Laura Geggel, 2021, p. 8), that Spinosaurus' sail might be a display feature ("Results: Dorsal and Caudal Sail Function" p. 7), and that Spinosaurus was a terrestrial and aquatic hunter, not just an aquatic one ("Results: Environmental Factors" p. 5).
Hone's and Holtz's Spinosaurus Neotype Design by Genya Masukawa (Figure 1):
Figure 1: Spinosaurus wading design (Top) (standing in water with head above the waves) is said to be more accurate than the swimming design proposed by Ibrahim et al., (2020)(b) (Bottom). Black arrows represent disproven traits. Gray arrows represent traits that can go with, or against, either model. White arrows represent traits that can go with either model. Scale bar is 1 meter:
Figure 1 Description:
Judging from the Figure 1 diagram, Spinosaurus' legs and tail might've been able to help it swim, but according to Hone and Holtz, the animal seems to thrive better as a wader, not a swimmer. Nizar Ibrahim says that Hone and Holtz's paper doesn't change anything (
Laura Geggel, 2021, p. 17).
There are a few problems that I've found with Hone's and Holtz's paper:
1. They did state that the design of Spinosaurus' skull is in between terrestrial and fully aquatic but they chose to say it was more likely a terrestrial-like theropod skull.
2. They used a Spinosaurus model that had longer legs and not a "M"-shaped sail.
3. They questioned Spinosaurus' legs as being adapted for aquatic locomotion, despite being similar to a crocodilian's or a penguin's.
As for that last part, Nash said that Spinosaurus would have been an underwater walker, and the data from Hone's and Holtz's paper actually support this view as well. This would help Spinosaurus to save more energy, and keep itself balanced, while swimming. As for the tail, if it didn't help Spinosaurus to swim, then it would have helped it to keep its balance as it walked under the water. This is similar to an average theropod's tail as they walked on land. As for the sail, Spinosaurus' sail is similar to a sail fish's, so having the sail just for display doesn't really seem complete. Spinosaurus hunting both terrestrial and aquatic prey doesn't sound so bad, although trying to figure out how it did so on land would take some time. My best guess that Spinosaurus belly-slid onto land, lied down, and waited for a small terrestrial creature to come its way. Hone's and Holtz's Spinosaurus model shows that it could walk as a biped just fine, despite evidence presented by Ibrahim et al., 2020(b) (with a COM greater than the femur length). That is still problematic. Most Spinosaurus teeth are found in ancient aquatic environments where other terrestrial animals are very rare, or even absent. This doesn't sound like an animal that would spend most of its time near land. Maybe Spinosaurus ambushed some terrestrial prey from time to time, but it would have to be in the water. There's also a problem with the fact that Spinosaurus' skull wasn't designed to hunt giant prey. Perhaps Spinosaurus hunted pterosaurs, as Hone and Holtz (2021) hinted at ("Results: Environmental Factors" p. 2). However, Spinosaurus couldn't run on land, therefore it would have to be from the water, or as I've explained above, it would slide onto land, and then lie and wait for a small creature to appear. The design of Spinosaurus' body also doesn't make it friendly towards land. The dense bones also seem to be odd on an animal that would have spent time on land, while most other theropod dinosaurs have hollow bones. Hone and Holtz say that Spinosaurus would have been in competition with both land and aquatic predators ("Results: Environmental Factors" p. 5), but it would be easier for Spinosaurus to take on a giant crocodyliform in the water than a Carcharodontosaurus on land, among other terrestrial carnivorous theropods. (Also, even though Hone and Holtz state that Spinosaurus was a weird theropod that preferred aquatic environments, you can bet that more people will take this paper as saying that Spinosaurus was an average terrestrial theropod)
Hone and Holtz wrote a very detailed paper, summarizing all the previous studies on Spinosaurus and its kin. Since Dr. Holtz is my professor now, I want to be careful with what I say. However, as I've heard him say many times now, science is about being objective. Taking this into account, I do not think that this paper gets rid of a possible aquatic Spinosaurus, but I do think Hone and Holtz make some good points that definitely fits with Nash's underwater-walking Spinosaurus. If Spinosaurus didn't swim after prey, then walking in the water and stalking prey would be the next best bet. However, I don't think that Spinosaurus just being a wader works, especially with the massive amount of Spinosaurus teeth discovered with other fish fossils in an ancient riverbed. Evidence in the future might change this and put Spinosaurus as a terrestrial predator again, but as of right now, Spinosaurus still seems to be more at home in the water.
As I've said before, Hone's and Holtz's paper fits well with Nash's portrayal of Spinosaurus, so an underwater-walking Spinosaurus is the best bet for now. I am more open to Spinosaurus being bipedal and more terrestrial now, but the weight of the evidence (as far as I can tell) points to a dinosaur that spent most of its time in water and wasn't suited well for terrestrial habitation.
Update (2/15/21): Larramendi et al., (2020) addressed Henderson (2018) in their paper from 2020, and stated the problems that helped Henderson to get his results. They say that, if the remodel of Spinosaurus with its paddle-like tail is correct, then Spinosaurus would have been more stable in the water than suggested by Henderson.
Quote from Larramendi et al., (2020) on Henderson (2018) (3. Results and Analysis 3.2.9 Nonavian avepod theropod dinosaurs para. 3):
Larramendi et al., (2020) also state that, in Larramendi's and Paul's unpublished observations, Spinosaurus' "terrestrial performance" would have been "limited," due to its "small ilium and hindlimbs proportions relatively to its large BM (body mass)." This helps to support the "too short femur relative to its center of mass for a feasible terrestrial bipedal locomotion on land." They also state that isotopes collected by Amiot et al., (2011) help to support this as well (3. Results and Analysis 3.2.9 Nonavian avepod theropod dinosaurs para. 2):
Once again, this shows that Spinosaurus was not a typical terrestrial bipedal theropod. This also seems to support the idea of Spinosaurus being a competent swimmer. Even if it wasn't able to pursue after prey underwater, a Spinosaurus walking as a biped in the water is the best bet, as stated by Nash and still supported by the findings of Hone and Holtz (2021). It would stand still in the water, waiting for anything to come close to it, and when the time came to strike, it did. On land, however, it would be a belly-slider, similar to hesperornithiformes, gharials, and loons. The best way for it to hunt on land would be to lie and wait. When a small animal (more likely a pterosaur) unconsciously walks past a hiding Spinosaurus, the dinosaur would attack!
Update (3/28/21): Hummingbirds can't walk or hop, since their legs are small and not very strong. They can shuffle, but that seems to be it (Hummingbird Central, "Hummingbird Facts and Family Introduction: Flying ... and Walking," para. 2).
It looks like Hummingbirds were designed mainly for flying rather than for walking. The same can probably be said for Spinosaurus, which swam, or walked, in water rather than walked on land.
.png)
.png)