Tuesday, January 1, 2019

The Belly-Sliding Spinosaurus.

Note: More information that supports a belly-sliding Spinosaurus is found here:

https://psdinosaurs.blogspot.com/2017/09/spinosaurus-facts.html


Spinosaurus Skeleton (2014):
In 2014, it was announced that a new specimen of Spinosaurus, FSAC-KK 11888, a subadult and new-designated neotype skeleton of Spinosaurus, had been discovered. Ibrahim et al., (2014) was the paper that made this announcement. Instead of having a half-circle-shaped sail it was two half circles clumped together, had short limbs, most importantly, it's center of mass (COM) was closer to the middle of its body instead of near its legs. This meant that Spinosaurus would lean forward on land, and thus, would need support on land. This led Ibrahim et al., (2014) to hypothesize a quadrupedal stance for Spinosaurus.

 This hypothesis has been, and IS still, criticized to this day. However, Ibrahim et al., (2020) (Tail-propelled aquatic locomotion in a theropod dinosaur) have brought this hypothesis back. In the paleontologists' favor, two "professors of organismic and evolutionary biology from Harvard" built a model of Spinosaurus, and concluded that the animal would tip over while on land (Clea Simon, 2020). Therefore, the quadrupedal hypothesis seems to be more of a theory. Before I go into more detail, let's take a look at Spinosaurus' crazy design and locomotive history, and then I'll describe my own hypothetical method of locomotion for Spinosaurus while it was on land. I must admit though that another paleontologist thought of it first, so I'll give him most of the credit. However, I'm going to edit it a little bit. 

 Anyway, let's get started!

 Spinosaurus' Locomotive History:
It is interesting to note that when Spinosaurus was first discovered, it was thought that it was a quadruped. In fact, it looked like Dimetrodon (Donald F. Glut, 2001, pg. 82). 

 Spinosaurus from 1970's(?) as described in Donald F. Glut (2001) (Pg. 82):
However, after Suchomimus and Baryonyx were discovered, scientists realized that Spinosaurus looked like them (Glut, 2001, pg. 84) (Charig and Miller, 1997, pg. 55-57). They used Suchomimus' and Baryonyx's body design to come up with a new look for Spinosaurus (Glut, 2001, pg. 84), this time with a bipedal look (Charig and Miller, 1997, pg. 12 and 55) (Sereno et al., 1998, pg. 1300):
But after its re-discovery in 2014, Spinosaurus has been depicted as walking on all fours again. Primarily, it was shown walking on its knuckles like an anteater or sloth. The reasons for this was stated by Ibrahim et al., (2014), and by Paul Sereno in the YouTube video Rediscovering Spinosaurus: The First Semi-Aquatic Dinosaur. They stated that Spinosaurus' legs were not efficient enough for it to walk as a biped on land, and Nizar Ibrahim said that Spinosaurus would have used its arms to help support its weight. Spinosaurus' arms were about the same size as its legs. Sereno in the video also stated that, when creating a computer model of the animal, it could not support itself on its hind limbs alone.

More information on Spinosaurus' anatomy was explained in Ibrahim et al., (2014):

1. "We note here that Spinosaurus must have been an obligate quadruped on land, the first discovered among theropod dinosaurs, given the usual horizontal sacroiliac joint and the anterior location of the estimated center of body mass," (pg. 1615).

2. "The center of mass in a biped must be located over the middle one-third of the pes to generate a plausible mid-stance pose. In our flesh rendering of Spinosaurus, the center of body mass is positioned in front of both the hip and knee joints at a distance greater than femur length,
suggesting that forelimb support was required during terrestrial locomotion. Spinosaurus appears to have been poorly adapted to bipedal terrestrial locomotion. The forward position of the center of mass within the rib cage may have enhanced balance during foot-propelled locomotion in water," (pg. 1615). 

3. "Reduction of the pelvic girdle and hindlimb and the concomitant enhancement of axial-powered locomotion are common among semiaquatic vertebrates," (pg. 1616). 

Spinosaurus' Posture After Ibrahim et al., (2014) (David Bonadonna):
The 2014 reconstruction was met immediately with skepticism. It was argued that a quadrupedal Spinosaurus doesn't seem probable (Scott Hartman, September 13, 2014, p. 11), which led to other paleontologists to reconstruct Spinosaurus themselves. Some made it a biped (Andrea Cau, 2014 and 2015), while others stated that the legs were incorrectly scaled and should be longer (Scott Hartman, September 12, 2014). Some said that Spinosaurus could "combat crawl" (Duane Nash, August 16, 2014) (Duane Nash, September 14, 2014). This means that Spinosaurus, basically, would belly-slide on land like a loon. The internet basically attacked Ibrahim and his team (Scott Hartman, September 13, 2014, p. 13). However, the legs were proven to have been small (Hartman, September 18, 2014). In 2017, Ibrahim and his team discovered another Spinosaurus skeleton with the same small leg proportions, so the leg lengths are valid (Fabbri et al., 2017).

The reason for having Spinosaurus being portrayed as  quadruped originally by Ibrahim et al., (2014) was that, in the Supplementary Materials of their paper on page 24, it is shown that Spinosaurus' center of gravity (represented by a red dot) is in the middle of its body, not at its hips like other theropods. Therefore, Spinosaurus would be leaning forward a lot.

Spinosaurus' Center of Gravity (Red Dot) (Ibrahim et al., 2014, Supplementary Materials, pg. 24):
However, Donald Henderson (2018) wrote a paper saying that Spinosaurus' center of gravity was closer to its hips, like other bipedal theropods, allowing it to walk as a biped ("Abstract," Figure 1, "Results" p. 1). He gave 0.3182 for its center of mass (Figure 7), which is similar to other theropods (Figure 1).

Spinosaurus' Center of Gravity (Henderson, 2018, Figure 1):
From what I can tell, although this seems to be a "slam dunk" as to whether or not Spinosaurus could walk as a biped, Henderson (2018) does have some problems though. First, Henderson blamed the new body proportions given to Spinosaurus, by Ibrahim et al., (2014), as a result of incorrectly using other species of spinosaurids in reconstructing the skeleton ("Discussion," p. 7-8). Fabbri et al., (2017) proved that Spinosaurus did indeed have short legs, so this critique is incorrect. Second, Henderson got the center of mass for Spinosaurus closer to its legs (Figure 1, Figure 11), but he got the center of mass for Spinosaurus' sail in the middle of Spinosaurus' stomach (Figure 2). Shouldn't it be the same for the sail and legs? This seems to be ignored ("Results" p. 1, "Discussion" p. 2-3). 

Spinosaurus Center of Mass for It's Sail (Henderson, 2018, Figure 2):
Henderson used a pigeon and an ostrich to confirm this ("Discussion" p. 2-3, Figure 11), but Spinosaurus is not closely related to either of those animals like other theropods are. Once again, Fabbri et al., (2017) says that Spinosaurus is closer to penguins. Henderson did say that the penguin's center of mass was in the middle of its belly (Figure 4), so it should be the same for Spinosaurus, as Ibrahim et al., (2014) did (Supplementary Materials, pg. 24, Figure S3). 

 Third, Henderson compared Spinosaurus to other theropods like T. rex, Suchomimus, and even an alligator in order to see how well Spinosaurus could float. He said that Spinosaurus could only float just as good as those other two dinosaurs (Figure 5, "Discussion" p. 4), and that Spinosaurus would tip over in the water unlike an alligator based on its center of mass and metacentric height ("Results" p. 6, Figure 7). However, Ibrahim et al., (2014) said that Suchomimus and T. rex were not like Spinosaurus, due to the short hind limbs and thickness of the bones. Those two dinosaurs seem to have had a more terrestrial lifestyle than Spinosaurus did. Also, Spinosaurus' bones were more dense than an alligator's, so it should have floated just as good as an alligator (Supplementary Materials, pg. 14-15). Henderson got an alligator's center of mass close to its legs (Figure 3), but alligators are still quadrupedal (Reilly and Blob, 2003, "Abstract"). Alligators can do a trot on land, but this is because their tails are dragging on the ground. Also, their tails and limbs have "individual reaction forces" that help them, which "is consistent with the more caudal location of its center of mass" (Willey et al., 2004, "Summary") (Reilly and Blob, 2003, "Abstract"). Dinosaurs didn't drag their tails (David Hone, 2012), so Spinosaurus couldn't drag its tail to walk like a biped on land. Also, trotting is something that is done on all fours, as defined by Merriam-Webster. Crocodilians do have large feet, which allows the "ground reaction force to shift farther from the limb joints" in an upright posture (Reilly and Blob, 2003, "Abstract"). In 2012, a crocodile in Africa was photographed standing on its hind legs, but this was only to grab food held out by visitors. Once it got its food, it went back onto its stomach (Alex Ward, 2012, p. 9). However, Spinosaurus' feet are either about the same, or slightly smaller, than its hands, with pedal unguals 2-4 being the same size as pedal ungual/digit 1, which is the smallest toe on a theropod's foot. Usually for theropods, unguals 2-4 are larger than ungual/digit 1 (Ibrahim et al., 2014, pg. 1614-1615, p. 8-10). Also, if Spinosaurus' posture was similar to a crocodilian's, then it wouldn't be able to stand in a bipedal pose for very long at all.

On 4/24/20, the subadult Spinosaurus' tail, as described in Ibrahim et al., (2020) (Tail-propelled aquatic locomotion in a theropod dinosaur), was discovered. The tail prevented Spinosaurus from rolling over (Jackson Ryan, 2020, p. 9). This solves the "tipping over" problem mentioned by Henderson (2018) ("Results" p. 6, Figure 7). 

 Spinosaurus' Tail (Ibrahim et al., 2020, Tail-propelled aquatic locomotion in a theropod dinosaur, pg. 2, Figure 1):
Also, it has been said, once again, that Spinosaurus needed help walking on land (Will Dunham, 2020, p. 10). Ibrahim et al., (2020) (Tail-propelled aquatic locomotion in a theropod dinosaur) also redid Spinosaurus' center of mass/gravity, and it appears to be closer to the middle of its body again, as reported in Ibrahim et al., (2014) (Supplementary Materials, pg. 24). In Ibrahim et al., (2014), the center of mass was 1.04 m. In Henderson (2018), it was 0.3182 in Figure 7, but the authors give 0.48 m. Now it's 0.725-0.825 m (Ibrahim et al., 2020, Extended Data Figure 8, "B"-"D"). This seems to back up what was said in Dunham (2020) about needing support for walking (p. 10). Jason Treat and Mesa Schumacher (2020) says that "Spinosaurus' center of gravity leans forward, which aids swimming, and its curved claws are more suited for catching prey in the water than for walking on land." Ibrahim et al., (2020) (Tail-propelled aquatic locomotion in a theropod dinosaur, 
Supplementary Materials, pg. 31, "Body mass, segment masses, and centre of mass (CoM)") says that  the new COM supports their 2014 conclusion that Spinosaurus needed support for walking on land, and that "a facultative, if not completely, quadrupedal gait on land" is necessary. Also, an animal that has a COM that is greater than their femur length will hinder bipedal locomotion, and even standing, on land.

Quote from Ibrahim et al., (2020) (Tail-propelled aquatic locomotion in a theropod dinosaur; 
Supplementary Materials, pg. 31, "Body mass, segment masses, and centre of mass (CoM)"):
Spinosaurus' Center of Mass (Ibrahim et al., 2020) ("B" is new COM, "C" is Henderson (2018) COM, and "D" is Ibrahim et al., (2014) COM):
Of course, some people say otherwise. Mark Witton (2020) says that the new COM from Ibrahim et al., (2020) means that Spinosaurus was closer to being a biped, and a speculative quadruped ("Spinosaurus 2020: thoughts for artists," "Posture and balance" p. 1-2). This is not what Ibrahim et al., (2020) said. They said that Spinosaurus was more than likely a quadruped (look above). Witton also thinks that more testing could result in a strictly bipedal Spinosaurus altogether ("Spinosaurus 2020: thoughts for artists," "Posture and balance" p. 1-2). However, Witton did some calculations of his own, and says that Spinosaurus' legs wouldn't be able to carry an animal that was over four tons, and the animal's legs would have to be straight and not bend in order to support itself as a biped ("Posture and balance" p. 3):
The neotype is speculated to weigh 3.6-4.6 tons (Ibrahim et al., 2020, Supplementary Materials, pg. 31). If it was in a vertical posture, as proposed by Andrea Cau in 2014-2015, and didn't bend its legs, then it could support itself. In any other posture, Witton says that its legs wouldn't be able to support its weight ("Spinosaurus 2020: thoughts for artists," "Posture and balance," p. 2-3). My question is, if the neotype's legs wouldn't be able to support its body unless it was in a vertical posture and its legs were straight, and its mass was not above four tons, then how would the even larger specimens like MSNM V4047 be able to support themselves? Henderson (2018) gave MSNM V4047 a weight of 6,500 kg (7.2 tons) (Table 2). What about the neotype being 4.6 tons instead of just 4? This, along with the "completely straight legs" argument, makes a "strictly bipedal" Spinosaurus seem unrealistic, despite some people desperately wanting it to be. This was stated by Duane Nash in 2016 (March 9, 2016, p. 5-8).

Also, here's more proof that the scientists behind Ibrahim et al., (2020) said that Spinosaurus leaned more towards quadrupedalism rather than bipedalism. Stephanie E. Pierce and George V. Lauder, the two "professors of organismic and evolutionary biology from Harvard" I mentioned before from Clea Simon (2020), built a computer model of Spinosaurus. Pierce said that Spinosaurus' center of mass would have made Spinosaurus "tip forward if it were on land, a dynamic that fits with the short legs" (p. 10):
Just in case, I decided to measure the ilium and femur of FSAC KK 11888 in Ibrahim et al., (2020)(b) (p. 2, Figure 1), and I got 70 cm for the ilium and 60 cm for the femur. I measured them again in Ibrahim et al., (2020)(a) (Figure 129), and I got 71 cm for the ilium and 66 cm for the femur. Then in Ibrahim et al., (2014) (p. 1614, Figure 2), I got 79 cm for the ilium and 62 cm for the femur. These sizes are either almost the same size, or only slightly larger than, the ilium of Allosaurus specimen USNM 4734 (72 cm) (Gilmore, 1920, p. 66), but its femur is 85 cm (p. 69). USNM 4734 is 7.9 meters long. The ilium of Baryonyx is 83.5 cm (Charig and Miller, 1997, pg. 47), and its body length is 7.6 meters long. FSAC is 10.3 meters long. The leg muscles of Spinosaurus wouldn't be able to carry its weight on land. Paleo-artist Luis V. Rey suggested this first on his blog ("Spinosaurus Revisited Part 2. Spinosaur hysteria!").

Otero et al., (2019) studied the locomotive history of the sauropodomorph Mussaurus. After studying different specimens from various ontogenetic ages (young to adults), it seems that the young were quadrupedal but the adults were bipedal. When they're young, Mussaurus' center of mass (CoM) was positioned in the middle of the animal's body, and the CoM was greater than the animal's femur length. This would make the young Mussaurus a quadruped. As they grew into an adult, their CoM would get shorter than the length of their femur, and closer to their hips, making it a biped. In fact, the CoM has to be less than one of the animal's femur length in order for it to be bipedal ("Abstract," Figure 2, "Discussion" p. 1 and 4). Spinosaurus' CoM (0.725-0.825 m) is still greater than one length of the animal's femur (0.625 m) (Ibrahim et al., 2020, "Body dimensions, body body mass, body segment masses, and whole body centre of mass" pg. 1).

Therefore, with its center of mass/gravity tipping it forward, and its claws not suited for walking on land, it seems that my loon/penguin mode of transportation for Spinosaurus could help solve this. Walking by putting one foot forward while its belly is on the ground/belly-sliding, Spinosaurus wouldn't need to use its hands and it would have had the support it needed to maneuver on land.

Of course, some people say otherwise. Mark Witton (2020) says that the new COM from Ibrahim et al., (2020) means that Spinosaurus was closer to being a biped, and a speculative quadruped ("Spinosaurus 2020: thoughts for artists," "Posture and balance" p. 1-2). This is not what Ibrahim et al., (2020) said. They said that Spinosaurus was more than likely a quadruped (look above). Witton also thinks that more testing could result in a strictly bipedal Spinosaurus altogether ("Spinosaurus 2020: thoughts for artists," "Posture and balance" p. 1-2). However, Witton did some calculations of his own, and says that Spinosaurus' legs wouldn't be able to carry an animal that was over four tons, and the animal's legs would have to be straight and not bend in order to support itself as a biped ("Posture and balance" p. 3):
The neotype is speculated to weigh 3.6-4.6 tons (Ibrahim et al., 2020, Supplementary Materials, pg. 31). If it was in a vertical posture, as proposed by Andrea Cau in 2014-2015, and didn't bend its legs, then it could support itself. In any other posture, Witton says that its legs wouldn't be able to support its weight ("Spinosaurus 2020: thoughts for artists," "Posture and balance," p. 2-3). My question is, if the neotype's legs wouldn't be able to support its body unless it was in a vertical posture and its legs were straight, and its mass was not above four tons, then how would the even larger specimens like MSNM V4047 be able to support themselves? Henderson (2018) gave MSNM V4047 a weight of 6,500 kg (7.2 tons) (Table 2). What about the neotype being 4.6 tons instead of just 4? This, along with the "completely straight legs" argument, makes a "strictly bipedal" Spinosaurus seem unrealistic, despite some people desperately wanting it to be. This was stated by Duane Nash in 2016 (March 9, 2016, p. 5-8).

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. 

How Did Spinosaurus Walk on Land?
With the information I have, and from what paleontologist Duane Nash said about his "combat crawl" for Spinosaurus, I think Spinosaurus used belly-sliding on land, similarly to a loon, crocodilians, otters, and penguins. Given that its habitat was a swamp or near a river, it would have had to deal with a muddy environment. Loons and crocodilians live in the same habitats. Loons always stay near water (Alina Bradford, 2016, "Habitat"), and crocodiles live near "lakes, rivers, wetlands, and even some saltwater regions" (Bradford, 2014, "Where do crocodiles live?"). For loons, given that their short limbs that makes walking on land very hard (Alina Bradford, 2016, "Description") (U.S. Fish and Wildlife Refuge, "Common Loons," "Nesting"), they tend to "move one foot forward at a time" with their bellies on the ground, or they belly-slide to help get themselves in the water (West Pond Association, "The Common Loon," "Daily Life' p. 4). I think Spinosaurus would have done the same thing. Plus, penguins and loons have thick bones, and feet placed at the anterior part of their bodies, like Spinosaurus did (Loon Preservation Committee, "Common Loon Plumage and Appearance," p. 1) (International Penguin Conservation and Work Group, "Introduction to Penguins," p. 2). Penguins would also belly-slide, also known as tobogganing, rather than walk, since it conserves energy (Wilson et al., 1991, "Abstract") (Gill and Prevost, "Penguin: Natural History: Locomotion and orientation,” Encyclopaedia Britannica) (New England Aquarium, 2016, "Penguins Teacher Guide: Physical Characteristics” pg. 2 p. 3) (Sea World Parks and Entertainment, "All About Penguins: Physical Characteristics: Legs and Feet” p. 3). Since Spinosaurus and a penguin have similar center of masses and bone densities (Ibrahim et al., 2014, Supplementary Materials, pg. 24, Figure S3) (Fabbri et al., 2017) (Henderson, 2018, Figure 4), it seems more likely that Spinosaurus acted more like a loon or penguin on land, and belly-slide in order to conserve energy. At best, it probably walked by moving one foot forward at a time while having its belly on the ground.

Loon:
Penguin Toboganning:
Penguin's Center of Mass (Henderson, 2018, Figure 4):
For crocodilians, despite having a center of mass closer to its legs (Henderson, 2018, Figure 3), crocodiles use the belly crawl as their main mode of transportation on land (Adam Britton, 1996, p. 1). They put their whole bodies on the ground, and their legs help to push them across the ground (p. 2). They can either be slow, or move at "5 to 10 kph" (3.1-6.2 mph) (p. 1). At higher speeds, their bellies can be lifted of the ground a little bit to "reduce friction" (p. 4). This helps the crocodile to get into the water "when sliding down an embankment into a river, lake, or pond" (Marshall Cavendish, 20011, pg. 60) (Sally M. Walker, 2004, pg. 17), and when they're scared (Sally M. Walker, 2004, pg. 17). Gharials belly-slide on land because they have "weak leg muscles," which means they are "poorly equipped for locomotion on land." They do most of their transportation via water (Smithsonian National Zoo and Conservation Biology Institute, "Gharial," "Conservation," "Physical Description" p. 3).

Alligator Center of Mass (Henderson, 2018, Figure 3):
Crocodile Getting into Water on Its Belly:
Crocodile Getting Out of Water on Its Belly:
Otters, like the loon, penguin, and crocodilians, have an elongated body and short legs. It's long tail helps the otter to swim, along with its webbed feet. On land, they are fast but they can move faster by belly-sliding (The National Wildlife Federation, "North American River Otter," Description p. 1). They slide in ice and mud (The National Wildlife Federation, "North American River Otter," Description p. 1) (San Francisco Zoo, "North American River Otter," Fun Facts). They can do this for fun (The National Wildlife Federation, "North American River Otter," Description p. 1), or to escape from enemies (Ernest Thompson Seton, 1909, "The Canada Otter," pg. 829). This seems to make the otter faster than a human (Ernest Thompson Seton, 1909, "The Canada Otter," pg. 829). Using this information, it sounds more plausible that Spinosaurus, while belly-sliding through the mud, would be faster than, and escape from, the two-legged Carcharodontosaurus and Bahariasaurus.

Otter Belly-Sliding:
There are other spinosaurids that seem to have done this as well. Aureliano et al., (2018) describes a spinosaurid (Irritator?) specimen from South America, called LPP-PV-0042, that had a small tibia that was dense like Spinosaurus' (pg. 8 and 12). It seems possible that this spinosaurid might have been belly-sliding as well. Also, Evers et al., (2015) reports that the specimen "Spinosaurus B," which seems to belong to Sigilmassasaurus, had a tibia only 60 cm long. However, they said that if spinosauridae did have short limbs, then it did belong to "Spinosaurus B" ("'Spinosaurus B' and Sigilmassasaurus," pg. 64). Short legs have been proven true for spinosauridae (Fabbri et al., 2017, pg. 109), so it seems that Sigilmassasaurus had short hindlimb too ("'Spinosaurus B' and Sigilmassasaurus"). Therefore, it seems that Sigilmassasaurus was a belly-slider as well, or at least walked by moving one foot forward at a time.

Ibrahim et al., (2020) (Geology and paleontology of the Upper Cretaceous Kem Kem Group of eastern Morocco), and Smyth et al., (2020), conclude that "Sigilmassasaurus" and "Spinosaurus B" are synonyms of Spinosaurus now. Therefore, "Sigilmassasaurus" had short legs too, since it is Spinosaurus. (I also consider LPP-PV-0042 to be Irritator, since it's found in the same formation as the latter) 

We also have to take Spinosaurus' habitat into perspective alongside its body design. The beach, swamp, or watering hole would have been muddy and hard to walk through on two legs. Don't believe me, just look at these people:
These people need sticks (or a third limb) to help support themselves in the thick mud. In other words, two legs just doesn't cut it against mud and water. The small-legged Spinosaurus, for example, would have even more trouble walking in the mud just on its two small legs. This would help it get killed if a Carcharodontosaurus or Bahariasaurus decided to attack it. Therefore, Spinosaurus would get on its belly in the mud, position its arms on its sides, and propel itself forward with its hind legs. It could escape faster into the water while the longer-legged Carcharodontosaurus and Bahariasaurus struggled to catch up in the mud. Crocodilians, loons, otters, and penguins belly slide in terrain that is hard for them to walk in, mainly due to their elongated bodies and short legs. This might have been something Spinosaurus, and other short-legged spinosaurids, did as well.

Spinosaurus also spent most of its time in the water eating fish, and could use its tail to swim in water like a crocodile. This would have enabled Spinosaurus to travel by swimming. If that's the case, then maybe Spinosaurus didn't need to walk on land? It probably didn't go onto land unless it has to rest/sleep, and lay eggs, just like a loon (Alina Bradford, 2016, "Habitat") or even a crocodile (Bradford, 2014, "Where do 
crocodiles live?"). In that sense, Spinosaurus would have stayed near the water's edge predominately, maybe even all throughout its life, and probably used the water to travel instead of walking on land. When it needed to come onto land, it would belly-slide like a penguin (Wilson et al., 1991, "Abstract"). Which mode of transportation would cost Spinosaurus less energy? An awkward vertical, straight-legged bipedal pose? Or, a belly-sliding pose akin to other semi-aquatic animals? Perhaps new data will reveal a "strictly bipedal" Spinosaurus, but the data we have now shows that Spinosaurus would have needed help while walking on land. Therefore, belly-sliding is the best answer.

Update (10/27/20): Gharials are a genus of crocodilian that have weak leg muscles that prevent them from walking on land like other crocodilians. They have to push themselves across the land by belly sliding. They rely on swimming for transportation (Smithsonian National Zoo & Conservation Biology Institute, "Gharial," "Physical description" p. 3) (Animal Diversity Web, "Gavialis gangeticus Gharial," "Habitat," "Physical Description"). They are also have an aquatic lifestyle, and only come onto land to bask and nest ("Native Habitat" p. 2). Adults are primarily piscivorous, while the young eat a variety of smaller animals ("Food/Eating Habits"). 

I think this is another great example of what Spinosaurus' lifestyle would've been like, if not the best one, and it's another reason why I support belly sliding for Spinosaurus whenever it came onto land.

Update (2/13/21): Spinosaurus is not the first carnivorous dinosaur to be at home in the water. The hesperornithiformes were an order (Martin et al., 2012, 2. Systematic paleontology) of theropod dinosaurs that looked a lot like modern birds (UCMP, Introduction to the Hesperornithithiformes p. 4). They also spent most of their time in the water. What's more important to me is that they couldn't walk as bipeds on land. They belly-slided (Martin et al., 2012, 1. Introduction para. 2) (Martynuik, 2012, p. 170). Since their hind limbs were designed for swimming, they couldn't travel on land well. Some genera might have been capable of flight, but most hesperornithiformes swam to travel instead of walked (Bell et al., 2019, p. 10: 4.1. Ecological implications para. 2) (Martin et al., 2012, 1. Introduction para. 2, 3. Discussion para. 3). 

It seems that the more specialized an animal is for a semi-, or fully, aquatic lifestyle, their hind limbs don't function well on land anymore. If people can't, or even refuse, to see that this is what's happening for Spinosaurus, then how can the hesperornithiformes get away with this? We already have theropods that couldn't walk as bipeds anymore because they were specialized swimmers. As diverse as dinosaurs were, just accepting that one family of theropods could forgo bipedal locomotion on land for swimming and not another is narrowing our view of how these creatures looked and lived. Like I said before, if it's proven that Spinosaurus was capable of bipedal locomotion, then I'll recant my position on this whole matter. However, the physiology of Spinosaurus seems to point to belly-sliding, especially since another family of theropods have taken this route.

In summation, Spinosaurus is closer to the gharial, loon, and hesperornithiformes, in terms of terrestrial locomotion, not other terrestrial theropods. Spinosaurus was a belly-slider, not a biped.

Conclusion:
With its center of mass/gravity tipping it forward and its claws not suited for walking on land, it seems that belly-sliding on land and walking in the water, proposed by Duane Nash, for Spinosaurus, could help solve this, even if it couldn't swim in deep water. This could have been done by other spinosaurids that also had small hind limbs, such as Irritator (LPP-PV-0042).

Update 6/19-22/22: Sereno et al., (2022) have stated that Spinosaurus is a bipedal, non-aquatic, "bipedal ambush piscivore" (Abstract):

https://www.biorxiv.org/content/10.1101/2022.05.25.493395v1.full

However, Fabbri et al., (2022) said that Spinosaurus' extremely dense bones are akin to quadrupedal animals, and aquatic ones as well:

https://www.nature.com/articles/s41586-022-04528-0.epdf?sharing_token=rxUUwyZxDWQ24dJfwtIw89RgN0jAjWel9jnR3ZoTv0NEFj8DFZa3bazFWKdXldNTvT8T3daJQzYMUbPXaqso6c2KKBgthBeOpsV72_JOZHeSlOxZzzE9wUggHYItKT5ASyn5r0hTiRPfCQi_Cfe9RPf0tvCNFd3T4QXE2UU4r7wR-SYYL4_TSvBiBpniofeQoStgnv6yWzzkL81Gcy2g6hKT9nO8ozsufeY9DwX1VK-Vsw94pFBHTtBWnm2-q0bJ33Xx2cPSUh5t7T-nx3NDvtkT9MSkWBYPTw7aqWM5FRs%3D&tracking_referrer=www.sciencenews.org

Supplementary information:

https://static-content.springer.com/esm/art%3A10.1038%2Fs41586-022-04528-0/MediaObjects/41586_2022_4528_MOESM1_ESM.pdf


I've come to the conclusion that one must make their own decision when it comes to how Spinosaurus lived, and following the evidence, I am sticking with a belly-sliding Spinosaurus. 


Also, Caneer et al., (2021) found T. rex arm marks in Colorado that suggests that it rose from a quadrupedal stance into a bipedal one (Abstract; pp. 29-30; p. 33 Figure 6, C; p. 35 Figure 8). If T. rex could use its arms like that, then I believe that Spinosaurus using its (longer) arms to help pull its body forward on the ground in a belly-sliding posture would have worked.

T. rex arm trace fossils (Caneer et al., 2021, p. 33 Figure 6, C):
Drawing of T. rex showing how it used its arms to sit down, or even get up from the ground (p. 35 Figure 8):
Caneer et al., (2021):
Links:
Ibrahim et al., (2014): 
https://www.researchgate.net/publication/265553416_Semiaquatic_adaptations_in_a_giant_predatory_dinosaur
Supplementary Materials (Pg. 22):
https://science.sciencemag.org/content/suppl/2014/09/10/science.1258750.DC1/Ibrahim.SM.pdf
Ibrahim et al., (2020). Tail-propelled aquatic locomotion in a theropod dinosaur:
Clea Simon (2020):
https://news.harvard.edu/gazette/story/2020/04/new-paper-suggests-spinosaurus-may-have-been-aquatic/
Locomotion History:
Donald F. Glut (2001) (Pg. 82 and 84):
Charig and Miller (1997) (Pg. 12 and 55):
Sereno et al., (1998) (Pg. 1300):
Scott Hartman (September 12, 2014):
Scott Hartman (September 13, 2014):
Andrea Cau (2014):
Andrea Cau (2015):
Duane Nash (August 16, 2014):
Duane Nash (September 14, 2014):
Scott Hartman (September 18, 2014):
Henderson, Donald M. (2018):
Ibrahim et al., (2020). Tail-propelled aquatic locomotion in a theropod dinosaur:
https://www.nature.com/articles/s41586-020-2190-3.epdf?sharing_token=wElGAWkXZX3eB14Er_jbUdRgN0jAjWel9jnR3ZoTv0OcJuFkKXfvVfjOrYF9meV2qCJkOX1x2LjcUMb1Lb5lZ9chhU_Vqfej8-PBfY04xZnY48UXBKYSWhbFemIIs3mnslnJcMCkPcDsf4JmQim7ZWuw7gTuaSQgIH1NES8XsNEAQbuXpuNMgu2T0alEiU1nolCaK6s1p8TvLl3vrvhPiBE9R0sp6pL6T-Jdz-i53gDgBKDkO1M4-gD343aSCj8uA6Wk_OUCrH_JGGWbqhjD9_2bj7JSympkyTP7aZ9BtXc%3D&tracking_referrer=www.sciencenews.org
Link 2:
Supplementary Information (pg. 29, "Body mass, segment masses, and centre of mass (CoM)"):
"Body dimensions, body body mass, body segment masses, and whole body centre of mass" pg. 1:
https://www.readcube.com/articles/supplement?doi=10.1038%2Fs41586-020-2190-3&index=3
Otero et al., (2019):
https://www.nature.com/articles/s41598-019-44037-1
Jackson Ryan (2020):
Will Dunham (2020):
"A Swimming Dinosaur: The Tail of Spinosaurus":
Carolyn Gramling (2020):
Jason Treat and Mesa Schumacher (2020):
Image:
Article:
Mark Witton (2020) ("Spinosaurus 2020: thoughts for artists," "Posture and balance" p. 1-3):
Duane Nash (August 16, 2014):
Duane Nash (September 14, 2014):
Alina Bradford (2016):
Alina Bradford (2014):
U.S. Fish and Wildlife Refuge ("Common Loons," "Nesting"):
West Pond Association ("The Common Loon," "Daily Life' p. 4):
International Penguin Conservation and Work Group ("Introduction to Penguins," p. 2):
Loon Preservation Committee ("Common Loon Plumage and Appearance," p. 1):
Wilson et al., 1991 ("Abstract"):
Gill and Prevost. "Penguin." “Natural History: Locomotion and orientation.” Encyclopaedia Britannica: 
https://www.britannica.com/animal/penguin/Natural-history

New England Aquarium. "Penguins Teacher Guide." “Physical Characteristics” pg. 2 p. 3. 2016:
https://www.neaq.org/wp-content/uploads/2016/06/LEARN_3-5-2_Penguin_TeacherGuide.pdf

Sea World Parks and Entertainment. "All About Penguins." "Physical Characteristics: “Legs and Feet” p. 3:
https://seaworld.org/animals/all-about/penguins/physical-characteristics/

Adam Britton (1995):
https://crocodilian.com/cnhc/cbd-gb4.htm
Marshall Cavendish (20011) (Pg. 60):
https://books.google.com/books?id=fU25LOYnVokC&pg=PA60&lpg=PA60&dq=belly+sliding+crocodile&source=bl&ots=f9aHV-nWCk&sig=ACfU3U0UWZhNV4cKMCrePO2wp9G2-GBfxQ&hl=en&sa=X&ved=2ahUKEwjHo8O_7YnqAhXClXIEHSdEDnMQ6AEwHHoECAIQAQ#v=onepage&q=belly%20sliding%20crocodile&f=false
Sally M. Walker (2004) (Pg. 17):
https://books.google.com/books?id=MbqS-zNASiUC&pg=PA17&lpg=PA17&dq=crocodile+belly+crawl&source=bl&ots=RHr0w9Px_C&sig=ACfU3U21x1RMKo92aeARkwdfbN6VphH8og&hl=en&sa=X&ved=2ahUKEwjlt6-W74nqAhVhg3IEHTRXDTQQ6AEwHXoECAMQAQ#v=onepage&q=crocodile%20belly%20crawl&f=false
Smithsonian National Zoo and Conservation Biology Institute. "Gharial." "Conservation." "Physical Description" (p. 3):
https://nationalzoo.si.edu/animals/gharial
Evers et al., (2015) ("'Spinosaurus B' and Sigilmassasaurus," pg. 63-67):
Link 2:
Ibrahim et al., (2020). Geology and paleontology of the Upper Cretaceous Kem Kem Group of eastern Morocco. ("Age," p. 2, 4-6):
Smithsonian National Zoo & Conservation Biology Institute. "Gharial":

https://nationalzoo.si.edu/animals/gharial#:~:text=Adult%20gharials%20primarily%20eat%20fish,at%20fish%20in%20the%20water

Animal Diversity Web. "Gavialis gangeticus Gharial" ("Habitat," "Physical Description"):

https://animaldiversity.org/accounts/Gavialis_gangeticus/
UCMP. Introduction to the Hesperornithithiformes. P. 4:
https://ucmp.berkeley.edu/diapsids/birds/hesper.html
Martin et al., (2012). 1. Introduction para. 2, 3. Discussion para. 3:
https://www.sciencedirect.com/science/article/abs/pii/S1871174X12000066
Martynuik (2012). P. 170:
https://www.google.com/books/edition/A_Field_Guide_to_Mesozoic_Birds_and_Othe/b5_DyhNk7FcC?q=Hesperornis+walk&gbpv=1&bsq=Hesperornis%20regalis#f=false
Bell et al., (2019) P. 10: 4.2 Ecological implications. Para. 2:
http://lithornis.nmsu.edu/~phoude/Bell%20et%20al%20%202018%20Morphometric%20comparison%20of%20the%20Hesperornithiformes%20and%20modern%20diving%20birds.pdf
Overland definition:
Merriam Webster. Overland:
https://www.merriam-webster.com/dictionary/overland
Larramendi et al., (2020) (3. Results and Analysis: 3.2.9 Nonavian avepod theropod dinosaurs):
https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.24574
Hummingbird Central. "Hummingbird  Facts and Family Introduction: Flying ... and Walking," para. 2:
 https://www.hummingbirdcentral.com/hummingbird-facts.htm

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