1.) Did Spinosaurus have short arms?
Originally, I came to the conclusion that Spinosaurus would've been a quadruped. This was due to the fact that Spinosaurus had long arms but short claws that were not robust, trace fossils of theropod arm and hand marks, and I kept getting a large center of mass for the animal. However, I noticed a pattern regarding a spinosauroid manual ungual from Ibrahim et al., (2020a) and the reconstructed Spinosaurus arm from DinoLab a few years back. It led me to a new possibility: Spinosaurus probably had short arms! If Spinosaurus was bipedal, then this is probably the best hypothesis to make it so.
I've learned over time to let go of my biases in order to get to the truth of a particular matter. It's been a gradual process with Spinosaurus, but it's better late than never. Let's suppose that Spinosaurus wasn't a quadruped. Let's agree that Spinosaurus' arms weren't capable of supporting its weight. Suppose that it's center of mass was shorter than its femoral length, allowing it to be a biped. These conclusions came from Sereno et al., (2022). However, we're also going to add Ibrahim et al., (2020a) and the reconstructed arm from DinoLab into the equation. Sereno et al., (2022) showed that about 90% of the arm for the reconstructed skeleton is a composite (then again, the whole skeleton is a composite). Only one phalange, and one manual ungual, came from the neotype. The phalange belongs on the second finger (Sereno et al., 2022) (Fabbri et al., 2022). The manual ungual is on the third digit, and it is very small (Fabbri et al., 2022). The phalange is elongated and skinny (Sereno et al., 2022) (Fabbri et al., 2022), just like the phalanges on the reconstructed arm from DinoLab. So far, so good! I also noted that, in my original post announcement about the DinoLab arm, I stated that the arm looked smaller than the arms on the reconstructed skeleton made by Ibrahim, Sereno, and co. It should be noted that the metacarpals (not shown), phalanges, and the radius and ulna (they are partially complete), are real material. The humerus is a "cast"/"articulated." There is also some criticism towards the third finger being way too long, but it's not conclusive. This comes from DinoLab on their Facebook post, and Paleontologist Roberto Diaz Sibaja from Facebook as well. Terry from DinoLab said that some paleontologists have verified the bones (that are real) to be conclusive (Facebook), so we seem to be on the right path here. I will also ignore the radius from Goo (2022) for now.
DinoLab Spinosaurus arm (Facebook, 2021):
Compare the claws to the spinosauroid claw (NMC 41820) from Ibrahim et al., (2020a) (
Figure 111A-B):Full figure. Scale bar is 5 cm:Description of NMC 41820 (Ibrahim et al., 2020a, Theropoda: Manual ungual morphotype 1): Information on
Spinosaurus arm from
DinoLab (
Facebook, 2021):
Another pic of the
DinoLab arm (
The Zone @91-3, 2021):
Pics of DinoLab arm from Twitter-X (2020):
First:Second (Notice that the arm looks smaller here):Spinosaurus' skeletal design by Sereno et al., (2022) (Figure 1). The neotype's bones are blue. The phalange in D is not from the neotype though. It's from the first digit, not the second. Scale bar for D is 3 cm: skeleton from Fabbri et al., (2022) (Figure 1). The red bones are from the neotype:
Close up of the hand. You can see phalange 2-1, and manual ungual 3, in red:
The claws on the hand look small in Fabbri et al., (2022) as well, which might lend support to
Spinosaurus having small manual unguals.
If we compare the presumed unguals of Spinosaurus to Baryonyx/Suchomimus walkeri, for example, then we can see a huge difference. Baryonyx's thumb claw, or manual ungual 1, is 31.0 cm (310 mm) long
(Charig and Miller, 1997, p. 46):Baryonyx's manual ungual 1 (p. 47 Figure 35):
The ungual from Ibrahim et al., (2020a) is about 5 cm, or slightly longer (Theropoda: Manual ungual morphotype 1). The presumed manual ungual from the T. rex specimen FMNH PR 2081 ("Sue") is about the same size, albeit more of the tip is missing (Brochu, 2003, p. 101 Figure 87) (Scale bar is 5 cm): The carcharodontosaurid Meraxes has all three manual unguals preserved, and most of its forearm. All three manual unguals are smaller than 10 cm, with the first ungual (the largest one) barely reaching 10 cm (Canale et al., 2022, Figure 1J) (Scale bar is 10 cm): Full figure. Notice how small the arm is compared to the animal's body (Scale bar under tail is 1 meter. All other scale bars are 10 cm):Spinosaurus' presumed manual ungual is basically the same size as T. rex's, and Meraxes'! Baryonyx's claws were large, and robust, enough for dexterity. Spinosaurus' are not. In fact, the hand bones attributed to Spinosaurus in Sereno et al., (2022) (Figure 1) are similar in morphology to a baryonychinae than to a spinosaurid. This might suggest that there was a baryonychinae (
Sigilmassasaurus?) coexisting with
Spinosaurus. I will keep in mind that the ungual from Ibrahim et al., (2020a) could belong to any digit on the manus, so other claws could've been bigger, or smaller, than
NMC 41820. Comparing it to the arm from Dino Lab, the ungual could belong to the first digit. The third ungual is more recurved than the first two, so NMC 41820 could belong to the first or second digit. The overall morphology of NMC 41820 matches the first ungual, so I'll place NMC 41820 as the thumb claw for now.
So in total, we have a Spinosaurus arm that consists of a (probable) small and stocky radius and ulna, slender manual phalanges, and small manual unguals that were not large and recurved as typically seen in the other spinosauroids like the baryonychinae. The humerus is unknown, but we can speculate that it was probably smaller than the one from DinoLab and shaped more akin to a baryonychinae. Taking all of this into account, one could speculate that Spinosaurus' arms were probably shorter than typically reconstructed. I would imagine that Spinosaurus could've taken the tyrannosaurid and carcharodontosaurid route: Evolve larger skulls that replaced the necessity to have long arms. In return, the forelimbs would be reduced in size.
This is, by no means, the final say in the matter. There is something strange with the arms of Spinosaurus, which leads me to believe that they were probably not very useful to the animal in life. There are other factors that still go against a strictly bipedal Spinosaurus (see part 1 of this series). Until more bones are discovered (hopefully from the neotype specimen), I'll say that Spinosaurus could've been quadrupedal IF it had long arms. I can't see Spinosaurus dragging is arms across the ground while being bipedal. If it had short arms, then I will say that Spinosaurus was bipedal.
Links:
Part 1:
https://psdinosaurs.blogspot.com/2023/03/what-did-spinosaurus-look-like-part-1.html
Ibrahim et al., (2020a):
https://zookeys.pensoft.net/article/47517/element/7/0/deltadromeus/
DinoLab:
1.) Instagram (Gramho):
https://gramho.com/media/2535465340537444071
2.) Facebook:
https://m.facebook.com/dinolabinc/posts/our-spinosaurus-arm-is-still-on-display-we-arent-sure-how-much-longer-we-are-goi/885524165571071/
3.) Twitter-X:
https://mobile.twitter.com/DinoLab_Inc/status/1322305140020269058
4.) The Zone @91-3:
-Photo:
https://images.app.goo.gl/FYeo7rjhQr6cPLja7
-Website:
https://www.thezone.fm/2020/09/02/geekout-dino-lab-spino-arm/
Sereno et al., (2022):
https://www.biorxiv.org/content/10.1101/2022.05.25.493395v1.full
Fabbri et al., (2022):
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
Mr. Sibaja (Palaeos):
1.) Facebook Post:
https://m.facebook.com/PalaeosPag/photos/a.157631294723661/1059058504580931
2.) Blog:
https://palaeos-blog.blogspot.com/?m=0
Charig and Miller (1997) (PP. 46-47):
https://www.biodiversitylibrary.org/page/36949178#page/201/mode/1up
Brochu (2003) (P. 101 Figure 87):
https://www.researchgate.net/publication/249022959_Osteology_of_Tyrannosaurus_rex_Insights_from_a_Nearly_Complete_Skeleton_and_High-Resolution_Computed_Tomographic_Analysis_of_the_Skull
Canale et al., (2022):
https://www.cell.com/current-biology/fulltext/S0960-9822(22)00860-0
-Link 2:
https://www.sciencedirect.com/science/article/abs/pii/S0960982222008600
2.) Given up?
The more I tried to make Spinosaurus bipedal, the more problems I keep running into. My main goal was to make the arms shorter, and the legs longer. I used Suchomimus to try and do that because it's the only spinosauroid that has arm material that I can find, and so far it's not working. The arms for
Spinosaurus seem to be longer than Suchomimus' a little bit, and the legs are extremely short compared to Suchomimus'. I also tried to do the same thing with Baryonyx, and that didn't work either. This, along with the information from my first post in this series, makes me think that Spinosaurus is still a quadrupedal. At the very least, that's the best hypothesis that seems to be working so far. We NEED to have the arm bones of this animal. That's the only way we can find a definitive answer. If I find another way to make Spinosaurus a biped, then I'll post it here. For now, I'll have to go with Spinosaurus as a quadruped. I will say this about the manual unguals: They seem to be shorter, and less robust, than the baryonychinae manual unguals.
3.) I might've found a way!
I think I've found a way!
I should note that my plan requires the Baryonyx finger to be the second digit of the manus. Charig and Miller (1997) said that the digit could be the second or third one (p. 47), so if it's the third finger then my math will not work. If it is the second finger, then we can hypothesize the length of the arm of the Spinosaurus neotype.
The Spinosaurus neotype has the manual phalanx 2-1 preserved. It's 17.5 cm long (Ibrahim et al., 2014, Supplementary Materials, p. 33). Baryonyx's manus also preserves a relatively complete finger (the tip of the ungual is missing though), but it's unknown if it's the second or third digit. I will assume, for the sake of this experiment, that it's the second finger. The probable manual phalanx 2-1 bone is 13.2 cm long (Charig and Miller, 1997, p. 47). I'm going to use these two bones to speculate the length of the
Spinosaurus' neotype's arm and hand.
Math:
Spinosaurus neotype FSAC-KK 11888:
Manual phalanx 2-1: 17.5 cm (Ibrahim et al., 2014, Supplementary Materials, p. 33).
Femur: 62.5 cm (Ibrahim et al., 2020b, Supplementary Materials: Date File 2, Body dimensions, body mass, body segment masses, and whole body center of mass, p. 1).
Tibia: 66.8 cm (Ibrahim et al., 2014, Supplementary Materials, p. 33).
Baryonyx:
Probable manual phalanx 2-1: 13.2 cm.
Humerus: 46.3 cm.
Radius: 22.5 cm.
Probable manual phalanx 2-2: 9.1 cm.
Source: Charig and Miller (1997) (pp. 43, 45, and 47).
Manual phalanx 2-1:
17.5 - 13.2 = 4.3.
4.3/13.2*100 = 32.6% increase.
Spinosaurus neotype arm bones estimates:
Humerus: 46.3 cm + 32.6% = 61.4 cm.
Radius: 22.5 cm + 32.6% = 29.8 cm.
Maual phalanx 2-2: 9.1 cm + 32.6% = 12.1 cm.
Complete arm length:
1.) Humerus + radius = 91.2 cm (61.4 + 29.8).
2.) Humerus + radius + manual phalanx 2-1 = 108.7 cm.
3.) Humerus + radius + manual phalanx 2-1 + manual phalanx 2-2 = 120.8 cm.
4.) Humerus + radius + manual phalanx 2-1 + manual phalanx 2-2 (120.8) + hypothetical manual ungual from Ibrahim et al., (2020a) (8.8 cm; 9.4 cm at whole? [Measured and estimated by me]) = 129.6 to 130.2 cm.
Leg length:
1.) Femur (62.5) + tibia (66.8) = 129.3 cm.
2.) Femur + tibia + metatarsal 1 (10.5 cm [Ibrahim et al., 2014, Supplementary Materials, p. 34]) = 139.8 cm.
3.) Femur + tibia + metatarsal 1 + pedal phalanx 1-1 (11.5 cm [Ibrahim et al., 2014, Supplementary Materials, p. 34]) = 151.3 cm.
Percentage of arm compared to leg:
*1.) Humerus + radius - femur + tibia:
129.3 - 91.2 = 38.1.
38.1/129.3*100 = 29.5% decrease.
100% - 29.5% = 70.5% of the leg.
*2.) Humerus + radius + manual phalanx 2-1 - femur + tibia + metatarsal 1:
139.8 - 108.7 = 31.1.
31.1/139.8*100 = 22.3% decrease.
100% - 22.3% = 77.8% of the leg.
3.) Humerus + radius + manual phalanx 2-1 + manual phalanx 2-2 - femur + tibia + metatarsal 1:
139.8 - 120.8 = 19.
19/139.8*100 = 13.6% decrease.
100% - 13.6% = 86.4% of the leg.
5.) Humerus + radius + manual phalanx 2-1 + manual phalanx 2-2 + possible manual ungual - femur + tibia + metatarsal 1 + pedal phalanx 1-1:
151.3 - 129.6 cm = 21.7.
21.7/151.3*100 = 14.3% decrease.
100% - 14.3% = 85.7% of the leg.
Or:
151.3 - 130.2 cm = 21.1.
21.1/151.3*100 = 14% decrease.
100% - 14% = 86% of the leg.
Hypothetically speaking, the arm of the Spinosaurus neotype would be about 70.5-86.4% of the leg (at best).
Links:
Charig and Miller (1997):
https://www.biodiversitylibrary.org/page/36949178#page/201/mode/1up
Ibrahim et al., (2014):
https://www.researchgate.net/publication/265553416_Semiaquatic_adaptations_in_a_giant_predatory_dinosaur
-Supplementary Materials:
http://science.sciencemag.org/content/suppl/2014/09/10/science.1258750.DC1/Ibrahim.SM.pdf
Ibrahim et al., (2020b):
4.) Another complete Spinosaurus arm!?
I don't know how I missed this, but someone on Reddit, called Super-Masterpiece-34, posted a picture of another complete Spinosaurus arm. It's a private specimen though, but it reveals what I said about the arm from Dino Lab: The humerus would've been shorter! The third finger is also shorter than the second one, contrary to the third digit of the Dino Lab arm. The manual unguals are the same as the ones in the Dino Lab, Ibrahim et al., (2020a), and Fabbri et al., (2022), which means that Spinosaurus' manual unguals are shorter than the baryonychinae's.
Photo of the private Spinosaurus arm (Reddit, Super-Masterpiece-34, May surprise many people):
.jpg)
Notice that the third finger is smaller than the first two fingers. Perhaps the Dino Lab arm is missing a phalanx in the middle finger, or one of the bones is in the wrong position, etc. The humerus seems to be about the same size as the radius and ulna. It also looks kind of similar to the Dino Lab humerus. It looks smaller than the Dino Lab humerus, so maybe it's authentic? From what I can guess, the arm would've been about 2/3 of the length of the man in the photo. We can also see this in a photo of the Dino Lab arm with paleontologist Dr. Philip Currie:
The arm, with a proper humerus, would've been about 2/3 the size of Dr. Currie, and probably the lady in the photo as well. Here's another picture of the arm with the same lady present:This suggests to me that Spinosaurus' arms were smaller than a fully-grown adult human. It's definitely smaller than the reconstructed arm in the 2014 skeleton.
Arms of the reconstructed 2014
Spinosaurus skeleton (Kenneth Chang, 2014):
The arms of the reconstructed skeleton are the same size, if not a little longer, than the man in the photo. This is not the case with the Dino Lab, and the private, Spinosaurus arms. I also noticed that Sereno et al., (2022) stated that the arms of their Spinosaurus model are shorter than the model used by Ibrahim et al., (2020b) (Figure 9):
Description of figure:
So, if the arms of
Spinosaurus were smaller than previously stated, then perhaps this means that it was bipedal after all? Perhaps the center of mass (CoM) is really towards the pelvis instead of the stomach. There is the problem of body mass. In my first post of this series, that seemed to be a constant bother regarding a fully bipedal
Spinosaurus.
Spinosaurus had to have been about 2-3 tons or so in order for the femur to support the animal's eight. If that's true, then the animal still would've been a quadruped. Still,
Spinosaurus bones have been found inland (Sereno et al., 2022, Figure 7A), so the animal had to have been able to traverse across the land somehow.
I'll say this: Based on what I've found so far, my math tells me that the arms of the
Spinosaurus neotype would've been about
70.5-86.4% of the leg length at best. I made a small drawing using a scale bar from Lucas et al., (2005) (Figure 1), and there is a small gap between the hands and the ground. This gap is represented by the arrow:The gap is not as large as I would've liked, so I don't know if it's definitive proof of a bipedal Spinosaurus. Perhaps it's enough for the animal to be bipedal without its hands brushing against the ground? Maybe. As for the weight of the animal, if it turns out that
Spinosaurus was too heavy to support its weight just by its femur/legs alone, then it was a quadruped. It's best to have an open mind. I really want paleontologists to find the rest of the bones in the neotype's arms. My answer: It could've been either-or.
Links:
Reddit. Super-Masterpiece-34. May surprise many people:https://www.reddit.com/r/Dinosaurs/comments/15ocfp3/may_surprise_many_people/
Kenneth Chang (2014):
https://www.nytimes.com/2014/09/12/science/a-nomads-find-helps-solve-the-mystery-of-the-spinosaurus.html
Photo:
https://images.app.goo.gl/19ScuGkueXHXFEFB9
Sereno et al., (2022a):
https://elifesciences.org/articles/80092
Lucas et al., (2005) (Figure 1):
https://www.researchgate.net/figure/Skeleton-of-New-Mexicos-official-state-fossil-Coelophysis-bauri-after-Paul-1993_fig1_255459322
5.) Suchomimus to the rescue?
Just when I thought I ran out of ideas, I find another way!
I went back to Suchomimus, and Suchomimus' manual phalanx 1-1 does make Spinosaurus' arms smaller. Granted, I had to take a few things into account:
1.) Suchomimus' and Spinosaurus' manual phalanxes from Sereno et al., (2022) aren't from the neotype, or Suchomimus holotype. They're separate specimens.
2.) However, Sereno et al. used them to finish skeletal diagrams of Spinosaurus and Suchomimus. I will assume that the manual phalanxes were scaled to size with the rest of the skeletons.
The Spinosaurus manual phalanx 1-1 used is UCRC PV8, and the Suchomimus manual phalanx 1-1 used is MNBH GAD503. They are "D" and "H" in Sereno et al., (2022).
Figure 1 (Sereno et al., 2022) (Scale bars for D and H are 3 cm):
Spinosaurus' manual ungual 1-1:
Suchomimus' manual ungual 1-1:
Manual phalanx 1-1:
-
Suchomimus (MNBH GAD503 [NOT holotype]): 14.4 cm (at best) (Measured myself).
-
Spinosaurus (UCRC PV8): 14.6 cm (Measured myself).
Sereno et al., (1998) for Sucho holotype (MNBH GAD500):
Humerus: 56.0 cm.
Radius: 25.5 cm.
Femur: 107.5 cm.
Tibia; 94.5 cm.
Spino neotype:
Femur: 62.5 cm (Ibrahim et al., 2022b).
Tibia: 66.8 cm (Ibrahim et al., 2014, Supp. Mat.).
1.) Arm length:
14.6 - 14.4 = 0.2.
0.2/14.4*100 = 1.4% increase.
Humerus:
56 cm + 1.4% = 56.8 cm.
Radius:
25.5 cm + 1.4% = 25.9 cm.
Spino:
-Humerus + radius: 56.8 cm + 25.9 cm = 82.7 cm.
-Humerus + radius + manual phalanx 1-1: 56.8 cm + 25.9 cm + 14.6 cm = 97.3 cm.
-Humerus + radius + manual phalanx 1-1 + possible manual ungual (incomp.): 56.8 cm + 25.9 cm + 14.6 cm + 8.8 cm = 106.1 cm.
-Humerus + radius + manual phalanx 1-1 + possible manual ungual (poss. comp. length): 56.8 cm + 25.9 cm + 14.6 cm + 9.4 cm = 106.7 cm.
Hypothetical Spinosaurus arm length: 106.1 to 106.7 cm.
Hypothetical Spinosaurus leg height: 151.3 cm.
This is smaller than the 129.6- to 130.2-cm length I obtained using Baryonyx. I also added this length to my previous diagram ("S" is for Suchomimus, and "B" is for Baryonyx):
.jpg)
Perhaps, this could help support a bipedal, and a smaller-armed,
Spinosaurus. Still have to shrink
Spinosaurus' weight down, and I still want the arm bones for the
Spinosaurus neotype discovered.
Links
Sereno et al., (1998):Sereno et al., (2022):
https://www.biorxiv.org/content/10.1101/2022.05.25.493395v1.full
6.) Too much weight to carry?
Well, I might've hit another roadblock... Possibly two of them. I found Myhrvold et al., (2022), and another figure of UCRC PV8. This time, the scale bare is 5 cm instead of 3 cm as in Sereno et al., (2022). I decided to remeasure the specimen.
UCRC PV8 from Myhrvold et al., (2022) (Figure 1d):
Length: 28.4 cm (at best).
Using Suchomimus manual phalanx 1-1 from Sereno et al., (2022):
28.4 - 14.4 = 14.
14/14.4*100 = 97.2% increase.
If Suchomimus' manual phalanx 1-1 was actually 23 cm (at best), and UCRC's manual phalanx 1-1 was 24.5 cm (at best) (Both were remeasured on 4/21/25 in Sereno et al., 2022. I assumed that the scale bars for both bones were 5 cm instead of 3 cm):
24.5 - 23 = 1.5.
1.5/23*100 = 6.5% increase.
56 cm + 6.5% = 59.64 cm for UCRC's humerus.
Using the 28.4-cm estimate for UCRC:
28.4 - 23 = 5.4.
5.4/23*100 = 23.5% increase.
56 cm + 23.5% = 69.16 cm for UCRC's humerus.
Now, after re-reading Sereno et al., (2022) and Myhrvold et al., (2022), it is stated that UCRC PV8 is 28 cm long. The authors scaled up UCRC PV8 to the length of the largest Spinosaurus specimen, MSMN V4047. UCRC PV8 was 20% smaller than MSNM V4047. Using UCRC and Irritator/Angaturama's manual phalanx 1-1, they estimated that MSNM's manual phalanx 1-1 was 35 cm long (Sereno et al., 2022, Table 1). If I upscaled Suchomimus' humerus using the 97.2% increase I obtained from UCRC, then we get a length of 110.4 cm for the humerus. This would just be for UCRC PV8's hypothetical humerus. I wasn't satisfied with this, so I went back to Sereno et al., (2022) and remeasured both manual phalanxes again. I assumed that the scale bars for both bones were 5 cm instead of 3 in order to see if I could make Suchomimus' manual phalanx longer. I got 23 cm (at best) for Suchomimus, and 24.5 cm (at best) for UCRC PV8. This would've made UCRC's humerus 6.5% larger than Suchomimus', resulting in a humerus length of 59.64 cm for UCRC's humerus. Since UCRC's manual phalanx is actually 28.4 cm long, this would result in a 23.5% increase. UCRC's humerus would be 69.16 cm long. The humerus is still growing. If MSNM's manual phalanx 1-1 was an estimated 35 cm, then the humerus would've been even longer for the largest adult Spinosaurus specimen. No matter what I do, the arms of Spinosaurus keep growing longer... The arm length I obtained for the neotype specimen, using Baryonyx, is probably more accurate than the numbers I obtained using Sereno et al., (2022). The Spinosaurus neotype, FSAC-KK 11888, doesn't have a preserved manual phalanx 1-1, so I cannot do any comparisons.
There's one last move that I could do in order to estimate the probable length of the FSAC's humerus. I can use the femur lengths of Suchomimus, and FSAC, to guess a humerus and radius length for FSAC.
FSAC's femur length: 62.5 cm.
Suchomimus' femur length: 107.5 cm.
107.5 - 62.5 = 45.
45/107.5*100 = 41.9% decrease.
56 cm - 41.9% = 32.5 cm for the hypothetical humerus.
25.5 cm - 41.9% = 14.8 cm for the hypothetical radius.
Arm lengths for FSAC:
32.5 cm + 14.8 cm = 47.3 cm.
Leg lengths:
62.5 cm + 66.8 = 129.3 cm.
As much as these measurements would be satisfactory for a bipedal Spinosaurus, I have to remember that we have the manual phalanx 2-1 for FSAC. As shown above, using Baryonyx's possible manual phalanx 2-1 because the Suchomimus holotype doesn't have the manual phalanx 2-1 preserved (sheesh!), we get a humerus length of 61.4 cm for FSAC. Perhaps Spinosaurus had long hands, but a short upper and lower arm? Either way, no matter what I do, the arms of Spinosaurus are suggested to have been very long. This leads me to suggest that Spinosaurus had long arms and hands, but short manual unguals.
Not only are Spinosaurus' arms getting longer, but the weight of the animal seems to be a big hinderance for a purely bipedal Spinosaurus. In Persons IV et al., (2019), Spinosaurus' femoral length is 83 cm. The weight of the animal is 1,645 kg (1.81 tons) (Table 2). This seems to be for the specimen MSNM V4047. How can a 14-meter long animal weigh only 1.8 tons!? Persons IV et al. couldn't comprehend this either. They used the "minimum femoral cross-section area (MCF)" method from Campione et al., (2014) to calculate the weight of MSNM, and they're obtained a weight of 1,645 kg for the specimen. They stated that the weight of the animal, and "the cross-sectional strength" of the femur because Spinosaurus has a closed medullary cavity, must've been greater (pp. 665-666). They also suggested that Spinosaurus being a quadruped or semi-quadrupedal, as suggested by Ibrahim et al., (2014), could've helped in the distribution of the expected greater weight of Spinosaurus, especially if Spinosaurus' femur couldn't hold its weight by itself. This is due to Ibrahim et al. stating that Spinosaurus' legs were "reduced in all dimensions relative to its body mass." Thus, obtaining an accurate body mass estimate for Spinosaurus is tricky (pp. 666-667).
This is interesting because paleo-artist/paleontologist Hank Sharpe on Twitter-X also used Campione et al., (2014) to estimate the weight of Spinosaurus, and came to the same conclusions. He said that Spinosaurus' femur wasn't large enough to support its weight (Sharpe, 2023, "Think Spinosaurus' legs look kinda wimpy for its size?..."). Larramendi et al., (2020) also stated that Spinosaurus' legs weren't capable of holding its weight alone (3. Results and Analysis 3.2.9 Nonavian avepod theropod dinosaurs para. 2). Even paleontologist Dr. Witton, who supports the strictly bipedal posture for Spinosaurus, said 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. If Spinosaurus was in a vertical posture, as proposed by paleontologist Dr. Cau in 2014-2015, and didn't bend its legs, then it could support itself. In any other posture, Witton said that Spinosaurus' legs wouldn't be able to support its weight (Spinosaurus 2020: thoughts for artists: Posture and balance, para. 2-3). All that I'm getting here is that Spinosaurus' legs weren't capable of supporting its weight as a 14-meter theropod. This beast should've been up to about 10 tons or so. Sereno et al., (2022) said that an adult Spinosaurus was 14 meters long and weighed 7,400 kg (8.2 tons) (Conclusions, number 1 and 5), so I cannot imagine a 80-cm long femur supporting a 14-meter creature that weighed 8 tons.
Interestingly, according to Sereno et al., (2022), Ibrahim et al., (2020b) said that Spinosaurus' CoM as an adult was 95.7-108.9 m (Table 9):
This is very close to the results I got in Part 1 of this series. It seems that Spinosaurus' CoM grows as its body grows.
So in the end, what do we know?
1.) We need the rest of FSAC's forelimb bones to be excavated.
2.) Based on what we do have, Spinosaurus' arm bones suggest that the animal had long arms and short manual unguals. The arms seem to have been as long as the animal's legs.
3.) If I use the femoral lengths of FSAC and the Suchomimus holotype, we get relatively small arms. However, this contradicts the very long manual phalanx 2-1 that FSAC has. UCRC PV8 also has a very long manual phalanx 1-1, correlating with the trend that Spinosaurus had long hands and possibly long forelimbs in general. Either Spinosaurus had short arms with long hands, or using the femoral lengths of Spinosaurus and Suchomimus do not yield accurate results for a hypothetical arm length for Spinosaurus.
3.) The weight estimates of Spinosaurus do not correlate with the size, and width, of its femur. An adult Spinosaurus was 14 meters and 8 tons, but it has been noted that its femur couldn't hold an animal that weighed that much. This suggests that Spinosaurus was too heavy to walk as a biped after all.
4.) The center of mass (CoM) of Spinosaurus seems to have grown as the animal's body grew.
For now, I have to stick with a quadrupedal Spinosaurus. The hands of the Dino Lab, and private, specimens do look long and sturdy enough to have given some support for Spinosaurus while it walked.
Links:
Myhrvold et al., (2022):
https://www.biorxiv.org/content/10.1101/2022.04.13.487781v1.full.pdf
Sereno et al., (2022):
https://www.biorxiv.org/content/10.1101/2022.05.25.493395v1.full
Persons IV et al., (2019):
https://onlinelibrary.wiley.com/doi/epdf/10.1002/ar.24118?tracking_action=preview_click&r3_referer=wol&show_checkout=1
-V2:
https://anatomypubs.onlinelibrary.wiley.com/doi/am-pdf/10.1002/ar.24118
-V3:
https://www.gbif.org/species/159236947
-Abstract:
https://onlinelibrary.wiley.com/doi/pdf/10.1002/ar.24118
Sharpe (2023) ("Think Spinosaurus' legs look kinda wimpy for its size?..."):
https://twitter.com/Paleoartologist/status/1622728136403337216
Larramendi et al., (2020):
https://anatomypubs.onlinelibrary.wiley.com/doi/abs/10.1002/ar.24574
Witton (2020):
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