There shall be no such good feelings and consensus with these posts as I am wallowing back into the treacherous waters of Spinosaurus once again!!
|credit Duane Nash
My idea posted here that Spinosaurus was not fully bipedal, nor classically quadrupedal either, but is best characterized as a belly slider continues to get a lot of page views. I will embellish my case for that locomotion in these posts as well as some other possible locomotory modes used during the creatures' ontogeny. More importantly I will explore how this creature likely moved in its predominant habitat - the water - which is more important imo than how it moved on land anyways. If you are still holding out that spino was primarily terrestrial and only went into the water sporadically we obviously differ right from the get - go in which case you will probably not be convinced by my arguments here which treat Spinosaurus as primarily aquatic as a given. I have discussed the sail before here but my thoughts on the sail have changed pretty radically - which will also be detailed in these posts.
Anyways, I hope you have as much fun with this post as I had writing it!!
It is now officially October which basically translates here in southern California where I live to asking yourself is it cool enough that I should think twice about putting on shorts and sandals today? Over on the east coast they experience a little bit more of a seasonal bite, with Florida occasionally getting a twinge of frost by winter. What this means for people over there is that they actually have to dress for the seasons. But what this change in seasons means for a particularly interesting marine mammal is a question of life or death.
|credit Ahodges7. Florida manatee @ Sea World Orlando CC3.0
The West Indian manatee (Trichechus manatus) is an obligate tropical water marine mammal. Sustained exposure to cold water (below 20 C or 68 F) will cause mortality. It is this sensitivity to cold water and not predators or even boat propellors that is the chief limit to the population of this species. This means that the Florida population - usually referred to as the Florida manatee - which can range as far north as Massachusetts must migrate south before the advent of substantial water cooling. Anthropogenic warm water refugia in certain enclaves of north/central Florida has created unique opportunities for these Florida manatees to remain in waters year round that they normally would have not persisted in.
However the question (unless you already know the answer) in your head should be why are these animals so deathly cold sensitive - shouldn't their blubber layer allow them to survive in cold conditions? The answer to this query is that manatees have only a slight, incipient fat layer. Despite their rotund appearance their blubber quotient is a far cry from the blubbery status of pinnipeds/cetaceans and other cold water tolerant marine mammals. What gives manatees that "fat" appearance is actually a very thick dermis i.e. they are thick skinned.
It could be argued that skin does not get the attention it deserves despite it being the most visible and exposed organ in vertebrates. It is also usually the largest organ. In fact if you ever want to show off your biological knowledge at a social event asking "what is the largest organ in the human body?" (snark, snark) is a good conversation starter because it is in fact skin.
So why do manatees (and, as I will discuss later, other aquatic tetratpods) develop such a thickened dermis? Manatees don't engage in combative behavior nor do they suffer from intensive predatory pressure. The reason for the dense and thick dermis is related to buoyancy control. While dense heavy bones (pachyostosis) have long been recognized as aides in achieving negative buoyancy in water what is less recognized is that a heavy, dense dermis is also an adaptation for achieving negative buoyancy.
In fact a paper (Kipps et. al., 2002) addresses this very issue. Titled Skin density and its influence on buoyancy in the manatee (Tricheus manatus latirostrus), harbor porpoise (Phocoena phocoena), and bottlenose dolphin (Tursiops truncatus) this paper compares and contrasts the relative level of skin density as an adaptation to buoyancy control in these three mammals. In both the harbor porpoise and bottlenose dolphin skin density was less than sea water density and contributed a net positive buoyancy force. Thinking in terms of where these animals live - relatively deep water - this makes sense to be a little bit lighter than water so they are not always expending energy to stay afloat. However the manatee is notably different in terms of buoyancy. Its thickened dermis provides a net negative buoyancy to its body.
From the study:
What should be restated is that the skin of the manatee:
"equaled 70% of the negative bouyant force of their dense, pachyosteosclerotic ribs."
That is substantial. Also of interest is that the manatee's buoyancy changes with water depth - which is a known phenomena in diving marine animals:
"The manatee is positively buoyant at the surface and negatively buoyant at depths of less than 10 m."
Now think about this in terms of what and where a manatee is going to be living and foraging. When they are just resting, breathing, or just lolly-gagging about at the surface it makes sense for them to be just slightly buoyant at the surface. However since they are herbivorous and much of their food will be foraged below the surface of the water it also makes sense that once they dip below the surface they are negatively buoyant so that they are not expending loads of energy staying deep. But as manatees are not predatory but herbivorous they are not going to go much beyond the photic zone in terms of foraging which is why that 10 meter mark is so interesting - most of the plant mass will be in the upper, shallower levels where sunlight penetrates.
If you are still a little confused about how the densities of different body tissues can change with water depth maybe this paragraph can help you from the paper:
Additionally it is important to realize that lipid (i.e. fat or blubber) and air filled tissues will compress with increasing pressure i.e. depth. What this means is that in shallow water a blubbery tetrapod can be positively buoyant but with increasing depth both lungs and fat tissue will compress so that the diving animal becomes neutral or even negatively buoyant. As a general pattern the neutral or "gliding" phase of diving tetrapods is both energetically and ecologically optimal similar to basically moving around in a zero gravity environment, essentially outer space.
However in aquatic tetrapods that are at most living in 10's of meters of water as opposed to 100's or 1000's meters of water depth, the pattern of the manatee - dense dermis and dense skeletal system - promises more optimization for that habitat choice. With a dense skeletal and dermal adaptation they do not have to expend energy diving - unlike positively buoyant deep divers which must expend at least some energy to get past the buoyant threshold of depth. Furthermore, since they are only living in water depths measured in the 10's of meters of depth it is not a challenge to push off the bottom or swim a bit to get a gulp of air.
At this point it should be apparent to readers where I am going with this. Spinosaurus, as most likely an inhabitant of estuaries, large river systems, possibly coastlines squarely falls more in the pattern of the manatee as opposed to the dolphins in terms of typical water depths (10's of meters) and therefore skin and skeletal density. Long story short give your Spinosaurus depictions a thick skin - looking at the skin of hippos and manatees is a great reference. And so the artists who speculated on the "Mr. Big" Spinosaurus countenance may have hit it closer to the mark than they even realized.
|credit Duane Nash
The Hippo: Can't Swim and Not Really Fat
Although I can't find any relevant literature on hippo skin it is fairly common knowledge that there skin is exceptionally dense and thick. While such a thick skin is no doubt useful to defend against predators and intraspecific combat I am going to suggest that such a thick dermis evolved in concert with those reasons to help the hippo achieve and maintain negative negative buoyancy.
The series Inside Natures Giants takes a look at hippo anatomy and anatomist Joy Reidenberg has some choice words describing hippos skin (start at about the 11:45 mark).
"That's really thick skin... and very, very tough. It feels almost like elephant skin but a lot, lot thicker. I think that's the toughest skin I ever had to cut."
When, after much effort, she finally gets a plug of skin removed you really get a sense of the thickness. She really makes a point that it is not fat but is all skin. The "fatness" of hippos is due to the thick skin. large stomach, and barrel chested torso. Don't fat shame hippos - they probably have a lower BMI than you do!!
Here is a really interesting photo-essay documenting an intrepid male hippo that got into a pretty dangerous situation moving through some rough coastal surf along the Mozambique coast of South Africa. The young male was observed fighting through strong surf and finally scrambling over sharp rocks for over 2 hours to make it around a point into a calm bay.
|credit Angie Gullan, Dolphin Encounters & Research Center. Nat Geo
|credit Angie Gullan, Dolphin Encounters & Research Center. Nat Geo
Underwater locomotion is the topic for another post and I am getting a little ahead of myself. What should be noted is that the correlation of thick skin and shallow water diving should be noted in both the manatee and the hippo.
Are there other aquatic/semiaquatic tetrapods that might suggest shared anatomical features in congruency with an aquatic habit?
Well, I am glad you asked because yes there are several strong candidates displaying various levels of dedication to a watery lifestyle.
|Walrus public domain
|Malayan tapir. credit Sepht CC2.5
Although more rigorous anatomical and behavioral studies are needed I think there is a strong suggestion of a trend in the animals I have listed. As we move from tapir to hippo to walrus to manatee this should become apparent. Increasing aquatic adaptation in these animals is associated with increasing skeletal density and skin density. At the lower end of the scale would be tapirs and at the other end of the scale walrus and manatee. I suspect Spinosaurus sat somewhere between the hippo and walrus in terms of dedication to the water. Spinosaurus therefore likely had an extremely thick and dense skin. This dermis would have "fattened" the appearance of the animal, offered protection from intraspecific combat, predators, and retaliatory prey (sawfish rostrums for instance). I highly endorse the Mr. Big model for Spinosaurus. And if walrus and hippo can pack on over 1,000 lbs of skin on their frame, Spinosaurus with a much larger frame likely packed on a skin weight measured in the tonnage!!
In my next post I will further look into the way(s) Spinosaurus moved in the water. Can't promise when that will be and I apologize if I do not respond to comments quickly as I will be in Dallas this week... and if you read said abstract list for a certain convention you will note there is a study on spino swimming ability. As a little hint for next post I predict that spino will be shown to be just an ok swimmer - probably less efficient than a croc. But this does not imply that spino did not have other ways of moving rapidly in the water (hippo punting hint, hint) ...
N. Ibrahim, P. Sereno, C. Dal Sasso, S. Maganuco, M. Fabbri, D.M. Martill, S. Zouhri, N. Myhrvold, D.A. Iurino (2014). Semiaquatic adaptations in a giant predatory dinosaur. Science 26 September 11, 2014
Kipps, E.K., McLellan, W.A, Rommel, S.A., & Pabst, D.A. 2002. Skin density and its influence on buoyancy in the manatee (Tricheus manatus latirostrus), harbor porpoise (Phocoena phocoena), and bottlenose dolphin (Tursiops truncatus). Marine Mammal Science 18(3) 765-778. July 2002
"A Long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom". Thomas Paine
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