Well now... a lot has transpired since my last post in which I suggested that Spinosaurus likely fit within a range of aquatic and semiaquatic tetrapods in having a very thickened skin to aide in ballast. I went to to SVP in Dallas where Paul Sereno discussed his findings on Spinosaurus' swim speed and likely neutral buoyancy in the water. Let's say that his findings - preliminary as they are and no doubt controversy will ensue upon publication - did nothing but bolster my contentions regarding how Spino sat, moved, and generally behaved as a physical body in the water. Based on Sereno's preliminary findings ol' Spino was just an ok swimmer at about 3 meters/second. That is just a tad bit faster than Michael Phelps but quite a bit slower than your average croc. Keep in mind that crocodiles for the most part are not actively chasing and pursuing fish in the water column like an otter or seal but are more often than not using sit & wait tactics. stalking, and using physical obstacles to corral fish. So the notion that Spino was actively paddling and dashing after fish in open water may not make much sense. That is not to say that Spino didn't have a way to actively pursue fish (underwater running hint, hint) but that will have to be a future post. Also - based on Sereno's preliminary findings - Spino sat low in the water in terms of buoyancy. In fact as I have been saying since the week the Ibrahim new look Spino was unveiled the WHOLE DARN SAIL WAS UNDERWATER when Spino was in deep water!! So I gots to feelz myself a bit here - told ya so, told ya so, told ya so. This really puts a damper on the prevailing notion, that in my opinion, has just kinda been accepted and not really questioned rigorously - that the primary use for the sail on Spinosaurus' back was for social signalling. Additionally keep in mind that Sereno's work on buoyancy - and I talked to him at SVP - does not infer that Spino likely had a thickened epidermis that further added density - his calculations were just using the bones. What I am trying to say is that Spino potentially sat a lot lower - like literally right on the bottom - in the water column. What this suggests is that some of the most startling and amazing artwork that came out with the Ibrahim reveal such as this piece by Davide Bonnadonna (used w/permission) might already be dated.
Instead a more likely image for Spinosaurus I would argue is the one below (used w/permission)- created by Christopher DiPiazza of the blog Prehistoric Beast of the Week.
Now I have to hand it to Christopher for being very prescient in doubting the ability of Spino as a swimmer but inferring it instead as an underwater runner and being one of the first artists to depict it as such. The only knock I can give it is that the skin should be a tad thicker (as discussed in my last post) and overall morphology more rotund ala hippo...
And here is my version of the underwater running Spino which is a bit of teaser for an upcoming post on underwater running... (yes, yes I wanted to do just 2-3 posts on Spino but it keeps growing)
So now that that stuff is out of the way (again the sail & underwater running will be future posts) I want to get to what this post is really about and that is two separate topics that might at first appear mutually exclusive but I will link them together at the end of the post: the Evers et al. paper that has spurred on some interesting thought but also loads of misinterpretation on the "popular" front; and, stimulated by discussions provided by LeeB from my last post, the interesting topic of island hopping hippos.
I have waited a bit to really address the Evers paper and, to be perfectly honest, I did this because the paper is just too darn long!! Seriously if the Ibrahim et al. paper got knocks for being overly abbreviated this paper deserves to get some critique on being overly verbose and less than easy to follow as a document. I really doubt most of the people heralding it as the savior of the long-legged Spinosaurus have even read it.
It did not have to be this way as the primary take home message is that Sigilmassasaurus brevicollis is its own beast!! That is about it when it is all said and done. Keep in mind that for Sigilmassasaurus we are talking about vertebrae - no appendicular elements, no skull material, not even a rib. It should also be added that Spinosaurus maroccanus (vertebrae & premaxilary material) is now - according to this study - sunk into Sigilmassasaurus brevicollis. As you may recall the Ibrahim et al. study cleaned house on all North African spinosaurid material and just lumped it all into S. aegyptiacus. The Evers paper could have been much more effective if they just presented their character analysis of the vertebrae, made their arguments for what bones they represent in the vertebral series, and made a succinct and clean argument for North Africa hosting at least two spinosaurids in the Kem-Kem and Bahariya respectively.
This beefy 100 page paper can best be summarized with this paragraph here:
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This paper should have just focused on retrieving S. brevicollis as a unique species and not have - I don't know how else to put it - used its bully pulpit to cast doubt on the unique morphology of Spino B and FSAK-11888 - especially since the monograph of the latter is not even published and we don't even have the former anymore. Instead, among other critiques of Ibrahim et al., the authors question the synonymy of Stromer's spinosaur material with Ibrahim et al's. FSAK-11888 and reject their claim of a neotype.
They also insist on describing the retrieval of FSAK-11888 as coming from an "allegedly" associated partial skeleton several times. Understandably this is because the monograph is not yet prepared addressing the taphonomy and collection of said specimen. At the same time Evers et al. seem to really run with this "chimeric" interpretation of FSAK-11888 casting doubt on its validity. Is it chimeric due to Moroccan fossil dealers pulling together disparate skeletal elements in order to make a sell? If this is the case then why give the animal diminutive pelvic elements? Andrea Cau has addressed this problem on his blog here. The second possibility is that the elements are chimeric not due to anthropogenic forces but due to taphonomic forces, a possibility the authors raise citing several studies alluding to the Kem-Kem having a notorious reputation for mixed elements. However let us reason this scenario out. In both Stromer's Spino B and the Ibrahim FSAK-11888 we have diminutive pelvic elements associated with lengthened dorsal vertebrae, and neural spines... what are the chances of that happening twice with either taphonomic or anthropogenic interventions? Again these critiques are not new, they should be addressed, but I think that they really obfuscate the prime - and very interesting - point of the Ever's paper: that is North Africa hosted two Cenomanian spinosaurids.
Evers et al. also fail to mention a very important layer of evidence in favor of FSAK-11888 representing a single individual. This line of evidence bears repeating - in fact it should have got trumpeted from the hill top loudly a long time ago - and that is that the vertebral and appendicular bone histology of FSAK-11888 both signal a relatively young, sub-adult ontogenetic age . Yup both the diminutive pelvic elements and vertebral column/ribs/gastralia suggest an immature individual. This is not what we should expect if the elements are chimeric. Let's review that bit from the supplemental materials section of Ibrahim et al.:
"Neurocentral sutures preserved in the vertebrae do not exhibit coocssification, nor is there coossification between sacral centra or between the ilium and sacral vertebrae." (pp.13 supp.)
"Two long bones (femur, fibula), a possible gatralium and the proximal end of a dorsal rib were selected for histological thin sections.... (pp. 13 supp.)
"An external fundamental system is not found in any of the four bones sectioned, and vascularization is still prsent in the circumferential layer. We infer a subadult ontogenetic stage for the neotype specimen. This interpretation is also based on the high amount of Haversian systems in the inner cortex, the decrease in density of vascularization towards the surface of the cross section and the decrease in spacing between LAGS toward the outer cortex. Maximum adult size would likely have entailed many years of subsequent growth." (pp. 14 supp.)
Not addressing these histological lines of evidence in the Evers et al. paper is a major omission in my view. As I have said before this paper should have just dealt with Sigilmassasaurus as a unique taxa and by going after Ibrahim et al. in totality in a none too circumspect way they really do themselves a disservice. If you are going to make critiques you can't just omit the stuff that you don't like i.e. the very pertinent bone histology congruency between vertebral and appendicular elements suggesting just one individual. Especially if your paper is 100+ pages long. But Evers et al. are not solely too blame on this front. If you go back and review the several very storied and infamous online critiques of Ibrahim et al. including - Switek's, Headden's, Hartman's, and Witton's - you will see that all four of them fail to mention the bone histology data suggesting FSAK-11888 is one, subadult individual. For all of these very vocal and visible blog postings not to mention this line of evidence is very interesting. Make of that what you will.
The unfortunate effect of this paper is that it adds fuel to the fire for those in the "popular realm" (i.e. JP3 Spino fanboys) who are hellbent on retrieving an upright bipedal Spinosaurus at any cost. "Look at this 100 page paper in PeerJ - the authors shoot down the Ibrahim paper!! Whoopee long legged Spino lives again!!"
Again, while the new paper might be correct in there being two or more spinosaurids in N. Africa (maybe even a species complex of Spinosaurus?) and that their critique of Ibrahim et al. lumping all N. African spino material into S. aegyptiacus may be valid- the morphology for Spinosaurus that the Ibrahim et al. paper gave did not hinge on the Sigilmassasaurus or S. maroccanus material anyways (which is mainly isolated vertebrae). We still, at the end of the day have the congruence of morphology in Spino B and FSAK-11888 standing tall (pun intended).
The authors even begrudgingly concede this in the last sentence below:
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Furthermore, on a wider note, this movement to retrieve a long-legged, bipedally striding Spinosaurus (Hartman's skeletal is usually the one bandied about - but not a fan of it myself or his spinosaurids in general - too gracile imo) both on the popular and scientific front I predict will fall flat on its face eventually (pun intended). Two specimens - as controversial and questionable as they may be - both displaying similar gross morphology in the pelvic area > zero specimens of Spinosaurus showcasing "normal" sized theropod pelvic anatomy. One set, the one we do still have, shows ontogenetic congruence between the vertebral and appendicular elements i.e. not likely a chimera. That's right, there is exactly 0.0% verifiable evidence for a long-limbed, classically bipedal Spinosaurus or even spinosaurid in North Africa. None. Any assertions for this interpretation over something more in the Ibrahim et al. ballpark of gross morphology - without skeletal evidence - is an assertion of a mythical creature over something we do have (controversial as it is) evidence for. It's high time we flip the script on long-legged Spinosaurus being the de facto representation - there is no evidence for that animal at all.
Now with no clever segue at all (although I promise it will relate back to above discusssions) I want to go into into island hopping hippos.
What really started this off was me stating - as others notably the San Diego Zoo - that hippos can't, in the truest sense of the word, actually swim. They are denser than water and don't really have large flippered limbs that can grab and push back large volumes of water. The reason for their density is - and I will go into this more in depth in a future post - that their primary mode of locomotion is best achieved via their ability to stay low in the water column. Hippos use a foot propelled running/trotting locomotion "underwater punting" in which the foot pushes off the substrate to achieve long and slow gliding phases or when hippos want to move faster they actually take shorter and quicker steps. At first this might seem counter intuitive but try it yourself. Go into a body of water at least 2/3 up your chest. Now experiment with running underwater to achieve maximum velocity. You will quickly realize that short/rapid punting steps outpaces long, gliding paces. Therefore for hippos to take advantage of moving quickly in the water via short/punting steps they need to be substantially denser than the water or else gliding phases (which are slower) will take over due to relatively increased buoyancy.
Before we take a look at the various island hopping hippos and other lines of evidence that are cited in favor of hippos being able to swim let's take a closer look at an animal that I neglected to mention in my last post but offers much utility to discuss here - the pygmy hippo (Choeriopsis liberiensis).
The pygmy hippo is a startling contrast to its larger more celebrated cousin. Although of the same family they are not closely related being of different genera and perhaps diverging as long ago as 8 million years ago (Boisserie, 2005). Pygmy hippos are very much smaller, more reclusive, and less aggressive or boisterous than their larger relatives. Their range is very limited: this is likely, I muse, because of the general loss of dense forested habitat in Africa since the Pleistocene that is their preferred habitat. Which brings me to my next point - pygmy hippos are very much forest animals, another contrast between their more grassland/savanna loving larger counterparts. In fact the animal most often posited as ecologically analogous to the pygmy hippo is the tapir.
Let that last bit about the tapir analogue sink in a bit - as you should recall in my last post I suggested tapir provide a good proxy for an animal that is not yet quite as aquatic adapted as a hippo. Tapirs can underwater punt, they have thickened skin that likely aides as ballast, and a barrel shaped torso for underwater streamlining. They are likely more bouyant than a hippo and might be able to achieve a swimming stroke or even float in saltwater. To put it another way tapirs are just getting their feet wet in terms of dedication to an aquatic existence. All of this is less than rigorous but is worthy of more testing mind you...
Going further if we take this analogy to tapirs for pygmy hippos and the general observation that they appear less massive skeletally , have less skin, and can potentially float or even actively swim - then maybe pygmy hippos actually did colonize offshore islands via swimming/floating. This may have especially been the case when pygmy hippos had a more wider distribution in the past.
When I took the question of pygmy hippos actually being able to swim into my laboratory (i.e. youtube clips) I very quickly - within the first 2 or 3 videos - saw some behavior suggestive of swimming. Mind you I am yet to see any good footage of big hippos swimming in any form...
Follow this link here to see some pretty good footage of a newborn pygmy hippo calf doing some ok swimming. It does appear to achieve limb assisted swimming in several instances in this clip. You will also notice that as soon as it stops moving its limbs it sinks like a stone. I know young hippos of the big species have been suggested to be a little more bouyant than their parents but am not quite sold and them being able to self propel across 10's or 100's of kilometers of open ocean. I am a bit skeptical of them having the strength and stamina to pull off such feats. But then again maybe saltwater provides just enough buoyancy to overcome their densisty...
Here is another youtube clip of an infant pygmy hippo that appears to get a little bit of limb assisted swimming going on... but as soon as it stops pumping those legs it sinks like a stone.
For comparison here is an endearingly cute video of an infant hippo (H. amphibius, larger species) moving in water. It always appears to sink like a stone after pushing off the bottom. There is one sequence towards the end where it looks like it is trying to paddle but doesn't really get anywhere.
Malagasy Hippos
Now, I will concede that pygmy hippos show a little bit more promise for swimming or floating across deep saltwater bodies than their larger cousins which leads me to my next point. The famed Malagasy dwarf hippo - which represents several species of hippo that only recently went extinct on Madagascar - may actually potentially be part of the same genus as the pygmy hippo. I am not going to pretend to be an overnight expert on Malagasy hippo taxonomy but let's just say that it is unresolved and that the putative ancestor(s) of the various - highly terrestrial or even mountainous - recently extinct hippos of Madagascar might have been quite a bit removed from the classic hippo (H. amphibius) we all know and love. Alternatively H. lemeriei is often compared to H. amphibius so it is possible that both genera of hippos made it to the island. So until the taxonomy of the species of hippos that were endemic to Madagascar - some of which may have been a lot closer to pygmy hippos than regular hippos - is sorted out (great grad student project btw) I can't really accept positing the existence of various hippo species on Madagascar as proof positive that hippos similar to modern H. amphibeus actually swam or floated there - but rafting is my favored scenario in any case. However as I conceded earlier I do hold more hope for dwarf hippos to actually swim or float in saltwater than H. amphibeus.
The Hippos of Center Island, Lake Turkana
I have to admit that this one was pretty interesting and a little perplexing. How did hippos colonize Center Island in the middle of Lake Turkana which is the largest desert lake in the world? Investigating this did get me to finger through my old copy of Eyelids of Morning: The Intermingled Destinies of Crocodile and Man which I always wanted to blog about. It's a great book btw and I don't see books like it really written anymore - pick it up. Back then Lake Turkana was called Lake Rudolph and in the book there are several mentions of hippo in the lake and violent confrontations with the locals. Its an eerie place for hippos because it is a true volcanic desert and testimony to the adaptability of these animals as long as they have water.
Lake Turkana is the largest of the eastern rift valley lakes. It is also a relatively recent lake in terms of origin - 200,000 years old is the most commonly cited age. And research suggests that what we see today - the largest desert lake in the world - might have been in fact two or more smaller lakes and dried up completely as recently as 7,500 years ago. Follow this link if you want a good overview of the complex and dynamic geological history of this lake Paleogeography of Lake Turkana. Even before "Mega-Lake Turkana" nearly dried up 7,500 years ago the Lake Turkana basin hosted several ghost lakes and rivers. Long story short we have a more parsimonious solution to why there are hippos on Center Island. They need not have swam, floated or rafted there - they most likely were always there - a hold out from when the lake level was a lot lower as has happened several times in the lakes geologic history. Which might happen again as the main incoming tributary for the lake - Omo river - is set to be dammed.
Mediterranean Isle Hippos
Shaking my head on this one.... the problem as I see it is the taxonomy of these beasts is a mess. Yes, we have definite "dwarf" island hippos on Crete, Malta, Sicily & Cyprus. But what can definitely be said about the hippo(s) that they evolved from? Most of the literature I see linked to these animals dates to the early 19th century or even 18th century. They need a revision for sure!!
Compounding the issue is that not only do we have fuzziness on the ancestry of these beasts is that the geological history of the Mediterranean basin is horribly complex. Events like the Messinian salinity crisis of the Miocene in which the whole basin literally dried up could have allowed the ancestor(s) of the Mediterranean "dwarf" hippos to literally walk up to the various isles. High evaporation rates in the Mediterranean also may have provided just the amount of buoyancy needed to float a putative ancestral hippo type beast - especially if not built as heavily as modern H. amphibius and possibly more along the lines of Archaeopotamus.
What can be said about the Mediterranean isle hippos species for sure? From what I gather not much definitely. We can't be precise in terms of who they evolved from or when they evolved? Are they really dwarfed? Do they represent a more ancestral stock of hippo and just evolved in convergence to look superficially like modern hippos? What can be said definitely in terms of their biogeographic origin? Not much really only that they got on those islands somehow.
My final thoughts on the debate of island hopping hippos is somewhere between Mazza (2015) and Van der Geer (2015). If you follow the links at the end of the post you can read their interesting exchange. Neither paper addresses the geological history of Lake Turkana which suggests that the hippos that live on Center island need not have swam their but arrived when lake levels were lower. Furthermore neither paper addresses the many unresolved issues in hippo taxonomy and that - especially when variable of skin thickness and bone thickness are taken into account - there may have been a spectrum of densities and therefore differing buoyancy in fossil hippos. Some of the Malagasy hippos might be closer to pygmy hippos. Who knows about the Mediterranean hippos and what they evolved from and how the geological history of that area affected their biogeography. Basically it is not so simple to look at what modern H. amphibius does or does not do and infer that fossil hippos had similar capabilities or limitations. Especially so in light of the real possibility that pygmy hippos might have a different buoyancy than H. amphibius.
Hippos Best Equipped Large Bodies Mammals For Rafting
All things being equal - and even if in especially high salinities or invoking putatively >less dense< ancestral hippos that could float or dog paddle - I still prefer rafting to colonize far flung islands as opposed to deliberate swimming and/or floating to them. Rafting solves the buoyancy issue which let's remind ourselves that no hippo of either species adult or immature has ever been observed to float or swim in fresh or saltwater for any length of time - that fact can't be ignored especially for a species so common in captivity and well observed as H. amphibius. Hippos are known for a relatively slow metabolism - especially for a mammal - and this preadapts them for oceanic voyages floating on rafts. Because hippos are known to live in estuaries and saltwater beaches they are in direct line of fire for the occasional, but inevitable, mega tsunami. mega-flood or mega-typhoon. The study of these events and how they reshuffle the deck ecologically is in its infancy. The historical and scientific record is not deep enough to document many of these events. But the geological record is. And over the course of long enough periods of time such events become not happenstance but statistical eventualities. Hippos, as long as they can get secured on a floating pile of debris or large tree, are well equipped - perhaps the best equipped out of all large mammals - to survive and take advantage of such events sporadic as they are.
The Spinosaurus Biogeography Question
Now I finally want to tie in the two seemingly disparate narratives of this post - a potentially diverse fauna of spinosaurids in N. Africa and the likely rafting capabilities of hippos. As I mentioned in my last post I find it likely that Spinosaurus probably plots pretty close to hippos in terms of buoyancy i.e. spino is likely a sinker and not a floater. My reasoning is that underwater running/walking is most efficient with a heavy/dense body. It is quite possible that Spinosaurus did not only swim poorly - it did not even swim at all. This creates a bit of biogeographical issue as well because it could not likely disperse over land that well either. At least in the adult morph bipedalism can't currently be retrieved and it most likely moved in awkward belly sliding shuffle. Therefore Spinosaurus was limited in dispersing itself actively over deep channels of water and over long patches of dry ground.
However just because Spinosaurus could not move itself over deep channels of water or across long patches of terra firma does not mean that other geologic agents could not do the moving for it... Keep in mind that the Cenomanian was a hothouse world and the mega-typhoons/mega-floods/mega-tsunamis and just general slosh between the land/intertidal/ocean realms must have been unprecedented. All Spino had to do was hold onto that floating clump of debris/tree and sail away perhaps even to South America (hello Oxalia).
What I predict was going on is that each major river system and adjacent estuaries/mangroves would start to evolve its own unique "flavor" of spinosaur. Whether or not you would call these different species or subspecies... its a bit subjective. Once in a while environmental perturbations would shuffle up the groups so that new colonists would be spit out and formerly isolated populations would reengage. This might be what we are seeing in the differences between Stromer's spino & Spino B and FSAK-11888 that Evers et al. note - different members of a "species complex". Sigilmassasaurus might be its own "species complex" and it is interesting that Evers et al.'s work shows it close to several baronychines . So North Africa might just have its own giant, bipedal spinosaur that is not Spinosaurus proper!! It might also eventually prove useful to designate FSAK-11888 as a separate species from Stromer's S. aegyptiacus but still cogeneric.
However, especially in light of the taxonomic mess that is hippo taxonomy, I wouldn't hold my breath expecting Spinosaurus or N. African spinosaurid taxonomy to be sorted out neatly any time in the near future or even in your life time.
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(2015) A reappraisal of the morphology and systematic position of the theropod dinosaur Sigilmassasaurus from the “middle” Cretaceous of Morocco. PeerJ 3:e1323 https://dx.doi.org/10.7717/peerj.1323
Mazza, PD (2015) To swim or not to swim, that is the question: a reply to Van Der Geer et al. Lethaia Focus V-48 pp 288-290
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
Van Der Geer AAE, Anastasakis G, Lyras GA (2015) If hippopotamuses cannot swim how did they colonize islands: a reply to Mazza. Lethaia Focus V-48 pp 147-150
Hippopotamus, H. amphibius & Pygmy Hippopotamus, Choreopsis liberiensis 2001. revised 2011. San Diego Zoo Hippopotamus Fact Sheet
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Cheers!!
http://phenomena.nationalgeographic.com/2014/09/11/the-new-spinosaurus/
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| credit Davide Bonnadonna (c) used w/permission |
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| credit Christopher DiPiazza used w/permission |
And here is my version of the underwater running Spino which is a bit of teaser for an upcoming post on underwater running... (yes, yes I wanted to do just 2-3 posts on Spino but it keeps growing)
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| Underwater Running Spino "glowstick" (c) Duane Nash use w/permission |
I have waited a bit to really address the Evers paper and, to be perfectly honest, I did this because the paper is just too darn long!! Seriously if the Ibrahim et al. paper got knocks for being overly abbreviated this paper deserves to get some critique on being overly verbose and less than easy to follow as a document. I really doubt most of the people heralding it as the savior of the long-legged Spinosaurus have even read it.
It did not have to be this way as the primary take home message is that Sigilmassasaurus brevicollis is its own beast!! That is about it when it is all said and done. Keep in mind that for Sigilmassasaurus we are talking about vertebrae - no appendicular elements, no skull material, not even a rib. It should also be added that Spinosaurus maroccanus (vertebrae & premaxilary material) is now - according to this study - sunk into Sigilmassasaurus brevicollis. As you may recall the Ibrahim et al. study cleaned house on all North African spinosaurid material and just lumped it all into S. aegyptiacus. The Evers paper could have been much more effective if they just presented their character analysis of the vertebrae, made their arguments for what bones they represent in the vertebral series, and made a succinct and clean argument for North Africa hosting at least two spinosaurids in the Kem-Kem and Bahariya respectively.
This beefy 100 page paper can best be summarized with this paragraph here:
This paper should have just focused on retrieving S. brevicollis as a unique species and not have - I don't know how else to put it - used its bully pulpit to cast doubt on the unique morphology of Spino B and FSAK-11888 - especially since the monograph of the latter is not even published and we don't even have the former anymore. Instead, among other critiques of Ibrahim et al., the authors question the synonymy of Stromer's spinosaur material with Ibrahim et al's. FSAK-11888 and reject their claim of a neotype.
They also insist on describing the retrieval of FSAK-11888 as coming from an "allegedly" associated partial skeleton several times. Understandably this is because the monograph is not yet prepared addressing the taphonomy and collection of said specimen. At the same time Evers et al. seem to really run with this "chimeric" interpretation of FSAK-11888 casting doubt on its validity. Is it chimeric due to Moroccan fossil dealers pulling together disparate skeletal elements in order to make a sell? If this is the case then why give the animal diminutive pelvic elements? Andrea Cau has addressed this problem on his blog here. The second possibility is that the elements are chimeric not due to anthropogenic forces but due to taphonomic forces, a possibility the authors raise citing several studies alluding to the Kem-Kem having a notorious reputation for mixed elements. However let us reason this scenario out. In both Stromer's Spino B and the Ibrahim FSAK-11888 we have diminutive pelvic elements associated with lengthened dorsal vertebrae, and neural spines... what are the chances of that happening twice with either taphonomic or anthropogenic interventions? Again these critiques are not new, they should be addressed, but I think that they really obfuscate the prime - and very interesting - point of the Ever's paper: that is North Africa hosted two Cenomanian spinosaurids.
Evers et al. also fail to mention a very important layer of evidence in favor of FSAK-11888 representing a single individual. This line of evidence bears repeating - in fact it should have got trumpeted from the hill top loudly a long time ago - and that is that the vertebral and appendicular bone histology of FSAK-11888 both signal a relatively young, sub-adult ontogenetic age . Yup both the diminutive pelvic elements and vertebral column/ribs/gastralia suggest an immature individual. This is not what we should expect if the elements are chimeric. Let's review that bit from the supplemental materials section of Ibrahim et al.:
"Neurocentral sutures preserved in the vertebrae do not exhibit coocssification, nor is there coossification between sacral centra or between the ilium and sacral vertebrae." (pp.13 supp.)
"Two long bones (femur, fibula), a possible gatralium and the proximal end of a dorsal rib were selected for histological thin sections.... (pp. 13 supp.)
"An external fundamental system is not found in any of the four bones sectioned, and vascularization is still prsent in the circumferential layer. We infer a subadult ontogenetic stage for the neotype specimen. This interpretation is also based on the high amount of Haversian systems in the inner cortex, the decrease in density of vascularization towards the surface of the cross section and the decrease in spacing between LAGS toward the outer cortex. Maximum adult size would likely have entailed many years of subsequent growth." (pp. 14 supp.)
Not addressing these histological lines of evidence in the Evers et al. paper is a major omission in my view. As I have said before this paper should have just dealt with Sigilmassasaurus as a unique taxa and by going after Ibrahim et al. in totality in a none too circumspect way they really do themselves a disservice. If you are going to make critiques you can't just omit the stuff that you don't like i.e. the very pertinent bone histology congruency between vertebral and appendicular elements suggesting just one individual. Especially if your paper is 100+ pages long. But Evers et al. are not solely too blame on this front. If you go back and review the several very storied and infamous online critiques of Ibrahim et al. including - Switek's, Headden's, Hartman's, and Witton's - you will see that all four of them fail to mention the bone histology data suggesting FSAK-11888 is one, subadult individual. For all of these very vocal and visible blog postings not to mention this line of evidence is very interesting. Make of that what you will.
The unfortunate effect of this paper is that it adds fuel to the fire for those in the "popular realm" (i.e. JP3 Spino fanboys) who are hellbent on retrieving an upright bipedal Spinosaurus at any cost. "Look at this 100 page paper in PeerJ - the authors shoot down the Ibrahim paper!! Whoopee long legged Spino lives again!!"
Again, while the new paper might be correct in there being two or more spinosaurids in N. Africa (maybe even a species complex of Spinosaurus?) and that their critique of Ibrahim et al. lumping all N. African spino material into S. aegyptiacus may be valid- the morphology for Spinosaurus that the Ibrahim et al. paper gave did not hinge on the Sigilmassasaurus or S. maroccanus material anyways (which is mainly isolated vertebrae). We still, at the end of the day have the congruence of morphology in Spino B and FSAK-11888 standing tall (pun intended).
The authors even begrudgingly concede this in the last sentence below:
Furthermore, on a wider note, this movement to retrieve a long-legged, bipedally striding Spinosaurus (Hartman's skeletal is usually the one bandied about - but not a fan of it myself or his spinosaurids in general - too gracile imo) both on the popular and scientific front I predict will fall flat on its face eventually (pun intended). Two specimens - as controversial and questionable as they may be - both displaying similar gross morphology in the pelvic area > zero specimens of Spinosaurus showcasing "normal" sized theropod pelvic anatomy. One set, the one we do still have, shows ontogenetic congruence between the vertebral and appendicular elements i.e. not likely a chimera. That's right, there is exactly 0.0% verifiable evidence for a long-limbed, classically bipedal Spinosaurus or even spinosaurid in North Africa. None. Any assertions for this interpretation over something more in the Ibrahim et al. ballpark of gross morphology - without skeletal evidence - is an assertion of a mythical creature over something we do have (controversial as it is) evidence for. It's high time we flip the script on long-legged Spinosaurus being the de facto representation - there is no evidence for that animal at all.
Now with no clever segue at all (although I promise it will relate back to above discusssions) I want to go into into island hopping hippos.
What really started this off was me stating - as others notably the San Diego Zoo - that hippos can't, in the truest sense of the word, actually swim. They are denser than water and don't really have large flippered limbs that can grab and push back large volumes of water. The reason for their density is - and I will go into this more in depth in a future post - that their primary mode of locomotion is best achieved via their ability to stay low in the water column. Hippos use a foot propelled running/trotting locomotion "underwater punting" in which the foot pushes off the substrate to achieve long and slow gliding phases or when hippos want to move faster they actually take shorter and quicker steps. At first this might seem counter intuitive but try it yourself. Go into a body of water at least 2/3 up your chest. Now experiment with running underwater to achieve maximum velocity. You will quickly realize that short/rapid punting steps outpaces long, gliding paces. Therefore for hippos to take advantage of moving quickly in the water via short/punting steps they need to be substantially denser than the water or else gliding phases (which are slower) will take over due to relatively increased buoyancy.
Before we take a look at the various island hopping hippos and other lines of evidence that are cited in favor of hippos being able to swim let's take a closer look at an animal that I neglected to mention in my last post but offers much utility to discuss here - the pygmy hippo (Choeriopsis liberiensis).
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| Pygmy hippo. public domain. Mt Kenya Wildlife park |
Let that last bit about the tapir analogue sink in a bit - as you should recall in my last post I suggested tapir provide a good proxy for an animal that is not yet quite as aquatic adapted as a hippo. Tapirs can underwater punt, they have thickened skin that likely aides as ballast, and a barrel shaped torso for underwater streamlining. They are likely more bouyant than a hippo and might be able to achieve a swimming stroke or even float in saltwater. To put it another way tapirs are just getting their feet wet in terms of dedication to an aquatic existence. All of this is less than rigorous but is worthy of more testing mind you...
Going further if we take this analogy to tapirs for pygmy hippos and the general observation that they appear less massive skeletally , have less skin, and can potentially float or even actively swim - then maybe pygmy hippos actually did colonize offshore islands via swimming/floating. This may have especially been the case when pygmy hippos had a more wider distribution in the past.
When I took the question of pygmy hippos actually being able to swim into my laboratory (i.e. youtube clips) I very quickly - within the first 2 or 3 videos - saw some behavior suggestive of swimming. Mind you I am yet to see any good footage of big hippos swimming in any form...
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| infant pygmy hippo swimming a little |
Here is another youtube clip of an infant pygmy hippo that appears to get a little bit of limb assisted swimming going on... but as soon as it stops pumping those legs it sinks like a stone.
For comparison here is an endearingly cute video of an infant hippo (H. amphibius, larger species) moving in water. It always appears to sink like a stone after pushing off the bottom. There is one sequence towards the end where it looks like it is trying to paddle but doesn't really get anywhere.
Malagasy Hippos
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| Choreopsis madagascariensis w/ H. amphibius skull. public domain 1923 |
The Hippos of Center Island, Lake Turkana
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| Lake Turkana. CC3.0 |
Lake Turkana is the largest of the eastern rift valley lakes. It is also a relatively recent lake in terms of origin - 200,000 years old is the most commonly cited age. And research suggests that what we see today - the largest desert lake in the world - might have been in fact two or more smaller lakes and dried up completely as recently as 7,500 years ago. Follow this link if you want a good overview of the complex and dynamic geological history of this lake Paleogeography of Lake Turkana. Even before "Mega-Lake Turkana" nearly dried up 7,500 years ago the Lake Turkana basin hosted several ghost lakes and rivers. Long story short we have a more parsimonious solution to why there are hippos on Center Island. They need not have swam, floated or rafted there - they most likely were always there - a hold out from when the lake level was a lot lower as has happened several times in the lakes geologic history. Which might happen again as the main incoming tributary for the lake - Omo river - is set to be dammed.
Mediterranean Isle Hippos
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| composite mounted skeleton H. minor. George Lyras 3.0 |
Compounding the issue is that not only do we have fuzziness on the ancestry of these beasts is that the geological history of the Mediterranean basin is horribly complex. Events like the Messinian salinity crisis of the Miocene in which the whole basin literally dried up could have allowed the ancestor(s) of the Mediterranean "dwarf" hippos to literally walk up to the various isles. High evaporation rates in the Mediterranean also may have provided just the amount of buoyancy needed to float a putative ancestral hippo type beast - especially if not built as heavily as modern H. amphibius and possibly more along the lines of Archaeopotamus.
What can be said about the Mediterranean isle hippos species for sure? From what I gather not much definitely. We can't be precise in terms of who they evolved from or when they evolved? Are they really dwarfed? Do they represent a more ancestral stock of hippo and just evolved in convergence to look superficially like modern hippos? What can be said definitely in terms of their biogeographic origin? Not much really only that they got on those islands somehow.
My final thoughts on the debate of island hopping hippos is somewhere between Mazza (2015) and Van der Geer (2015). If you follow the links at the end of the post you can read their interesting exchange. Neither paper addresses the geological history of Lake Turkana which suggests that the hippos that live on Center island need not have swam their but arrived when lake levels were lower. Furthermore neither paper addresses the many unresolved issues in hippo taxonomy and that - especially when variable of skin thickness and bone thickness are taken into account - there may have been a spectrum of densities and therefore differing buoyancy in fossil hippos. Some of the Malagasy hippos might be closer to pygmy hippos. Who knows about the Mediterranean hippos and what they evolved from and how the geological history of that area affected their biogeography. Basically it is not so simple to look at what modern H. amphibius does or does not do and infer that fossil hippos had similar capabilities or limitations. Especially so in light of the real possibility that pygmy hippos might have a different buoyancy than H. amphibius.
Hippos Best Equipped Large Bodies Mammals For Rafting
All things being equal - and even if in especially high salinities or invoking putatively >less dense< ancestral hippos that could float or dog paddle - I still prefer rafting to colonize far flung islands as opposed to deliberate swimming and/or floating to them. Rafting solves the buoyancy issue which let's remind ourselves that no hippo of either species adult or immature has ever been observed to float or swim in fresh or saltwater for any length of time - that fact can't be ignored especially for a species so common in captivity and well observed as H. amphibius. Hippos are known for a relatively slow metabolism - especially for a mammal - and this preadapts them for oceanic voyages floating on rafts. Because hippos are known to live in estuaries and saltwater beaches they are in direct line of fire for the occasional, but inevitable, mega tsunami. mega-flood or mega-typhoon. The study of these events and how they reshuffle the deck ecologically is in its infancy. The historical and scientific record is not deep enough to document many of these events. But the geological record is. And over the course of long enough periods of time such events become not happenstance but statistical eventualities. Hippos, as long as they can get secured on a floating pile of debris or large tree, are well equipped - perhaps the best equipped out of all large mammals - to survive and take advantage of such events sporadic as they are.
The Spinosaurus Biogeography Question
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| Early Cretaceous. Albian |
However just because Spinosaurus could not move itself over deep channels of water or across long patches of terra firma does not mean that other geologic agents could not do the moving for it... Keep in mind that the Cenomanian was a hothouse world and the mega-typhoons/mega-floods/mega-tsunamis and just general slosh between the land/intertidal/ocean realms must have been unprecedented. All Spino had to do was hold onto that floating clump of debris/tree and sail away perhaps even to South America (hello Oxalia).
What I predict was going on is that each major river system and adjacent estuaries/mangroves would start to evolve its own unique "flavor" of spinosaur. Whether or not you would call these different species or subspecies... its a bit subjective. Once in a while environmental perturbations would shuffle up the groups so that new colonists would be spit out and formerly isolated populations would reengage. This might be what we are seeing in the differences between Stromer's spino & Spino B and FSAK-11888 that Evers et al. note - different members of a "species complex". Sigilmassasaurus might be its own "species complex" and it is interesting that Evers et al.'s work shows it close to several baronychines . So North Africa might just have its own giant, bipedal spinosaur that is not Spinosaurus proper!! It might also eventually prove useful to designate FSAK-11888 as a separate species from Stromer's S. aegyptiacus but still cogeneric.
However, especially in light of the taxonomic mess that is hippo taxonomy, I wouldn't hold my breath expecting Spinosaurus or N. African spinosaurid taxonomy to be sorted out neatly any time in the near future or even in your life time.
(2015) A reappraisal of the morphology and systematic position of the theropod dinosaur Sigilmassasaurus from the “middle” Cretaceous of Morocco. PeerJ 3:e1323 https://dx.doi.org/10.7717/peerj.1323
Mazza, PD (2015) To swim or not to swim, that is the question: a reply to Van Der Geer et al. Lethaia Focus V-48 pp 288-290
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
Van Der Geer AAE, Anastasakis G, Lyras GA (2015) If hippopotamuses cannot swim how did they colonize islands: a reply to Mazza. Lethaia Focus V-48 pp 147-150
Hippopotamus, H. amphibius & Pygmy Hippopotamus, Choreopsis liberiensis 2001. revised 2011. San Diego Zoo Hippopotamus Fact Sheet
Cheers!!
http://phenomena.nationalgeographic.com/2014/09/11/the-new-spinosaurus/