Sunday, August 31, 2014

Surf OR Turf: Can A Terrestrial/Aquatic Switchitter Really Exist?

Following on the heels of my Spinosaurus post I have been thinking about the aquatic/amphibious thing a lot lately and looking at the various levels of engagement in either realm animals partake in and have come to the conclusion that, at least for a terrestrial limbed tetrapod, being equally competent in the land and water at a high level is pretty darn hard to do if not impossible. (In the title I used the word switchitter to refer primarily to baseball players that are equally competent as hitters with both arms leading in case you miss the translation)

As a qualifier I stated this animal has to have limbs as numerous snakes are great in both realms - what comes to mind are garter snakes. I also stated that the animal has to be terrestrial i.e. not a bird as the dipper bird makes for a good case of being equally competent in both realms.

CA red-sided garter snake
I should also qualify my statement by clarifying what I mean when I say "equally competent". I would consider a predator that can both run down prey and swim down prey at high levels "equally competent".  Not just any prey though - this animals must outpace the fastest prey items in both respective realms. Or a prey species that could elude the fastest predators in either realm. Such an animal would be able to approach the cursorial capacity of canids or felids on land and in the water be on par with the fastest scromboids and dolphins.

For sure there are plenty of animals that hunt down and kill prey in both realms - bears, lots of mustelids, monitors, crocs, frogs - but when you really look at these animals they generally are geared primarily to one environment or the other. And in many cases they are utilizing stealth or elements of topography to gain an advantage. Remember I am looking for that animal that can capture - not by stealth but by speed - the quickest and most elusive prey in either environment with equal ease and efficiency.

I am starting to think there is very good reason such an animal does not exist nor could it ever.

The reason being that aquatic versus terrestrial movement asks the limbs to do very different things. Aquatic propulsion will tend to select for shorter and stouter limbs. Shorter and stouter limbs, being closer to the body, will create less drag than longer limbs during the recovery stroke. So if speed is something of importance in amphibious tetrapods shorter and stouter limbs will be selected for. This is seen very well in skeletons of otters. In addition a long torso works a bit like a hull of a boat to help cut through and displace water. Just look at the unique proportions of Michael Phelps - long arms and torso, but relatively short (read powerful) legs and flexible flipper like feet.


Now contrast this with what it takes to be an excellent cursor ; long legs, especially the lower leg elements; a deep thoracic cavity to house large heart and lungs; and compact torso for quick maneuvering. Seen against what is called for to be an efficient amphibious tetrapod it quickly becomes apparent that the two forms of locomotion are diametrically opposed. And also why Phelps, although a world class swimmer, would make a relatively poor runner. Compare and contrast the dimensions of Phelps with the ideal body dimensions of a runner and you will see that humans mirror the pattern seen in animals.


So although we have lots of examples of animals that dominate the aquatic and terrestrial realm in fiction - for example Dinocroc, Godzilla, Cloverfield Monster, Beast From 20,000 Fathoms, and the semi-mythologized JP 3 Spinosaurus - being equally proficient at a high level in both realms I don't see happening for a tetrapod.









Saturday, August 16, 2014

A Very Dated Picture of Baronyx from Way Back in the late 90's

I never date or sign my pictures - that way  I can create havoc for art dealers of the future when I am a bright shiny star worth gabillions - so you just have to take my word for it that I drew this back in the late 90's - maybe 97/98. But as evidence I offer the very dated look for a putative spinosaur for the given time table of rendering. I think I was going for Baryonyx walkeri - back in those days if I recall that was all we really had going for good remains of a spinosaurid dinosauri. Anyways Andrea Cau of the blog Theropoda left a comment on my last post that reminded me of this pic so I wanted to put it up in all of its embarrassing detail.


Gobbling up cute, little aestivating lungfish.

Did Bakker Get Spinosaurus Right After All?

We all know and love the new Spinosaurus aegyptiacus reconstruction right? "Disenchanting" is how Douglas Henderson described it on facebook. And it does fall a little bit shy of the impressive dual land/water arch predator it has so often been posited to be, especially since a little movie came out (JP3 duh) and our friend the Spino terrorized everything from T-rex to estranged parents. Truth be told it looks a little clunky, a little too front heavy to work right - at least the way we expect theropods to work  since GSP started drawing them as svelte athletes rip roaring across the page. So, yeah I do want to comment on the new Spino reconstruction and yeah there are a lot of people - very knowledgeable ones at that - that would definitely disapprove of what I am going to do before publication of the new material. But I will argue that my interpretation on what was going on with Spino is in fact presaged by what we already know from other members of spinosauridae and yes, as suggested by the title, I think Bakker got the jump on all of us by suggesting way back when that Spinosaurus was a lot more aquatic than generally portrayed. But I will offer my little Antediluvian Salad spin on Spinosaurus and hopefully you find it interesting. And leave a comment.


Moss Landing/Elkhorn Slough US Army Corps. public domain
Before I get to Spino itself I want to tell a little story to help illuminate my point. A little tale about kayaking. I do like to kayak and one of the funner places to do so in California is a place called Elkhorn Slough in the Monterey Bay. It is a tidally influenced estuary and there is always open access to the ocean, especially because of the harbor at the entrance having to be maintained. It is noted for the unique population of sea otters that forage in an estuary as opposed to the ocean - often times within view of the highway that crosses the estuary. These estuarine sea otters have been shown to predate heavily on crabs and, in a classic trophic cascade, have bolstered stands of eel grass in the estuary. In addition to the otters, there are loads of harbor seal, water birds and miles of watery inlets to explore. It is the type of place that lets you imagine the California that once was - when coastal lagoons and estuaries dotted the entire coast, flocks of birds blackened the skies and grizzlies patrolled everywhere. Elkhorn Slough is so spellbinding in fact that as I kayaked it I lost track of the depth of water I was in and became grounded in mud in shallow water. This sucks I thought to myself as I considered the possibility of having to get out and push my kayak back into deeper water. Thankfully cooler heads prevailed. I did not make the rookie mistake of getting out of the kayak where I would have likely got mired waist deep in tidal mud in a sulphurous mess. In our best impersonation of saltwater crocodile plowing through tidal flats we shimmied left and right and paddled with short quick thrusts to get back to deep water. And if you imagine the long rounded body of the kayak as equivalent to the long low body of a crocodile, seal, or otter the analogy of moving through deep/sticky mud in a kayak is spot on. And if you think of large amphibious tetrapods moving through the mud, and I am talking deep/miring mud, they all get low and down in it and use their body surface area to prevent sinking too deep. Go look at saltwater crocodiles steaming through tidal mud flats in Queensland.



Large tetrapods that do not use this technique in deep mud risk getting mired. For large bipeds the risks are even more extreme as all of the weight is concentrated on just one limb when pushing off. As the picture above attests a very large American crocodile has some advantages over a bipedal hominid in deep mud (he survived btw). Whether you are a trained soldier or a simply like to take the Spartan Challenge one is quick to see the benefits of getting down into the mud to get through it. It might seem counter intuitive at first but just ask this guy.


Now following this line of thought of how to traverse muddy areas as a large tetrapods let us revisit Spinosaurus aegyptiacus - especially with respect to the new reconstruction c/o Paul Sereno.

Nat Geo
So we got a couple things going on here. A very long, low body with some ridiculous looking shorty-shorts legs. Especially reconstructed in the Paulian push off mode the beast looks seriously in danger of tipping over. Combined with the strangely down turned head and the heavy arms the question is begged is this manoeuvre even possible? To further complicate the issue let us put this animals in its habitat - tidal/deltaic wetlands. (insert discussion on spinosauridae habitat choice, diet, you can find the evidence is compelling!) Now we have the issue of a massive, bipedal animal in a habitat full of mud - as discussed earlier large bipeds in thick mud can be an issue. What was the poor spino to do? What I am going to suggest is that Spinosaur and its ilk went to boot camp and learned how to combat crawl. Not entirely quadrupedal but not entirely bipedal but a bit of a compromise between the two. Lower the torso, push off with the short back legs - a bit like a penguin actually - and the coup de grace those bizarre and massively boned and muscled forelegs. Simply fold them in the manner all theropods are now known to do. The radius and ulna take the weight and the clawed digits are curled inwards. Take a look at this pic c/o Dave Hone of a Suchomimus arm. Spinosaur arms were robust but not as elongate as often depicted. But when the spino goes into this belly slide I will suggest the arms can assist in kind of a "combat crawl" motion. The elbow is where the movement happens and the radius and ulna take the weight.

Seen in this manner the long low torso, short legs, short but powerful forearms, and downward sloping neck shared by all spinosauridae now start to make sense. Chasing after stuff on terra firma these  animals were not. But if you were in the water, or a hapless dinosaur mired in the mud, spino could get the jump on you.

Duane Nash
Above is a very quick and punk rockish sketch I did yesterday of a spino moving through thick tidal flat mud. The black color on the underside is not some type of reverse countershading but the thick/clingy mud you find in such environments. I have been drawing theropods on and off for over 20 years but drawing spinosaurids you have to sort of unlearn what you are used to. Not tall and gracile, no graceful S-curve to the neck, not a compact torso but a long barrel shaped torso. They seem to go against the "birdy, svelte, uber athletic" theropod bauplan that has dominated reconstructions essentially as a result of Gregory S. Paul's influence. Hence the culture shock at the new reconstruction of Spinosaurus. In hind sight maybe we should have seen this coming. All of the known relatives of Spino tip forward a bit, have short heavy arms, and well just look oddly proportioned compared to other theropods. Spinosaurs took these attributes to the extreme. I would suggest all spinosauridae would utilize belly crawls in muddy situations and perhaps occasionally even on dry land. If you look at the list of large amphibious tetrapods that do not utilize the belly slide in thick mud the list is basically.... nil? Maybe hippos don't belly slide, but they do have four short powerful limbs and a long torso. Capybaras?


Below is a depiction I did of Spino with just the body showing and not obscured in the mud. It is a little difficult to draw as such and work kinematically where everything goes. But I think such a style of movement is a fair compromise considering what we know from these critters anatomically and ecologically.

Duane Nash
And again let us just look at the skeletons of other related spinosaurids. Like I said earlier some take objection to me writing about Spino before the publication but if you look at these skeletons in light of the argument I just put forth for belly sliding/combat crawling there is no reason to say that they may have all been doing it. And I seriously doubt Sereno and Nat Geo are going to suggest this mode of movement because, you know, they put their mount in a Paulian push off extreme cursorial stance. And finally Robert Bakker was suggesting that these guys were a lot more aquatic than generally portrayed. And usually ridiculed for his opinions. Last laugh may go to Bakker on this one.

As for how they moved in the water I think more or less like a hippo but bipedal running underwater. Not tail driven.

Subadult Spino. wiki. legs were likely shorter. Creative Commons Funkmonk
Baronyx walkerii. CC Funkmonk. Note heavy, but short forearms

Above is a skeletal by blogger/artist/researcher Jaime A. Headden. You will notice that of the material we have, the lower legs are missing. Never the less, as in this skeletal, they are usually rendered fairly long and gracile. But they may have been quite a bit stumpier.

Suchomimus tenerensis. CC AStrangerintheAlps
Again with Suchomimus we see that tendency to fall forward a bit. Almost as if it is begging to go belly slide in some primordial ooze.

Suchomimus tenerensis CC Belinda Hankins Miller

Getting low, getting low.





Friday, July 18, 2014

Scrummy for Scromboids: How Do Great White Sharks Catch Tuna Anyways?

Well all righty then todays post might seem a bit all over the place but bear with me there is a valuable point to be made out of all of it.

Monterey Bay Aquarium. tuna & great white

I have had a bit of a predilection with all things aquatic lately as you may have noticed - plesiosaurs, pliosaurs, ammonites, and all things gishy, gushy, and squidy - have been on my mind of late. And here in the states it is summer and that means one thing and one thing only - SHARKS!! An immature great white shark has already bit a swimmer who blundered into it as the shark was trying to free itself from a fisherman's hook in southern California. And Discovery Channel is set to break new highs (lows?) in shark carnage in their annual Shark Week marathon. The ambassador to sharkdom, great white sharks, have a bit in common with T- rex in that the attention and coverage both animals receive is a bit fantastical and overshadows other shark/theropod species in popular media. That is not to say I personally have or will ever tire of learning new insights into both species. So I say hogwash to notions of overexposure as both species are stupendously cool and actually quite unique in the evolutionary framework of their respective pedigrees. But other types do need more attention and to let you know now, I will be talking about great whites in terms of one specific aspect of their predatory behavior that is little explored. But to set the stage I need to talk about something seemingly completely incongruous with great white sharks - spotted hyena predation.


The book above was one of the foundational pieces of natural history literature that really captured my attention as a youngster. I recently became reacquainted with the book. At the research institution where I work at doing data entry, The Western Foundation of Vertebrate Zoology (despite the name the museum  specializes in birds eggs), was cleaning out old books one of which was Innocent Killers. So I quickly scooped up the free copy of this brilliant text only to discover that it was a copy signed by author Jane Goodall . And yes that is the Jane Goodall I am talking about - the Jane Goodall best known for her work with primates. Less well known is that for several years she worked with wildlife photographer Hugo Van Lawick documenting the social and predatory ecologies of spotted hyenas, wild dogs, and jackals in east Africa. While common knowledge now what Jane revealed concerning spotted hyenas was an intensely social, intelligent, and capable predator that operated largely under the cover of darkness. This data went contrary to previous assertions of spotted hyenas as consummate scavengers - characterizations due to diurnal observations and entrenched negative cultural views of hyena. I know the decline in the quality of natural history documentaries has been well noted (mermaids, Megalodon) but the Nat Geo channel has produced a very interesting film entitled Night Stalkers - Hyena Gangs which is on youtube check it out.


The take home message is that a significant misunderstanding of spotted hyena predatory behavior and ecology went unchecked right up until the 1970's when actual long term field studies, including nocturnal observations, were made. And this is for a particularly well known large predator that lives in open habitats.

Check your privilege, mate

With this example in mind what might we be missing in terms of behavior for great white sharks at night? The challenges of documenting the behavior of this massive, complicated predator are even more  compounded by its aquatic realm - where even during the day turbidity and lack of vision are par for the course. And this is a bias that I think can't be overstated to your typical arm chair naturalist (myself included) - bad visibility, even in tropical waters, is the rule rather than the exception. When you watch undersea documentaries you are seeing the rare, exceptional times where visibility is good. Otherwise tides, currents, plankton blooms, and storms muck up the water a good part of the time - and give predators a decided advantage. Just as hyenas and lions have a decided advantage over their ungulate prey at nightfall.

As a kid in the eighties I watched every documentary I could find on great white sharks in hopes of getting a glimpse of their predatory behavior. But all I saw where rather sedate looking whites gathered around human induced feeding opportunities. I could not figure out how such a languid looking fish captured smart, agile, and quick pinnipeds. Boy was I wrong!! Nat geo captured white shark predatory behavior via stationary underwater cameras in the 90's but it was not until the "air jaws" phenomena off South Africa via Chris Fallows that we really saw unadulterated white shark predatory behavior. What's more not only were white sharks doing acrobatic full breaches, extended chases - they were also hunting this way at night!!

Nat Geo
And I now I want to get to the heart of this post. We have good footage and data on the ways that white sharks go after and capture pinnipeds, even varying their tactics based on the type of pinniped. But how do they capture the other, other white meat? I am talking about those hyper-streamlined, hyper vigilant, super charged, warm blooded super-fish - tuna. Of the genus Thunnus these fish, most notably bluefin, albacore, and yellowfin are super tasty to certain hominids as well as anything else that can get a jump on these most speedy of pescados.

Now I love to quiz fishermen on bits of natural history that they have observed - fisherman's tales notwithstanding they are a treasure trove of knowledge and experience. Lucky enough my uncle regularly goes on charter trips to catch yellowfin tuna at Guadalupe Island off of Baja California, Mexico. And any great white shark groupie should know that island well, it is a preeminent spot for eastern pacific white sharks. Indeed my uncle testifies the white sharks there are a bit of a nuisance, and snatch hooked tuna so often that they are dubbed "the taxman" (experienced surfers on California refer to white sharks as "men in grey suits" alluding to how commonly they encounter them with no incident spooky still though). A quick perusal of you tube clips verifies my uncles' testimony.


Also check out this vid of what is claimed to be an 18 footer at Guadalupe. Based on the girth that might not be an overestimate.

But when I ask my uncle if a great white can catch a free-swimming yellowtail he is quick to say no... only when they are hooked can the whites get a yellowtail as they are too fast.


At first this makes sense, tuna are very fast - faster than whites and faster than the pinnipeds that whites have been documented to hunt. But then again tuna are always put forth as a food of great whites, you have accounts of great white sharks actively following bluefin tuna, especially in the Mediterranean, or getting into tuna aquaculture pens as recently happened in Mexico where a massive shark was killed by tuna ranchers, and then you have loads of accounts of tunas being pulled out of the stomachs of dead great white sharks. Of special significance are the great white sharks of the Mediterranean, said to follow schools of tuna through the straights of Gibraltar and sometimes even being encountered in the traditional matanza tuna killing festivals. Interestingly the rare Mediterranean white sharks, including some of the largest ever captured, may subsist primarily on tuna - seals not occurring there in any large numbers. Here is a good blog post going deeper into this little known population of white sharks. Additionally the Mediterranean might be from a relict Australian population that got misdirected about half a million years ago.

So how does a great white shark go about hunting a tuna? Well I suspect, at least in the case of bluefin tuna, we are not witnessing the white shark in hunt mode because white sharks may only have the advantage at night. I want to draw your attention to this study: Electroretinographic Analysis of Night Vision in Juvenile Pacific Bluefin Tuna (Thunnus orientalis) published in the Biological Bulletin 2009. Basically, although the bluefin tuna has good vision and is a sight orientated predator, it's vision is geared towards diurnal activities and shows a marked decline for nocturnal vision. So marked in fact that collisions at high speed against underwater pen walls is a significant mortality factor in pen raised bluefin tuna. If this decline in nocturnal vision goes across the whole genus of tuna Thunnus, might this be the small window of opportunity that allows the larger, slower but nocturnally capable white sharks to predate speedier tuna? And hopefully now you see why I started with a discussion of hyena nocturnal habits and how for so long that animal was misinterpreted. It is not hard to imaging how the nocturnal habits of a large, rare oceanic predator might go unrecognized as well.



The 19 foot, 5000 pound female great white has been following the school of bluefin tuna for several weeks now but has not attempted an attack. During the day it stays below and behind the school, sometimes letting the school get several miles ahead of it as it slowly maintains course and conserves energy, living off of its fatty liver. But it is gravid and needs a good feed for its growing pups. Tonight is a new moon and under the cover of darkness the white shark will make its move. A few of the larger fish have become aware of something large following the school for several weeks now and are careful not to swim at the periphery of the school as they have experienced attacks before. But other fish are naive to white sharks. They have  ran into small blues and makos but had outgrown these predators in size and strength. This female white shark is experienced in bluefin hunting and knows that only a certain congruence of factors allows her to make a kill of the speedier tuna. Cloud cover, lack of moonlight, and a bloom of phytoplankton gives her the cover to slip into the school of tuna. Although technically not asleep, they are swimming in a sort of waking dream - like race cars at idle. Amazingly the white shark is able to blend into the school by mimicking the swimming rhythm of the school and evade detection. They may sense another large fish but register it as just another tuna. When the shark picks a victim the dash is barely over a distance of 10 feet, but conditions are so murky that even the great white shark misses its mark on a disabling bite to the tail. With the school now alerted it is like a hundred ferraris going from 0 to 50 mph. But the white shark knows that it can still exploit this chaos and when one startled tuna accidentally rams into another tunas gills, momentarily stunning it, the shark has its meal.




Cheers!!!




Sunday, July 6, 2014

Pliosaurus kevani - Business in the Front, Party in the Back

So a recent paper on the biting and feeding mechanics of Pliosaurus kevani has been published, Funtional anatomy and feeding biomechanics of a giant Upper Jurassic pliosaur from Weymouth Bay, Dorset, UK. Brian Switek wrote about it here and you can access the full paper here. As usual I encourage you to read Brians' article as well as the original paper before you go further.

Using bite force estimates, finite element analysis, and beam theory the authors arrived at an interesting ecomorphological compromise reflected in the massive (nearly 2 meter) skull of the Weymouth pliosaur. The back of the skull was indicative of tremendous power and bite force, indeed Pliosaurus kevani ranks right up there with the supreme chompers of all time in terms of bite force. However the front of the skull is decidedly less robust, narrower, and suited for less extreme bite forces. Intuitively this makes sense, even in our own mouth if you really want to chomp down on something you use your rear molars for maximum bite force leverage. Crocodiles also test significantly higher in rear tooth bite pressure and when pulverizing tough prey will preferentially use the rear of the jaw. The front of the skull reflects a compromise of sorts, sacrifice strength for a more efficient streamlined profile and quicker strike that allows for a more generalized foraging modus operandi of both large and small prey items - which is supported by bite marks, stomach remains etc etc.

So far so good, I find this analysis makes a lot of sense. But here is where the paper, which Brian echoes in his own article, falls down for me a bit. The authors suggest that due to the more gracile build of the front of the jaw large prey could neither by shaken violently or twisted (death roll style) for fear of mechanical breakage. Instead large prey, the size of which the authors arbitrarily limit to one-half the length of the pliosaur for reasons I do not fully grasp, was tattered to bits by the rearward part of the mouth until small enough to swallow whole. A feeding methodology, mind you, that no modern vertebrate uses exclusively to dismember large prey. Just think about that for a second. You have a large body in your mouth and start chomping on it until smaller parts start coming off. Do you release the large body to retrieve and swallow the smaller bits? Remember that you are in water so stuff will start sinking and floating away as well... all in all it just seems a little cumbersome and awkward a feeding strategy.

First of all I want to address the claim of the front of the skull being weak and fragile. If you follow the lines on the graph of the pliosaur (solid black) they line up rather closely with those of C. niloticus (nile crocodile) in green.


Especially pertinent is the last graph Log J on the bottom row (size corrected) which measures torsional resistance, i.e. death rolling, where the pliosaur follows the nile crocodile pretty much tit for tat. Basically what can be gleaned is that the skull is not quite so strong as alligators and caimans, but stronger than gharials and pretty much equal to that of a nile crocodile. I do not think weak or fragile are the best terms to describe the jaw, or even the tip of the jaw which still had to be able to withstand several thousand pounds of bite force.

So I do not know about you but I would not characterize nile crocodiles as particularly dainty processors of food, nor do I think they hesitate to shake, twist, tear, gnash, chomp, and clamp onto their prey with reckless abandon. And they certainly do not follow any kind of dictum of only preying on animals 1/2 of their own length.



Beam theory and finite element analysis aside, just look at the damn skull. It is a massive skull and even though the jaw tapers at the end it is still an impressive organ of destruction.


We have to remind ourselves that what animal, apart from another adult Pliosaur, would offer the kind of resistance to break such a jaw? Ammonites, smashed to bits. Plesiosaurs... please, throttled to death. Ichthyosaurs, mince meat. Leedsichthys, mobile feasts. Again back to modern crocodiles, which pulverize, twist, and shake prey, they do actually sometimes have the tips of their jaws broken off. A longirostrine jaw is actually not the ideal morphology for resisting torsion. But this breakage is usually the result of a fight with a bigger croc. Or maybe, as the picture below is rumored to be from a fight with a hippo, but not verifiable. The point is modern nile crocs have mechanical weakness in their skulls but that does not stop them from death rolling, thrashing prey about, and generally being an archosaurian bad ass.





And I think the same held true for Pliosaurus kevani. There was a bit of functional weakness in the skull, but likely the only animal to exploit such weakness was another Pliosaurus kevani.


As another analogy let me offer you up the story of the Tomistoma schlegilii - better known as the false gharial. Like the gharial this relatively narrow snouted croc was long assumed to be pretty much a specialist of fish, avoiding larger vertebrates due to limitations imposed by its skull. Except the Tomistoma did not get the memo and is reported to dine on birds, reptiles, deer, and primates (including the bipedal type primate). In one particularly gruesome account a very large 5 meter female Tomistoma killed and ate a grown adult man which was pulled from the archosaurs stomach (picture above) in addition to an adult proboscis monkey and adult long-tailed macaque (reference croc specialist group newsletter). I wonder how the Tomistoma dismembered said human quarry? Did it pulverize the body to soften it up and then swallow? Engage in a little bit of death rolling? Perhaps slap the body against the surface of the water or perhaps the bank of the shore? My bet would be probably a little bit of all three. And that is also how  I bet Pliosaurus kevani would have dealt with its meals. An initial grasp with the quick biting end of the jaw, then ratcheting the prey towards the back of the skull several pulverizing bites to splinter shell, break bone, and collapse rib cages. Then, once dead, depending on the size of the quarry a couple of good thunks against the surface of the water, or if too heavy grab an extremity or soft spot and death roll off a bite sized piece. We can't rule out, given the likely habit of live birth in sauropterygian plesiosaurs, some type of social cohesion in which case rotational feeding was likely extremely probable. You know, just like nile crocs pulling apart a zebra today. Also check out how similar the lower jaw of the Tomistoma is to the lower jaw of Pliosaurus kevani.



Foffa D, Cuff AR, Sassoon J, Rayfield EJ, Mavrogordato MN, Benton MJ. Functional anatomy and feeding biomechanics of a giant Upper Jurassic pliosaur (Reptilia: Sauropterygia) from Weymouth Bay, Dorset, UK, (2014) Journal of Anatomy



Sunday, June 22, 2014

Bear-ly Sustainable: Can Polar Bears Find Refuge in the Genome of Brown Bears?

Tobias Bidon
Ok today I want to write about a more contemporary topic - the genetic legacy of bears. And I want to direct your attention to a paper dealing with the complicated genetic history of modern bears - abstract here and the more accessible science daily article: Evolutionary history of bears: It's complicated. Of course you should go read the articles yourself but what the gist of the story is that the genetic history resulting in the "species" or better yet "species groups" of modern bears is vastly complicated with numerous inbreeding occurrences and hybridization events. It appears many, if not most, bear species are kissing cousins. Central to the story is Beringia, the now submerged land bridge joining Asia and North America. According to the research Beringia may have served as a bit of mixing grounds from which the modern trajectories of bears into Asia and the Americas have since went their respective ways. Of course the bear family is just one of several carnivorous mammal families (felids, canids) that readily hybridizes. There is even a wikipedia web page on the subject of ursid hybrids. This topic of course delves deeply into the surprisingly murky definition of what constitutes a species. The long and short of it from what I gather is that the definition of what constitutes a species depends on who you ask.

Now just thinking about bear hybridization events should get you thinking about the well documented hybridization of polar and brown bears. Two species that are closely related and overlap enough in range and habit that matings do and have occurred quite often it seems.

Polar/Brown hybrid. wiki
Rothschild Museum
Thinking out loud after reading these articles and with the prospect of polar/brown bear hybridization events occurring more frequently in an Arctic under the influence of AGW I wondered: instead of going extinct, might the genome of polar bears find refuge in northern populations of brown bears? That is if through increased meetings due to lack of sea ice, the polar bear gets absorbed by the larger brown bear population shifting it's range north - could such an admixture reverse in the future if climatic conditions shift back to a regime more ameliorative to polar bears? Granted that they are so close genetically anyways - why couldn't the polar bear reemerge in several thousand years from a stock of hybridized polar/brown bears? The lack of color, streamlined form, ice gripping claws, and carnivorous dentition being selected for  in such a scenario and the polar bear reemerging... why not? I think it is an interesting notion, interesting enough to write a blog post about anyways.


 

Monday, June 16, 2014

Lessons From the Leopard Seal

I have been wanting to write this post for a while now. I am using it as a bit of a foundational piece as it pertains to several of my own posts (and future posts) reconstructing feeding ecologies of extinct critters (long necked plesiosaurs, Deinocheirus). Essentially I want to deconstruct some of the regimented thought processes I come across, in my opinion, with regards to ideas of what it means to be a dietary specialist. And I am using the leopard seal, Hydruga leptonyx, for my argument.


It is a bit of a macabre fact that seals in Antarctica will sometimes wander up valleys, get lost, and die with their remains being mummified in the desiccating conditions for thousands of years. Shown to good effect in the pic above of a mummified leopard seal carcass (lifted from here) the dentition of the leopard seal is very interesting. The canids and incisors speak to the well known macropredatory habits of this pinniped. Almost any documentary on the wildlife of Antarctica is replete with scenes of leopard seal depredations on poor penguins. But what I find more interesting regarding the leopard seal is what the back teeth, the carnassials, are doing. While most other carnivorans use their carnassials to slice and dice meat in a scissors like fashion the leopard seal has turned its carnassials into krill straining devices. What we have here is a switch hitter, a dual specialist, in both large and small prey.


The pic above is a still from video by David Hocking and other researchers from Monash University Melbourne, Australia. What you see is that after sucking in a small fish from a device (pictured at the right) the leopards seal then expelled excess water - the fish chum you see floating around - and swallowed the fish whole. In their test California sea lions were also able to suck in small fish but would often lose the small fish as they expelled the excess water. The leopard seal, as has been often hypothesized, was for the first time demonstrated to use its strange teeth as a sieve.

Leopard Seals Suck Up Krill Like Whales

Now you may have read or heard about this study already, it's been out for a couple of years. What I want to elaborate on, in a bit of a thought experiment with a slight nod to All Yesterdays, is this: Imagine if leopard seals were extinct... would a paleontologist correctly infer the "dual specialist" lifestyle strategy of the leopard seal? Would a great debate ensue? I can imagine one camp championing the notion of leopard seals as specialist strainers of small prey versus another camp arguing for macropredatory habits. For sure the word "specialist" would be bandied about by both sides.

We don't have to look back to far into paleo-discourse to see such arguments such as the famed T-rex scavenger/hunter debate. Currently we have the debate of diplodocine sauropods acting as specialist "hoover vacuum cleaners" of ground cover ferns. Or were they high browsing branch strippers? Or both depending on what was most abundant/nutritious at the time? (For the record I prefer the mixed bag)

Now again, this is a conceptual piece and it relates to how we think about a problem: the concepts, examples, and precedents we keep in our heads and use to marshall in arguments. And I think the story of the krill slurping, penguin skinning leopard seal is a good example to keep in mind when thinking about how animals may have functioned in the past. Especially the Mesozoic. Why do I highlight the Mesozoic as a time when being a dual specialist was perhaps especially useful?

Because Mesozoic ecosystems may have often been boom/bust based on a dominant monsoonal climate regime. Torrential rains for several months followed by a significant greening up of the landscape commensurate with an explosion of small animals - insects, snails, lepidosaurs, protomammals, hatchling dinosaurs. Seasonal bodies of waters appear teaming with turtles, amphibians, fish and especially lungfish. A season of plenty followed by a season of wilt. Drying up of seasonal waterways. Small animals dying or going into estivation. Plants dying back, sequestering resources back into their root systems until the rains return. In such a regime it is quite easy to imagine how a certain flexibility in  diet, akin to the leopard seal which can hunt large prey when available but will readily live off of miniscule krill if need be, could have been par for the course.

Over at Theropoda Andrea Cau paints a vivid picture of Deinonychus antirrhopus switching dietary strategies to subsist in such an environment (text may sound a little awkward due to translation):

"During the short rainy season, the Deinonychus are used to avoid their conspecifics, except to mate. During times of plenty, this animal is a hunter shy small vertebrates, such as lepidosauri and mammaliformi, a stealthy predator on the capacities alone, ignoring their own kind. During the dry season, Deinonychus takes on a more aggressive behavior and violent, when forced by a shortage of food (its usual prey tend to aestivate during the dry season) to focus on the carcasses of dinosaurs regularly decimated by drought, and then, inevitably, is forced to interact with many of his peers."

Such a lifestyle is not without context even today. Arctic foxes following polar bears onto the ice to scavenge seal carcasses during the winter when rodent prey is unavailable. Wolves subsisting on small prey hunted in a solitary fashion during the spring and summer but switching to pack hunting of large prey during the winter.

Now some may think I am conflating dual specialization with opportunism and, admittedly I am muddying the waters a bit here. Ultimately what I am trying to convey is that taking part in several sources of food may or may not be reflected in the anatomy of the animal in question. Simply because an animal has specialized anatomical features that suggest one mode of feeding does not mean that the animal is limited to that type of food.

Unless you are an anteater. Then for sure you eat nothing but bugs.

Related Posts Plugin for WordPress, Blogger...