#paleotalk

When I was a kid, I was fascinated by snakes. I loved their slinky, slithery movements and bone-defying flexibility -- and I was also endlessly frustrated by my inability to tell whether they had very long legless bodies or just very long tails attached to a skull, or what.

(For the record, they’re mostly body, with short necks and tails. You can tell by looking at where the ribs are in a skeleton.)

As major reptile groups go, snakes are a very recent development -- they’re younger by far than lizards, crocodiles or turtles, having only emerged late in the Cretaceous. They've been around for less than birds or mammals have, too -- and there’s a reason for that, as it’s thought that snakes arose in part in response to arrival of mammals.

The precise origin and development of snakes isn’t extremely well-documented, because snake skeletons are fragile and easily destroyed and so don’t turn up too often in the fossil record. What we do know is that the first snakes were relatives of the early monitor lizards, as monitors are the snakes’ closest living relatives, and probably resembled them.

Reasonably well-preserved snake fossils start appearing in the tail end of the Cretaceous, only a very short time before the famous extinction. These creatures still had vestigial limbs, often reduced to a single hind pair, and were burrowers (this in part counteracts the fragility of their skeletons, as burrowing animals are typically in an excellent spot to become buried and preserved and tend to be well-represented in the fossil record -- there’s nothing quite so nice in paleontology as a self-burying fossil). It’s not that clear what their ancestors were like, but they were probably something not too different from the modern-day earless monitor, another burrowing species.

That burrowing habit, specifically, is quite likely what caused the snakes’ leglessness. Burrows, as a rule, are cramped spaces -- digging is tiring work, so most animals make tunnels just wide enough for their bodies. This in turn means it’s a good thing to have a compact body, because it cuts down on how much dirt you have to move. Burrowing mammals, like gophers or ferrets, achieve this by having short, powerful limbs and low profiles, keeping their legs tucked against their chests when moving and essentially turning themselves into furry little tubes. They can do this because their high metabolisms allow them to support themselves on legs kept erect beneath their bodies, a stance that requires a lot of energy to maintain.

Lizards, with lower metabolisms, cannot do this, and keep their legs in a sprawled position. This is a problem for burrowing lizards, because it drastically widens their profiles -- and in turn forces them to either waste a lot of energy in digging wider tunnels or deal with cramped, tight conditions and toes that smack against tunnel walls with every step. The early snakes couldn’t pull their legs underneath themselves, so they did the next best thing -- they got rid of them altogether.

This of course begs another question -- if lizards has so much going against them in a tunneling lifestyle, why did they adopt it begin with? There are a number of possible answers, and it’s likely that they all played a factor. Burrows offer shelter from predators, weather extremes and temperature fluctuations, to begin with, and these would all have benefited a lizard capable of heading underground. However, the late Jurassic and the Cretaceous also saw a new clade of animals come along that was characterized by, among other things, extensive burrowing habits and a tendency to feature on the menus of modern-day snakes.

                                      “I don’t like where this is going...”

Where the prey goes, predators follow -- and a sudden boom in tunneling prey followed by a boom in tunneling predators is probably not coincidental. Even if the early mammals weren’t the reason those early lizards headed underground to begin with, they certainly encouraged them to stay there.

Later snake species left their tunneling origins fairly early and adapted to a variety of different environments, but their burrow-dwelling origins likely did a lot to save their hides during the end-Cretaceous extinction. Burrowing animals tend to do comparatively well during such events, as the ability to retreat underground lets them ride out the brunt of climate and temperature extremes.

Having a diet that mostly consisted of another widely-surviving group probably didn’t hurt much, either.

After the metaphorical dust settled, snakes did fairly well for themselves. They colonized most terrestrial habitats in fairly short order, and some reached gigantic sizes -- Titanoboa, a genus that arose five million years after the dinosaurs died out, is thought to have reached or exceeded thirteen meters (nearly forty-three feet) in length. The origin of snake venom isn’t really all too clear, but probably happened in the early Cenozoic as well. This was a major development for snakes, as it made them much more efficient predators -- earlier species had relied on physically subduing their prey, which was both less likely to successfully kill their prey and more likely to get them injured.

Snakes also developed very specialized organ arrangements to suit their lifestyles, although to rarity of preserved soft tissues makes these difficult to trace and time. Their organs are thin and elongated, and snakes have only one working lung, the right one -- in many cases, the left lung is entirely absent.

                    A slow-worm, which is neither a snake nor a worm.

The basic snake body plan is evidently a very successful ones for their lifestyle, because multiple families of lizards developed into it independently of either true snakes or each other. Most, like the ancestral proto-snakes, are either burrow-dwellers or live in thick undergrowth. Very few are as specialized as true snakes, however -- many have vestigial limbs, whether stubby legs or simply claws, and almost all retain two functional lungs.

So, this is interesting! The basic concept here is that there’s been speculation for a few decades now that tyrannosaurids could have been social or semi-social animals that moved and hunted in groups, a theory primarily based on the occasional discoveries of mass death sites where large numbers of these creatures were all in one spot and died at the same time. In this case, either four or five specimens of Teratophoneus (a genus of tyrannosaurs that lived about ten million years before the famous one) seem to have all died together during a flood.

Social behavior is something that has evolved multiple times among different organisms, since it tends to be advantageous for both predators (who can cooperate in bringing down tougher or more elusive prey and in defending against other hunters) and prey (there’s safety in having a lot of pals to stand with or get lost among when something’s hunting you). Thus, since it’s generally accepted that dinosaurs were comparatively intelligent and active animals, it’s thought that it would have been likely that social behavior arose among them as well.

The primary arguments against this theory are that these mass death sites aren’t especially common (this is the third found since 1910) and that they don’t necessarily show that these creatures always lived in groups. Essentially, the sites only strictly show that these creatures died together, not that they lived together -- it’s possible, for instance, that they only gathered together because of environmental stresses such as a lack of food driving them to unusual behaviors. Whether they were solitary outside of these instances or whether they spent all their lives together isn’t something that the fossils can show us.

Islands, especially isolated ones, are something of a biologist’s paradise -- the isolation, numerous empty niches and often eclectic and unusual natures of the specific organisms that end up stranded on any given island have given rise to a truly amazing range of unusual and specialized lifeforms. This is especially clear when we look at the recent geological past, and the island endemics that lived during and after the ice ages.

One thing that pops up very often are dwarf versions of mainland animals -- dwarf elephants and hippos flourished on several islands and are some of the most iconic examples of island dwarfism, where large animal species shrink in size due to reduced space and food sources on island environments.

Dwarf elephants arose numerous times from both elephant and mammoth stock; they were widespread in the Mediterranean -- Sardinia, Sicily, Crete, Malta, Cyprus and the Greek islands were all home to populations of pigmy proboscideans.

Sea levels fluctuated throughout the ice ages as the glaciers advanced and retreated, so several islands were colonized on multiple occasions by full-sized elephants and mammoths who became stranded when the seas rose again. This resulted in multiple separate instances of dwarf proboscideans of different levels of dwarfism arising independently -- Sicily and Crete were home to several species each.

Skeleton of a Cretan dwarf mammoth.

Other dwarf proboscideans, mostly belonging to the genus Stegodon, lived in Indonesia. The dwarf mammoth Mammuthus exilis, descended from the much larger Columbian mammoth, also lived in the Channel Islands of California until about 13,000 years ago.

Mammuthus exilis compared to a human.

As I mentioned, dwarf hippos were also common. Dwarf hippopotami lived on Sicily, Malta, Crete and Cyprus during the Pleistocene, while three species coexisted in Madagascar until about a thousand years ago.

Examples of this also existed among ruminants; the Balearic Islands were, until about 5,000 years ago, home to the tiny Myotragus, a relative of goats and sheep that, besides being maybe half a meter tall, also had front-facing eyes (unlike the side-facing eyes of most herbivores) and incisors that grew continuously, like those of rodents and lagomorphs. It also had bones similar in structure to those of cold-blooded animals, suggesting it had adapted to have a much slower metabolism than other mammals in response to its home’s balmy climate and lack of predators.

Probably the most extreme example was the minuscule Candiacervus, a deer that shared Crete with the dwarf elephants and hippos and that stood no more than 40 cm at the shoulder.

Candiacervus ropalophorus next to the flightless owl Athene cretensis

A very ancient example of this is also known in the form of Europasaurus, a dwarf sauropod that lived on an island in a large archipelago that is now central-western Europe.

Europasaurus compared to its larger relative Giraffatitan

The opposite phenomenon -- island gigantism -- is also well-recorded, and happens when small creatures respond to environments with plenty of food and few predators by growing huge.

Flying animals also tend to lose their ability to fly -- flight and the growth and maintenance of the large muscles animals need to fly are both huge resource and energy drains. If an animal doesn’t need to fly on a regular basis (such as to escape predators or move between distant areas), then the growth of specialized flight organs can become a significant waste of resources, and natural selection will favor animals whose atrophied flight musculature lets them allocate those resources elsewhere.

When paired with island gigantism, this has resulted in creatures such as the late lamented dodo (its closest relatives are, in fact, pigeons, although dodos seem to have reached up to one meter in height), the moa-nalos (giant, flightless ducks) of Hawaii, the moas of New Zealand and the elephant birds of Madagascar.

(Four species of moa compared to a human)

All these factors -- gigantism, dwarfism, flightlessness -- naturally often occurred in the same places and in the same times as different species responded differently to their island environments. The result in the creation of multiple very peculiar and unique environments.

New Zealand is a fascinating situation, since its lack of any native mammals besides a few bats meant that the birds lately had it to themselves, and evolved to fill all sorts of niches that are normally filled by mammals -- the moas, for instance, occupied the niche normally taken by big and medium-sized ungulates.

Malta and Sicily, in addition to dwarf elephants, were also home to Cygnus falconeri, a giant island relative of the modern-day mute swan perhaps two meters long -- tall enough to loom over its pachyderm neighbors.

Modern-day Gargano, in southern Italy, seems to have been a large island during the Miocene, about 12 to 4 million years ago. It lacked large mainland predators outside of birds of prey and some crocodiles, and as such many of its native creatures developed gigantism and -- among the birds -- flightlessness. Natives included Hoplitomeryx, a genus peculiar, five-horned deer-like creatures; Deinogaleryx, giant carnivorous moonrats; the giant flightless anatid Garganornis; and a variety of both giant rodents and lagomorphs, including giant pikas, dormice and hamsters; and two species of giant barn owls, Tyto gigantea and T. robusta, which were likely among the island’s dominant predators.

Garganornis

Essentially, the original team to work on the new Spinosaurus remains argued that it was the first known semiaquatic theropod, basing their evidence of details such as dense bone that could have served as ballast, feet purportedly well-suited to paddling in the water, a barrel-shaped torso convergent with those of whales and the immense quantity of food in the deltas, lagoons and rivers of the Cretaceous Sahara region -- a region that was long known to have a lot more big predators than big herbivores, so its trophic chains were always a bit questionable. This theory, as most ambitious theories generally do, received its share of pushback on the grounds that, while Spinosaurus certainly wouldn’t have been especially agile on land, arguing for a fully amphibious lifestyle was premature and there wasn’t enough hard evidence to argue for the creature having been well-suited specifically for movement in the water.

The argument then largely stalled there. The idea of water-loving Spinosaurus was there, and had the potential to dramatically shape our understanding of theropod and specifically spinosaurid evolution, but a clinching argument to close things one way or another was lacking. Spinosaurus was certainly a shore-dweller -- but was it more? That wasn’t certain.

So, the actual news here. Over 2018 and 2019, the team responsible for the 2014 find returned to the Moroccan site where those bones were found to see if they could, ah, dig up something new. What they got included a lot of tail bones, all seemingly from one individual animal, which provided two interesting pieces of evidence:

First, the tail bones largely match those described from the first Spinosaurus fossils found in Egypt, suggesting that a single species would have ranged across all of North Africa.

Second, the tail vertebrae had a very interesting shape. So, vertebrae have a ridge growing from their top called a spinous process -- scientific terminology is nothing if not exhaustive -- which can be of varying length in different animals and, for that matter, in different vertebrae in the body.

(Image from Wikimedia Commons, here)

Spinosaurus’ famous sail was held up by very, very tall spinous processes. Now, these newly analyzed tail vertebrae also had very tall processes -- nothing like the back sail, but some were nearly two feet tall. As the rest of the tail was no wider than otherwise expected, this would have made Spinosaurus’ tail a shape much like an oar, or a paddle -- long and vertically flat. Looking back at the image, see the other growths on the vertebra’s sides? In life, the projections of different vertebrae interlock with one another; this helps keep the spine rigid. In Spinosaurus, these growths seem to have atrophied into nothing towards the end of the tail, giving it a lot of flexibility when blending from side to side.

The conclusion drawn from this was that this broad, flexible tail would have been used by the living animal to swim, beating back and forth to push against the water around it and propel the creature forwards; this isn’t that big a stretch -- that’s how crocodiles swim, for instance. This was tested using a robotic facsimile of a Spinosaurus tail, which proved that, indeed, the creature would have been able to propel itself through the water much more quickly and more efficiently than any other known theropod.

This, strictly speaking, only proves that Spinosaurus was a very efficient swimmer, especially when compared to other dinosaurs -- but it does play rather nicely with the view that it would been a chiefly aquatic creature.

The causes of the K-T extinction -- a massive asteroid impact some 65.5 million years ago and the resulting worldwide climatic disruptions and trophic collapses -- have been known for some time. Besides the rather dramatic and sudden change in what sorts of creatures turn in up in the fossil record -- the way large, diverse archosaurs dominated land environments and various sorts of other reptiles dominated marine environments until they all vanish at the same time got impossible not to notice after a while -- there are also the huge, buried crater in the Yucatan and the worldwide layer of iridium-rich deposits and tektites (small bodies of glassy minerals formed from material flash-melted and frozen mid-air during asteroid impacts).

Now, what had been missing -- until this March of 2019, obviously, or I would probably not be writing this to begin with -- was direct evidence of what happened during the disaster itself. Evidence of its effects is abundant -- direct evidence of what happened right at that moment, when the meteorite hit and its impact shook the world... not so much.

Our new evidence comes from Hell Creek, in North Dakota. It’s a fossil bed containing the jumbled remains of very diverse animals -- Triceratops bones, ammonites, mammals, tree material, fish and mosasaur remains have all been found in this single mass grave. Apparently, powerful waves swept through bodies of water and flung aquatic animals on land, devastating shoreline environments in the process. The mixed remains of aquatic and terrestrial life were then buried in sediment from successive waves.

(The waves, apparently, aren’t likely to have been direct tsunamis from the impact -- rather, the researches working on this site think them to have been seiche waves, which are caused by seismic tremors even very far from the source epicenter.)

The quickness of the event was apparently such that the remains were preserved with minimal damage -- the detritus would have flowed quickly and set just as quickly, allowing minimal scattering and crushing and thus permitting a high degree of preservation. Some of the remains are amazingly well preserved -- there’s even a fish that was clearly broken in half after hitting a tree.

Trace fossils are also present -- mammals burrows and dinosaur tracks were both preserved.

One of the most fascinating parts of this, to me, is the presence of tektites in the gills of the fish found in this bed, as well as in amber deposits within it. The causes are obvious enough -- as the glassy material rained down on the landscape, it would have fallen into the water where it would have been breathed in by fish and remain lodged in their gills, likely contributing to their deaths, and likewise on the forests that would have dominated the Hell Creek area at the time and lodged itself in tree sap that, over time, hardened into amber. These things, together, probably preservation the most specific view of a single moment in time -- the most specific event -- in this entire treasure trove.

Another thing to keep in mind: while the Hell Creek beds have generally yielded a very through cross-section of their ancient biodiversity compared to other sites, fossilization is always a very hit-or-miss process, and it’s extremely rare for a creature to die in just the right place and conditions to be preserved. The fossil record in inherently a very incomplete thing. Mass death events like, where an entire community is killed and preserved together by a single event, tend to be some of the most complete reputations of prehistoric environments by virtue of almost nothing having the chance to escape whatever disaster is burying almost every living thing at once -- think of the Burgess Shale. This mass grave, from what I understand, seems like it might end up yielding a few surprises once there will have been time to study it more thoroughly.

This is easily the most fascinating discovery of the past few years.

I found this phylogenetic tree on the web some time ago, and I wanted to share it. It’s a combination of being very well-drawn and actually fairly accurate that I don’t see very often.

The main division, as plainly visible, is between prokaryotes and eukaryotes. Prokaryotes are single-celled organisms without a nucleus in their cells -- the DNA floats free inside them in a single tangled chromosome -- and without any organelles. They used to all be classified as bacteria, but they were divided based on physical differences (bacteria use a substance in their cell walls that archaeans don’t) and genetic ones (archaeans are more closely related to eukaryotes than to bacteria). Eukaryotes have nuclei and organelles, as well as much larger cells than prokaryotes do, and occur as both single-celled and multicellular organisms.

What I love about this tree is the way it represents the relationships between organism groups. The eukaryotes come from two fused branches from the archaeans and bacteria -- eukaryotes are thought to have evolved from archaeans who “swallowed” oxygen-breathing bacteria, which over time became symbiotic and were eventually, so to speak, “stripped down” in a process called endosymbiosis until they became our mitochondria. A second branch connects to the plants -- endosymbiosis happened again, later, with photosynthetic bacteria known as cyanobacteria, which became chloroplasts. In this sense, eukaryotes are technically chimeras formed from the fusion of bacteria and archaeans.

The protists are a very loose classification of single-celled organisms from which plants, animals and fungi arose. The red leaf next to the big leaf labeled plantae (the plants, obviously enough), I assume, represents the red algae, a second group of photosynthetic organisms which use red pigments to catch light rather than green chlorophyll like green algae and true plants do. Red algae occur as both single-celled organisms and multicellular ones, and are always aquatic.

The little branch connecting the presumed red algae back to the protists is probably meant to represent secondary endosymbiosis, where eukaryotic red algae and green algae (this process is thought to have happened at least twice) were swallowed by other protists and became light-catching organelles themselves, with their own chloroplasts still visible inside them.

Fungi and animals are placed close together because they are far more closely related to each other than to plants. The reasons for this classification can be a tad complex if you’re not familiar with the literature, but it’s based on a number of traits like cells with flagella that move by pushing themselves forward by means of a flagellum attached to their hind end (as opposed to almost all other eukaryotes, which pull themselves forwards with one or more flagella attached to their front end), and the way they store sugars (plants form long chains of linked ring-shaped sugar molecules, while animals, fungi and their relatives do the same but with branched chains) and turn them into physical supports (plants use sugar molecules to make cellulose, animals and fungi and co. use them to make chitin).

This is one group I’m guessing most of you will recognize, at least by sight.

Trilobites are some of the most common fossils found in Paleozoic (541 to 252 million years ago) marine rocks, as they were extremely abundant animals and their hard shells preserved easily. They were around for nearly the entirety of the Paleozoic, and were both very widespread and very diverse.

I’ve also been looking at a lot of these guys for school lately, so I’ve been itching for a while to share my newfound appreciation for these beautiful shield-headed sea pillbugs.

I was browsing paleoart on Deviantart yesterday, and I found something I felt obligated to share.

Namely this handsome fellow:

(Art by Olorotitan on Deviantart)

This creature is Dollocaris. It was a thylacocephalan, one of a group of possible crustacean relatives that lived from the Silurian to the Cretaceous (which comes to be well over 300 million years). Dollocaris itself lived in the mid-Jurassic in what is now Europe (which was then a sizeable tropical archipelago) and was probably an ambush predator. It holds the distinction of having had some of the proportionally largest eyes of any living being -- its eyeballs made up a full quarter of its length.

Size-wise, it was around 20 cm (just shy of 8 in) long.