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I was watching a talk by Michio Kaku and he mentioned that crocodiles (or possibly alligators, I forget offhand) don't actually age -- they can die, but they essentially go through no aging process beyond adulthood
Can anyone link me any sort of detailed information on this?
Well, this needs to be broken down into two parts. Do Crocodilians age (undergo senescence), and are Crocodilians immortal (will only die of external causes)?
Are Crocodilians immortal? - No. They appear to live about as long as humans before they die.
Measuring crocodile age is unreliable, although several techniques are used to derive a reasonable guess. The most common method is to measure lamellar growth rings in bones and teeth-each ring corresponds to a change in growth rate which typically occurs once a year between dry and wet seasons. Bearing these inaccuracies in mind, the oldest crocodilians appear to be the largest species. C. porosus is estimated to live around 70 years on average, with limited evidence of some individuals exceeding 100 years. One of the oldest crocodiles recorded died in a zoo in Russia. A male freshwater crocodile at the Australia Zoo is estimated to be 130 years old. He was rescued from the wild by Bob Irwin and Steve Irwin after being shot twice by hunters. As a result of the shootings, this crocodile (known affectionately as "Mr. Freshy") has lost his right eye.
Do Crocodilians undergo senescence (show signs of aging)? Well, if this study from "Gerontology" written by Patnaik BK in 1994 is to believed… Maybe not.
Evidences and mechanisms of rapid or negligible senescence in reptiles are still fragmentary and unclear… neither the increase in mortality rate and accumulation of lipofuscin nor the reproductive senility have been shown conclusively in ageing reptile populations.
So, while Crocodiles and Alligators (both Crocodilians) definitely have a finite lifespan, because they just continue to grow it's hard to tell how long they have left until the day they die.
Morelet's crocodile was first described in 1850 in Mexico by the French naturalist Pierre Marie Arthur Morelet. The species was subsequently named after him.   It was long confused with the American and Cuban crocodiles because of similar characteristics and an ambiguous type locality. It was not generally accepted as a separate species until the 1920s.
Morelet's crocodile has a very broad snout with 66 to 68 teeth when they are fully mature. They are dark grayish-brown in color with dark bands and spots on the body and the tail. This is similar to other crocodiles, like the American crocodile, but the Morelet is somewhat darker. Juvenile crocodiles are bright yellow with some dark bands. The crocodile's iris is silvery brown. They have four short legs, giving them a rather sprawling gait, and a long tail, which is used for swimming. The hind feet of the crocodiles are webbed. They have very explosive capabilities because of their strong muscles and are fast runners. 
Morelet's crocodile is small compared to most other crocodiles. The males can become larger than the females. The average adult Morelet's crocodile is about 2.1 m (6 ft 11 in), with a typical length range of 1.5 to 2.7 m (4 ft 11 in to 8 ft 10 in) (the lower measurement representing the mean total length of a female at sexual maturity which is attained at roughly 7–8 years of age in the wild).   Almost all crocodiles in excess of 2.5 m (8 ft 2 in) are males and at this advanced stage of maturity, the male goes through a significant change in skull osteological morphology as the skull appears to increase in broadness and robustness.  Large adult males can attain a length of 3 m (10 ft), anything in excess of this is considered exceptionally rare for this species however the species has a maximum reported length of 4.5 m (14 ft 9 in), with two other outsized specimens reportedly measuring 4.1 and 4.3 m (13 ft 5 in and 14 ft 1 in), respectively.    Body mass in this species is often around 38–58 kg (84–128 lb), however this may include possibly overweight captive specimen and average adult body mass in wild adults from Belize was posited as approximately 27.7 kg (61 lb). The weight of a large 3 m (10 ft) wild male crocodile is estimated to average 83.9 kg (185 lb) in although mass is likely much more in outsized individuals.   Overall, this species is similar in appearance and morphology to the Cuban and the larger American crocodiles.
For crocodiles the mating season usually will begin in July or August. They are ready to mate when they are about 8-10 years of age. But that is calculated according to its size. Both sexes can be very territorial as well as aggressive in nature during this period of time. The females are going to make sure that other females aren’t coming into their area to find a place to put their nest. They are very picky when it comes to the male they will mate with. All others will be driven out of their location, forcefully if necessary.
The males tend to put on a great show of power and dominance with each other. The standoff can last for several hours before one of them will turn and walk away. It very rarely ever escalates to the point of a physical confrontation for the two males though.
When a male is ready to mate, he will very carefully and attentively approach a female. This is very different from the way in which a crocodile is often portrayed. He will try to rub his body against hers to see if she will return the affection or not. If she wants to mate with him she will be vocal about it. The mating will take place under the water. They may stay with each other for several days and mate again over that period of time.
Crocodiles offer nests of eggs. The females will dig at least 10 feet into the ground. They want to make a burrow where they can deposit their eggs and prevent other predators from consuming them. They will hide the entrance as well so that they can get back in there when they need to. These animals have a remarkable memory which helps them to successfully do so.
The number of eggs that a crocodile will deposit depends on the species, their location, and their size. It can be as few at 10 or as many as 100. All of the eggs can be deposited within a hour or so. The more eggs she can lay the better because only about 20% of them are actually going to produce offspring.
It takes about 80 days from conception for the young to hatch. If the temperatures are very warm they will hatch sooner. Likewise, it can be up to 100 days if the temperatures are cooler before they emerge.
The females are often seen with the eggs in their mouths. They aren’t eating them, but cracking the shells. By doing so they will help their young to be able to get out of the protective shell. She will also carry them into the water with her mouth. This is one of the few reptiles that actually is known to be a very good nurturer of their offspring.
The young will spend several months in the close proximity of their mother. While she will do all she can to offer protection and direction her efforts are often in vain. On average only about 2% of those that hatched will survive long enough to mate themselves.
One of the most amazing things about crocodile reproduction is that they don’t have chromosomes in the genes to create males and females. Instead, the temperature around the area will determine what the sex happens to be. As a result those that emerge are usually always only of one sex or the other.
New research explains why crocodiles are relatively unchanged since dinosaur times
If you look around, you’ll see all sorts of dinosaur descendants, although we call them birds nowadays. But if you want to truly see a dinosaur-like creature, just like it would have roamed the Jurassic age 200 million years ago, just find a crocodile. While some of the species have died out (previous ancestors also consisted of giants as big as the dinosaurs, plant-eaters, fast runners, and serpentine forms that lived in the sea), a lot of what you see now is what you got then.
In new research published in the journal Nature Communications Biology, scientists from University of Bristol explain how crocodiles follow a pattern of evolution known as ‘punctuated equilibrium’.
Crocodiles have many traits which gave them the ability to survive the past 200 million years (Image: Pixabay)
If it ain’t broke, don’t fix it
The rate of crocodile evolution is generally slow, but occasionally they evolve more quickly because the environment has changed and they need to adapt. Specifically, the research suggests that the crocs’ evolution speeds up when the climate is warmer (which it was in the age of the dinosaurs more than it is, today which could explain the different varieties that emerged then), and that their body size increases.
The secret to their slow aging is the limited diversity of crocs and their apparent lack of evolution. It seems the crocodiles arrived at a body plan that was very efficient and versatile enough that they didn’t need to change it in order to survive. They have the ability to thrive in or out of water — they can stay under water for up to an hour — and are able to live in complete darkness. They are also very robust, meaning they can survive horrible injuries.
Believe it or not, it actually isn’t that uncommon for a crocodile to lose a leg and then go on to live another 70 and 100 years.
So while the rest of their dinosaur contemporaries were done in by the asteroid which pancaked itself into the Gulf of Mexico 66 million years ago, crocodiles’ traits — such as the ability to draw energy from the sun, something others couldn’t do — allowed then to continue on life’s journey.
The crocodiles have had a much greater diversity of forms in the past. Examples include fast runners, digging and burrowing forms, herbivores, and ocean-going species. Image credits: University of Bristol.
“Our analysis used a machine learning algorithm to estimate rates of evolution,” said lead author Max Stockdale. “Evolutionary rate is the amount of change that has taken place over a given amount of time, which we can work out by comparing measurements from fossils and taking into account how old they are.”
For the study, the team measured body size — which is important because it interacts with how fast animals grow — how much food the crocodiles needed, the number of their populations, and how likely they were to become extinct.
“It is fascinating to see how intricate a relationship exists between the earth and the living things we share it with,” said Stockdale. “The crocodiles landed upon a lifestyle that was versatile enough to adapt to the enormous environmental changes that have taken place since the dinosaurs were around.”
Now, the next step for the researchers is to see why some crocodiles thrived through the millions of years, while others didn’t.
Genetic Analysis Shows Crocodile Evolution Was Rebooted by Ice Age Glaciations
Crocodiles are resilient animals from a lineage that has survived for over 200 million years. Skilled swimmers, crocodiles can travel long distances and live in freshwater or marine environments. But they can’t roam far on land. American crocodiles (Crocodylus acutus) are found in the Caribbean and Pacific coasts of the Neotropics but they arrived in the Pacific before Panama existed, according to researchers from McGill University.
Over 3 million years ago, the formation of the Isthmus of Panama altered global ocean circulation, connecting North and South America and establishing the Caribbean Sea. This resulted in widespread mixing of species on the continent and separation in the seas. On land, mammals from North America such as mammoths, sabre-toothed cats, horses, and camels invaded South America, and strange mammals like giant ground sloths, armadillos, and opossums from South America invaded North America. This event is known as the Great American Interchange, and the opposite happened in the seas, where new species of corals, clams, and fishes evolved in the separated Pacific and Caribbean waters.
Researchers captured and took blood samples of crocodiles from several populations living on both coasts of Panama. Credit: Luis Felipe Estrada
The question a group of McGill and Panamanian researchers asked was: how distant are the Pacific and Caribbean populations from each other and does it match the geological record? Researchers have long suspected that American crocodiles living on the Pacific coast should have diverged genetically enough from Caribbean populations to become unique species.
“We assumed we would detect significant genetic differences between Pacific and Caribbean crocodile populations that were isolated for the past 3 million years,” said José Avila-Cervantes, a recent PhD graduate of McGill University under the supervision of Professor Hans Larsson.
To test this, Avila-Cervantes captured and took blood samples of crocodiles from several populations living on both coasts of Panama. Back at McGill University, he sequenced their genomes to look for small variations in their DNA. He used the genetic differences to estimate how much evolutionary divergence and gene flow existed between populations. With this information, the team found that Pacific and Caribbean crocodile populations have been separated for only about 100,000 years.
“This time of separation is a far cry from the 3 million years we were expecting,” said Professor Larsson, Director of the Redpath Museum at McGill. “But it did match the last interglacial period of the Ice Age.”
At McGill University, researchers sequenced the genomes of crocodiles to look for small variations in their DNA. Credit: José Avila-Cervantes
Glacial and interglacial cycles in the Ice Age mark periods of peak polar glaciations separated by relatively warm times. These warm times caused sea levels to rise over 100 meters globally compared to present-day levels. Using the record of Ice Age sea levels, Avila-Cervantes was able to reconstruct what Panama would have looked like during these peak cold and warm periods of the Ice Age.
Coastal movements explained
“It surprised us to see that during the warm inter-glacial periods, most of Panama was underwater with the coasts separated by brackish lagoons, small rivers, and thin stretches of land,” said Avila-Cervantes. “These are the reasons why we think crocodiles were able to pass from coast to coast freely and explain why their oldest genetic signature of separation coincides with this time.” A second younger signature of genetic separation is timed to about 20,000 years ago and coincides with the last glaciation cycle that they found made Panama about twice as wide as it is today, and probably a good barrier for these crocodiles. “This is one of the first studies to implicate Ice Age glaciation-interglaciation cycles with the evolution of a tropical organism.”
Yet the researchers discovered there is some genetic divergence between the populations on each coast despite the frequent inter-glaciations, and this diversity is at risk due to habitat destruction from human development. “It was difficult to find any population living on the Pacific coast near the Panama Canal,” said Avila-Cervantes.
One of the best-preserved populations is in the middle of the Panama Canal on the Barro Colorado Island Nature Monument. “Preserving the population around this island may be our best chance to preserve the unique genetic signatures of Panamanian American crocodiles,” said Professor Larsson. “Our study not only highlights the resilience of crocodiles to ancient climate changes and their great capacity to survive large geological events, but also their vulnerability to our voracious need to modify their environments.”
Reference: “Effect of the Central American Isthmus on gene flow and divergence of the American crocodile (Crocodylus acutus)” by Jose Avila‐Cervantes, Carlos Arias, Miryam Venegas‐Anaya, Marta Vargas, Hans C. E. Larsson and W. Owen McMillan, 14 December 2020, Evolution.
How Long Do Crocodiles Live
Some crocodiles were known to have lived past a century, and even those that don’t can still exceed a human lifespan with ease. Myths are even going around claiming that crocodiles might have the potential of being immortal, as long as they can be protected from disease, hunters or starvation. Since crocodiles live such a long time, and since scientists have not been able to come up with a reliable means of measuring their lifespan accurately, getting a precise reading on how long crocodiles can live is difficult at best. However, separating myth and assumption from actual fact can still get us close.
Through actual time measurements performed on crocodiles living both in captivity and in the wild, it was determined that the average lifespan of these reptiles is about 30-40 years at the very least. Some larger species are estimated to live an average life of up to 70 years, with individual members reaching ages of 115 or even more than 140. A male crocodile named Kolya, held in a Russian zoo for about 80 years, was claimed to have been 115 years old at the time of death in 1995. A freshwater crocodile in Australia was known to have survived for nearly 140 years until 2010. Also, the Crocworld Conservation Centre claims to have a living Nile crocodile that is about 117 years old. Although these claims cannot be verified completely, they seem to confirm the theory that many species of crocodiles can live for more than a century under ideal conditions.
There are claims that crocodiles don’t age. While how the process of aging in these reptiles actually works is still widely unclear to scientists, crocodiles do show signs of aging. As they become older, they tend to lose their teeth and become weaker. Females start to lay fewer eggs, and all members of the species may be prone to cataracts and other disorders specifically associated with the aging process. The most accurate method to measure how long crocodiles have lived is to measure lamellar growth rings in their teeth and bones, a process that still leads to many inaccuracies.
Recent discussions and articles have led to the popularization of the myth that crocodiles might not age at all. According to some scholars, there is no evidence that the aging process even applies to the biology of crocodiles, and there is no difference between adult members of the species of different ages. One myth even states that, if it wasn’t for disease, famine and hunting, crocodiles would likely live forever, growing constantly but never really dying of old age. Experts have put this myth to rest as well, showing that crocodiles rarely even reach the age of 100.
Research Reveals Why Crocodiles Have Changed Little Since Age of Dinosaurs
(CN) — Birds and mammals have undergone numerous changes to better adapt to their environments over millions of years. Beaks curved over time to better pluck insects from tree bark and pelts grew thicker for animals to weather a colder climate.
Yet the humble crocodile has ambled along over the last 200 million years, relatively unchanged from its Jurassic period ancestors.
A study published Thursday in the journal Nature Communications Biology explains why crocodiles have followed an apparently slow evolutionary pattern.
As far as reptiles go, crocodiles are closely related to dinosaurs. But they’re incredibly complex biological organisms that survived the meteor impact that ended the Cretaceous period roughly 66 million years ago — and did in their dinosaur relatives.
So in a completely different era, why do crocodiles still look like mini dinosaurs that roam the earth?
According to researchers, the crocodile found the most suitable body type for its survival. They found crocodiles achieved a punctuated equilibrium, or a sort of stasis in their evolutionary journey.
This theory contrasts with the idea that evolution happens over a long period of time with gradual changes, which scientists can see in the fossil record.
Crocodiles, the study authors argue, are an example of a species that changed in response to their environment over short bursts.
The study authors used machine learning to measure the rate of evolution.
“Evolutionary rate is the amount of change that has taken place over a given amount of time, which we can work out by comparing measurements from fossils and taking into account how old they are,” said lead author Max Stockdale from the University of Bristol in a statement accompanying the study. “For our study we measured body size, which is important because it interacts with how fast animals grow, how much food they need, how big their populations are and how likely they are to become extinct.”
Long before the giant meteor set off the cataclysmic extinction event that killed all their cousins, crocodiles came in different types and sizes.
There were giant and iguana-sized crocodile relatives. Others were plant eaters, some ran quickly and others still were sea serpents able to thrive in the warmer climate of the Jurassic period.
Crocodiles, dinosaurs and winged pterosaurs all descended from the archosaur. But only the crocodile survived a post-meteor world, and its survival could be due to a complex system of senses and other traits that allowed it to become an apex predator.
Modern crocodiles can tolerate saltwater thanks to special salt glands that filter out minerals. They replace their teeth up to 50 times throughout their life and they also have a cerebral cortex, which is vital for perception, memory and consciousness. They can hunt prey by following movement patterns, like when an animal goes to a body of water for a drink. Some crocs have been observed using sticks to lure birds in for the kill.
In other words, crocodiles are flexible creatures.
The researchers next hope to uncover why some prehistoric crocodile species died, while others survived and thrived.
“It is fascinating to see how intricate a relationship exists between the Earth and the living things we share it with,” said Stockdale. “The crocodiles landed upon a lifestyle that was versatile enough to adapt to the enormous environmental changes that have taken place since the dinosaurs were around.”
Crocodiles have smooth skin on their belly and side, while their dorsal surface is armoured with large osteoderms. The armoured skin has scales and is thick and rugged, providing some protection. They are still able to absorb heat through this thick, rugged armour as a network of small capillaries push blood through the scales to absorb heat.
Body size metrics
The masses of fragmentary fossilised remains are clearly not representative of the body mass of the animal in life. Body mass of fossil taxa is often represented in analyses by a linear metric that is considered to correlate with overall body size 13,14 . We could not determine full body length or snout-vent length in many fossil pseudosuchians because most lack complete skeletons. Skull length has been shown to be an indicator of body size in dinosaurs 41 . Skull length has been used in a recent study of crocodilian macroevolution 29 . These authors 29 provided a linear regression of total skull length with total body length, but of the six taxa in their plot, the large-bodied examples are long-snouted, and the small-bodied examples are short-snouted. This is not a representative sample, since long-snouted and short-snouted forms are known across a range of body sizes for example, a highly elongated snout is observed in both the modest-sized Isisfordia duncani 42 and the giant Sarcosuchus imperator 43 . Likewise a short-snouted morphology occurs in the small-bodied Shamosuchus djadochtaensis 44 and the exceptionally large Razanandrongobe sakalavae 37 . Therefore, this regression does not address the possibility that skull length may over-estimate body size in long-snouted taxa, and under-estimate it in short-snouted taxa. It is difficult to determine whether the patterns observed 29 truly reflect the evolution of body size, or whether this pattern has been driven by the evolution of skull aspect ratios. An alternative measure, skull width, avoids contrasts of long-snouted and short-snouted forms, but would underestimate body size in small-headed forms such as aetosaurs.
The diameters of long bones have been used in body size studies in dinosaurs 14,15 . However, limb elements of pseudosuchians are not so abundant in the fossil record as skulls (Supplementary Data 1). Further, although the dimensions of a load-bearing skeletal element such as the femur or humerus are proportional to that load, non-avian dinosaurs were entirely terrestrial, with an exclusively erect gait. This is not true of the Pseudosuchia, in which some forms showed a sprawling or semi-erect gait 2 , and several clades of crocodylomorphs were partly or entirely aquatic 2 . These issues mean that femur length or diameter, for example, would not provide a reliable estimate of body mass or length across all taxa.
The body size proxy used in this analysis is the score of each taxon on the first component axis of a principal components analysis of 21 traits (Supplementary Data 1). This is based on the methodology of Smith 45 , which posits that variation in body size drives covariance across multiple traits. The analysis of Smith demonstrates that difference in size, or isometry, is indicated by the relative position of taxa on a trend line drawn through a bivariate space of two characters. Variation in body shape, or allometry, is indicated by the residual error of points above or below that trend line, and therefore this method is applicable to taxa of varying body proportions. A simplified demonstration of this rationale is shown on Fig. 5. Jolicoeur 46 expands upon this concept into a multivariate model using principal components analysis. Principal components analysis re-orientates a multivariate dataset into a corresponding array of orthogonal axes, with each axis accounting for a decreasing fraction of variance. These axes are derived through correlation tests of each variable with each other variable. Strong correlations contribute more substantially to a principal component axis than a weak correlation. The positions of the first two principal components in a bivariate context are shown on Fig. 5. Principal components analysis commonly uses log-transformed data 47 to standardise measurements and limit the effects of body size on interpretations of allometry. However, it has been observed that log transformations are not universally appropriate 48 , and in this study it is isometry, not allometry, that is of interest. Therefore standardising the data through log-transformation would be detrimental in this case. The approach of using principal components analysis to estimate relative body size has been applied in previous examples including birds 49 , bats 50 , rodents 51 , and insects 52 .
In this two-dimensional example, relative size is indicated by the position of points along the trend line, which corresponds to the first axis returned by principal components analysis. Variations in shape are indicated by position on the orthogonal second component. A positive correlation between traits may be driven by factors other than size, therefore this study uses 21 characters, rather than just two.
The dataset was assembled from the literature. Peer-reviewed articles featuring images of fossil specimens were sampled and measured using ImageJ 53 . Up to 21 characters were measured from each specimen, depending on their completeness (Supplementary Data 1). These included cranial, mandibular, humeral and femoral characters, selected to encompass the greatest diversity of body shape possible, without introducing an excessive amount of missing data. All the measurements were collected in centimetres. A non-parametric Spearman-rank correlation test was performed between all variables, and confirmed many positive correlations indicating positive covariance within the data (Supplementary Data 1). This covariance is likely to be driven by body size, because the variation within the data is extremely large. For example, skull width in the smallest and largest taxa differs by an order of magnitude. Therefore these data are suitable for body size estimation using principal components analysis.
The raw data were, necessarily, highly incomplete. The PCA was implemented with iterative imputation using PAST version 3.1 for Mac OS 54 . An advantage of this approach is that it is not limited by variable preservation of any single character. A dataset limited to only complete specimens would be vanishingly small, and unrepresentative of the clade as a whole. Likewise, using a single character as a body size proxy would limit the dataset to those specimens with that character preserved. This may introduce preservation bias, since the probability of preservation may vary between skeletal elements. Using iterative PCA enables relative body size to be estimated from multiple characters. If a given character is missing, its value is inferred from other characters that are preserved. Therefore, the analysis does not require any one character to be complete in every specimen, enabling relative body size to be estimated for a much larger number of taxa than would otherwise be possible. The final dataset included scores for a total of 280 taxa. The distribution of these 280 taxa along principal component 1 is shown on Fig. 6, including the position of some notable examples.
Points are coloured according to relative skull width. This demonstrates the relationship between PC1 and body size, with larger taxa having higher PC1 scores. Note the colour gradient is not fully continuous along the x-axis, indicating taxa with disproportionately wide or narrow skulls. Notable taxa are indicated: (a) is the smallest taxon in the dataset, Knoetsuchkesuchus guimarotae (b) Razandrongobe sakalavae, the largest notosuchian (c) Crocodylus thorbjarrsoni, the largest crown crocodylid (d) Purussaurus brasiliensis, the largest crown alligatorid (e) Machimosaurus, the largest thalattosuchian and (f) Sarcosuchus imperator, the largest pseudosuchian.
PCM studies of evolution require a phylogenetic tree. There is no published phylogenetic hypothesis that encompasses all Pseudosuchia, as well as molecular data from living taxa. Therefore, we estimated a new phylogenetic hypothesis for this study.
Previous studies of trait evolution in fossil taxa have been dominated by informal supertrees 29,55,56,57,58,59,60,61 . While informal supertrees have practical advantages, they have no underlying systematic basis and are therefore subjective. A matrix-based approach was also ruled out, because collecting character data for such a large matrix from the literature and vetting characters for redundancy would have been impractical. In addition, such a large matrix would have introduced a significant fraction of missing data, which could undermine the quality of a finished tree. The phylogenetic hypothesis used in this study is based on a formal supertree analysis (Supplementary Information). Formal supertrees use a systematic approach to assimilating multiple smaller topologies into a single tree. Such methods have been used previously in macroevolutionary analyses of fossil taxa 28,62 . Liberal formal supertree methods enable a well-resolved consensus topology to be estimated from source trees that are incongruent.
Here, the supertree (Fig. S4) was assembled using the matrix representation with parsimony method, an approach that has been validated in methodological comparisons 63 . The supertree was estimated from a sample of 175 source trees published since 2010, each re-analysed from their original source matrices using Bayesian inference and the MK model 64 . Only matrices containing morphological data were used the source trees relating to the crown-group were constrained to a published topology of extant taxa derived from molecular data. Supertrees generated by the analysis were evaluated using stratigraphic congruence, with the best example being retained for use in analyses. The supertree was dated using the equal method the dated supertree contained a total of 579 archosauromorph taxa, including 24 extant species. This tree was then trimmed to match the 280 pseudosuchian taxa included in the body size data. This phylogenetic approach was implemented to eliminate as many sources of error as possible. Every effort was taken to incorporate the most recent and comprehensive data from a diversity of morphological, stratigraphic and molecular sources. The best-performing phylogenies are retained at each step. However, like any phylogenetic comparative analysis, the results of the analyses presented here do depend on a correct phylogenetic topology. Therefore if a significant number of the source trees are incorrect, the resulting supertree may not be an accurate framework for phylogenetic comparative methods. This is a limitation that can only be conclusively overcome through further description of pseudosuchian fossils, and continued development of phylogenetic methods. Full details of the supertree analysis are presented in the Supplementary Information.
Phylogenetic models of body size were fitted using BayesTraits version 2 65 . A random walk model was fitted as a null hypothesis and was compared with other models (Ornstein-Uhlenbeck, Kappa, Lambda, Delta, directional trend, variable rates) using Bayes Factors. All analyses were run for two million iterations, and the first 10,000 iterations were deleted as burn-in. The likelihood profiles of each run were plotted to check for a uniform distribution, indicating convergence. Rate scalars returned by the variable rates model were computed using the BayesTraits online post-processing tool, and these were plotted on the phylogenetic tree to illustrate the relative evolutionary rate on each branch. Branch lengths were set to the scalar values returned by the variable rate model, and coloured according to a gradient from blue to red, denoting low and high rates, respectively. The rate scalars were also plotted as a time-series, calculated as averages per 1-million-year time bin, for which the taxa included in each increment were determined using the dated supertree. Bootstrapping was implemented as insurance against the effects of outliers and potential errors introduced by tip dating.
The phylogenetic models return a phylogenetically adjusted mean and root trait estimation, known as alpha (Table 1). An average alpha value was calculated using the values returned by the phylogenetic models that significantly outperformed Brownian motion. This average was weighted using the log Bayes factor of the respective model. For comparison, a simple arithmetic mean was also calculated from the raw data. These mean values are dimensionless, occupying the scale of the first principal component. For the purposes of illustration, the mean values were transformed back into an estimate of skull width in centimetres.
Trees were dated using first and last appearance dates from the Paleobiology Database (pbdb.org). Mean body size and body size variance were represented as time series using the scores of each taxon on principal component 1 as a body size proxy. This is not a straightforward exercise because of fossil preservation heterogeneity and uncertainty about the temporal ranges of taxa, many of which are known from very few specimens. Therefore, we implement three approaches to reconstructing time series of mean body size and body size disparity:
Empirical fossil ranges. The simplest time-series approach is to bin taxa by the stratigraphic stage in which fossils have been found, a commonly used method 34,35 . This assumes the known stratigraphic range was the true stratigraphic range and does not estimate originations and extinctions before and after the known range.
Phylogenetic adjustment of missing data. Using the same tip-dated formal supertree used for fitting phylogenetic models, we added ghost ranges in million-year increments (difference in first appearances of sister taxa) to the empirical data in method (a). This approach can only reconstruct ghost ranges and does not estimate beyond known fossil ranges of sister taxa. Further, the method assumes that the topology of the phylogenetic tree is correct an incorrect topology could significantly change the durations of fossil ghost ranges.
Addition of ancestral taxa. This method uses stratigraphy and the phylogenetic tree as in method (b), but also estimates traits of hypothetical ancestral taxa, following established methods 66,67 . Ancestral states at each node were estimated using the ACE function in the APE package in R 68 , and their geological age was inferred from the length of the corresponding internal branch of the phylogenetic tree.
A time series may also be biased by outlier taxa, or by inaccuracies in the dating of the phylogenetic tree. To mitigate this, all three versions of each time series were subjected to bootstrapping, in which multiple mean values were calculated from random 50% samples of taxa in each time-bin and repeated over 100 iterations in each case.
The time series of body size, body size variance and evolutionary rate were analysed for correlation with temperature using linear regressions, partitioned into Mesozoic and Cenozoic spans. This temporal split corresponds to the end-Cretaceous mass extinction and an apparent shift in the stability of body sizes. Mesozoic paleotemperatures come from a consensus curve of oxygen isotope data from a range of sources 69 , selected for its comprehensive temporal coverage. Cenozoic paleotemperatures come from a curve estimated from oxygen isotope data derived from benthic foraminifera 70 , which correlates with the less complete terrestrial temperature record 71 and has been used in previous macroevolutionary analyses of body size 72 . This paleotemperature record is limited to a global average. Similarly, body size data is also limited to a global average, since many species are known from too few specimens for their geographic range to be determined. Therefore the data is not sufficient to incorporate geographic variations in temperature and body size.
Statistics and reproducibility
Phylogenetic models were fitted using BayesTraits 65 . Statistical analyses were implemented using the R programming language and the APE library 68 . The dataset assembled as part of this study included 280 taxa. Time-series representations were subjected to a sensitivity analysis using 100 randomly sampled bootstrap replicates 73,74,75 .
Further information on research design is available in the Nature Research Reporting Summary linked to this article.
Did Crocodiles Descend From Dinosaurs?
Crocodiles are built to last. Evolving around 200 million years in the Mesozoic epoch, crocodiles have far outlived the dinosaurs. Scattered across more than 90 countries, 23 species of the Crocodylia order haunt freshwater rivers, streams and marshes. Those 23 crocodilian separate into three families: alligatoridae (alligators and caimans), crocodylidae (crocodiles) and gavialidae (gharials).
Voluble crocodiles exhibit a range of calls signaling anything from courtship to distress [source: Florida Museum of Natural History]. They swim through their habitats chomping on frogs, fish, turtles and other vertebrates. To help them float, many crocodiles will ingest rocks to weigh down their centers of gravity [source: Burton and Burton]. Evolutionary biologists suspect that crocodiles' aquatic nature may have helped them survive the theoretical asteroid impact that led to the extinction of dinosaurs.
But if crocodiles and dinosaurs were living in the same time period, how closely were they related? For starters, dinosaurs and crocodiles are both reptiles. They share cold-blooded circulations that rely on the environment to heat and cool. Physical similarities, such as their rubbery skin and fierce teeth and claws are also apparent. But if you've read How Dinosaurs Work, you know that many scientists agree that birds, not crocodiles, descended from dinosaurs. Yet, we also know that birds and crocodiles are the only extant species that share a common ancestor with dinosaurs.
If we take a closer look at the crocodile's family tree, some surprising animals are hanging out on the various branches. Crocodiles are members of the Reptilia taxonomic class with other creepy crawlers, including snakes and lizards. And according to paleontologists, birds are also lumped in that category. In fact, crocodiles are more closely related to birds than they are to snakes and lizards [source: University of California Museum of Paleontology].
With a little more investigation into this unlikely bond between croc and bird, we can find out where dinosaurs fit into the prehistoric family portrait.
How could a delicate robin or sparrow have any connection to a crocodile? It's all about common ancestors. Taxonomists organize the animal kingdom based on where species came from, or who descended from whom. All of the descendents of one common ancestor are referred to collectively as a monophyletic taxon. Those taxons can then be arranged visually into a cladogram, which illustrates how the various taxons are related to one another.
To understand how this works, let's think about corn. Imagine listing out every conceivable corn-based product: cereal, syrup, plastic, biofuels and so forth. The lists would contain wildly different items, but you could link them all with that single commodity. When it comes to crocodiles, birds and dinosaurs, a prehistoric creature called the archosaur serves our umbrella.
Archosaurs, also known as the ruling reptiles, originated about 250 million years ago during the Carboniferous period [source: University of California Museum of Paleontology]. The primary characteristic that unifies archosaurs is two openings on the side of their skulls. These tetrapods, or four-legged vertebrates, splinter into two groups: bird crocodiles (Ornithosuchia) and false crocodiles (Pseudosuchia). In a bizarre twist of taxonomical fate, crocodiles don't belong with the false crocodiles.
So what does all of this classification have to do with whether crocodiles came from dinosaurs? Alongside birds and other flying reptiles, dinosaurs are lumped into the Ornithosuchia branch. Though dinosaurs and crocodiles have the common ancestor with the archosaur, they evolved separately.
Today, habitat destruction threatens the livelihood of some crocodile species. Though they could weather an asteroid blast that altered the physical world, humans might be edging them out. The Chinese alligator and Philippine crocodile are the two most threatened crocodilian species [source: NOVA]. Both are on the International Union for the Conservation of Nature's red list, denoting their highly endangered status. Captive breeding may help the populations rebound before they die out completely.
If you've ever gotten pulled over by a police officer and turned on the waterworks, you're guilty of crying crocodile, or insincere, tears. The idiom isn't based on a bunch of weepy reptiles cruising through the bayous. Crocodiles do produce tears via the lachrymal gland, like humans, but they serve no emotional purpose [source: Florida Museum of Natural History]. Tears lubricate and clean dry eyes.