Dacentrurus = Miragaia

[Sim]

i've been under the impression for a long time now (a couple of years) that they were the same.  I didn't realize it hadn't been confirmed.  I too will miss the Miragaia name.  Flows off the tongue better.

The synonomy had been proposed before based on less good remains, but it was challenged.

oh, and why has the impression been all along that Dacentrurus was much larger than Miragaia if they are the same animal?  Is it possible that different populations in different geographical locations were different in size, the way South African lions and Painted Dogs are larger than their East African counterparts?

The size difference of Dacentrurus and Miragaia was based off of different specimens for each, while they were considered separate.

[andrewsaurus rex]

Right.  So does that mean the Portugal Dacentrurus was smaller than the Morrison Dacentrurus?  The "Miragaia" known specimens are considered to be fully grown but are significantly smaller than the Dacentrurus size of 8 meters (ie second biggest stegosaur).  As i said, different populations achieving different sizes?

[Sim]

There is no Morrison Dacentrurus currently.  Alcovasaurus was considered a Morrison Miragaia while the latter was considered valid, but now that Miragaia has been made a junior syonym of Dacentrurus, Alcovasaurus has gone back to being considered having its own genus.  The larger size for Dacentrurus was based on a British specimen.  Some animals keep growing for as long as they live, even after reaching adulthood.  I think it could be the case for Dacentrurus too, meaning the Portugal Dacentrurus could have reached the size of the British specimen.

[Flaffy]

Alternatively the British and Portugal specimens may very well be different species within the Dacentrurus genus. With how scrappy the British specimen is I certainly wouldn't be surprised.

[andrewsaurus rex]

If an animal grows all its life how can it be determined it's an adult from fossil remains?  The bones would exhibit signs of growing, just like a sub adult.

[Turkeysaurus]

"Miragaia" would be within normal size rage imo.

[Sim]

If an animal grows all its life how can it be determined it's an adult from fossil remains?  The bones would exhibit signs of growing, just like a sub adult.

The degree of bone fusion I understand is used to determine whether a fossil specimen is adult, as well as cutting bone from a specimen and examining its structure.

[Torvosaurus]

If an animal grows all its life how can it be determined it's an adult from fossil remains?  The bones would exhibit signs of growing, just like a sub adult.

Not exactly. Crocodilians grow roughly a foot a year for 4 years or so, and then slowly reduce the rate they grow until they reach full maturity, at which point they are growing only a few centimeters a year. A 15 foot croc is only going to reach 16 feet in 10 years or more, if at all. Crocs tend to get bulkier instead of longer. So they do grow longer, just not at a significant rate.

I'm sure dinosaur growth rates are along similar lines, though I've never looked into it.

Torvo

[Newt]

This is currently an active area of research, and there is no clear-cut answer. The fusion of the neural arches to the centra of the vertebrae is often used as a proxy for maturity in extinct archosaurs. In crocodylians and some extinct pseudosuchians, this fusion starts at the rear of the spine and proceeds towards the head as the animals ages. It has been assumed that this is the case for dinosaurs as well, but recently some researchers have questioned this. 

Very old individuals can often be identified by the obliteration of sutures between bones of the skull, and by ossification (transformation into bone) of tendons and ligaments.

On a more philosophical note, it's worthwhile to ask what "adult" means in an ever-growing animal. Even in humans it's a hazy concept - reproductive maturity (puberty), somatic maturity (cessation of long-bone growth, usually a few years after puberty), legal maturity (defined by statute), and social maturity (reached very young by some, never achieved by others) are all involved. 

The only one of these metrics that applies to dinosaurs is the first, but reproductive maturity in ever-growing animals, probably including most dinosaurs, is often reached when the animal is still rapidly growing and is a fraction of its potential maximum size. So a dinosaur judged to be a "subadult" or even a "juvenile" based on osteology might have already produced offspring. So if we are discounting reproductive maturity, and there is no clear-cut point of somatic maturity as there is in mammals, then "adult" is an arbitrary concept with probably not too much biological significance.

Very likely the age and size demographics of populations of many dinosaur species, or at least of long-lived dinosaurs such as sauropods, would have looked similar to those of many larger fish today, with vast numbers of juveniles, a smaller number of reproductively mature individuals with a 2- or 3-fold mass difference between small young "subadults" and older larger "adults", and a few "grandpa" individuals much older and larger than the typical adults.

Shorter-lived dinosaurs such as smaller ornithischians may have had more compressed demographics, but still no obvious distinction between reproductively mature but still rapidly-growing "subadults" and slow-growing "adults".

[andrewsaurus rex]

that's very interesting, Newt.  Animals that grow all their lives don't really have the more clear cut final phase of life that most mammals do (another example of the dangers of assigning mammalian traits to dinosaur behaviour).