#stellasaurus

Happy Star Wars Day!

Stellasaurus ancellae

posting more art i never posted, Behold some of the weirdest fanart i ever done,

Got bpjd on my mind and that extends to ocs. Been wanting to make a proper team but I’ve had no ideas until now, so here’s my second brave oc(s) Caine and Stellar.

Styracosaurus, anthon500 (deviantart)

Speciation, Cladogenesis and Anagenesis

Speciation is a term used by scientists to describe the process by which a population of organisms give rise to a new species. The idea of a species is wholly man-made, and its true definition varies, but for the purpose of this article:

A species is the largest grouping of individuals which can produce fertile offspring from a mating pair. A species can also be defined as having a specific karyotype (a unique arrangement of DNA chromosomes), or in the case where we don't have genetic data, by having enough unique changes to be significantly different from others in their genetic family.

Cladogenesis in Bears, Source

When we discuss speciation, usually it is Cladogenesis that comes to mind. Cladogenesis occurs when a portion of the population splits off and develops adaptations separately. Think for example, Brown Bears and Polar Bears, which we suspect originated from the same ancestor species.

Anagenesis is a different process, where the separation of species is not a difference in space and/or compatibility, but a difference in time. Think for example Homo sapiens and it's ancestors. I could probably trace an uninterrupted line of ancestors back 10s of millions of years ago, but if you dug up that ancestor's bones, palaeontologists wouldn't call it Homo sapiens.

Anagenesis in Hominids, Source

A population separated by nothing but time alone would still be referred to as different species, because they would have different physical traits and genetic make-up.

Both cladogenesis and anagenesis occurs in populations over time. One group may develop without any splintering for millions of years, and then a group splits off later to become a separate species.

Cladogenesis and Anagenesis, Source

Anagenesis in Ceratopsians

So with that explanation out of the way, let's get to the part that's actually interesting: dinosaurs. Like all organisms, speciation in dinosaurs would've occurred by populations splitting up and specialising, or dying out. But recently, the idea of speciation by anagenesis has been proposed in a certain group of dinosaurs.

Ceratopsians are a group mostly identifiable by their large imposing neck frills and face horns. New species are proposed when they differ enough in the shape and length of their eyebrow horns and their nose horns, the height and width of their neck frills, and the arrangements of parietal horns on and around their frill. There's lots of room for variety in ceratopsian face arrangements.

Styracosaurus, Victor Sales (Deviantart)

Styracosaurus is a fairly mainstream dinosaur, but what's probably not as well known are its close relatives. Ceratopsians such as Stellasaurus and Einiosaurus have been found in fossil beds nearby Styracosaurus. And although there may have been some niche partitioning or sexual selection allowing these dinosaurs to coexist in the same place, new research suggests they were the same group separated by time.

Top: Stellasaurus, HodariNundu (deviantart)

Bottom: Einiosaurus, Calisius (deviantart)

Sexual pressures can drive animals to rapidly develop extreme physical traits for no other reason other than to look attractive. Modern dinosaur species (birds) can have a variety of physically exaggerated traits that serve no purpose beyond attracting mates. In Cockatoos, it's erect feather head crests. In Peacocks, it's massive colourful tail plumes. So perhaps in Ceratopsians, it was face and frill ornamentation.

When Stellasaurus was first described as a species separate from Styracosaurus, some Palaeontologists decided that it represented a transitional species. The relatively straight nasal horn of Styracosaurus had developed into a thicker, more curved structure later in the lineage. The neck frill became sturdier and more robust, the frill horns curving in strange directions. The supposed endpoint in the lineage is Pachyrhinosaurus, who's nasal horn is now a solid plate of bone, and the neck frill is decorated with a variety of horns pointing in every which-way. It may be a less pointy dinosaur, but it was very heavily decorated. And sex sells.

Top: Styracosaurus Anagenesis, Christopher DiPiazza

Bottom: Pachyrhinosaurus, Raph Lomotan (deviantart)

The Centrosaurine Styracosaurus lineage is not the only subject of Ceratopsian Anagenesis. The Chasmosaurine line has also been suspected of transitional forms, with the recent discovery of Navajoceratops. But in that lineage, speciation pushed towards longer brow horns and taller neck frills in the younger species.

New discoveries Navajoceratops and Terminocavus, Ville Sinkkonen

It is important to note at the time of writing, these theories are not accepted by all of the Palaeontological community. And that also my explanation is not as detailed or accurate as someone with all the right background knowledge could give. So if you are intrigued as to how this theory could be better explained, here's some further reading:

Chris DiPiazza (2020) Stellasaurus: Beast of the Week

John Wilson et al (2020) A new, transitional centrosaurine ceratopsid from the Upper Cretaceous Two Medicine Formation of Montana and the evolution of the ' Styracosaurus-line' dinosaurs

Stephen Belletini (2020) Was "Rubeosaurus" Styracosaurus After All? YDAW Synapisode #6 (Youtube)

Stellasaurus

Stellasaurus (що означає “зіркова ящірка”; це як посилання на форму орнаментації голови, так і данина пісні “Starman” Девіда Боуї) – рід центрозавринових цератопсових динозаврів, що мешкав у штаті Монтана під час пізньої крейди. Типовий і єдиний вид – Stellasaurus ancellae. Його рештки були знайдені в формації Two Medicine, що відноситься до пізнього кампану. В цій…

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