Early Jurassic North American Sauropodomorphs were Migrants

The sauropodomorph dinosaur called Sarahsaurus was a migrant into North America just like the other North American sauropodomorphs that have been described to date.  That is the conclusion made by researchers from the University of Texas Austin, in a scientific paper published this week.  Recently, Everything Dinosaur has covered a number of technical papers that have featured the Suborder Sauropodomorpha (the sauropods and their direct ancestors).  The United States might be famous for dinosaurs such as Brontosaurus, Camarasaurus and Diplodocus, but surprisingly not much is known about the ancestors of these iconic, long-necked dinosaurs.  Writing in the open access journal PLOS One, the researchers from the University’s Jackson School of Geosciences, conclude that the handful of sauropodomorphs known from the Lower Jurassic of North America are not that closely related and they represent successive immigration waves into that part of the super-continent Pangaea.

Sarahsaurus and Other North American Early Jurassic Sauropodomorphs Do Not Form a Unique Clade

A life reconstruction of the North American sauropodomorph Sarahsaurus.

Picture Credit: Brian Engh

CT Scans and Phylogenetic Analyses

The researchers conducted the first detailed analysis of the fossils ascribed to the genus Sarahsaurus (Sarahsaurus aurifontanalis).  This dinosaur had been named back in 2010, from fossil material excavated from the Lower Jurassic Kayenta Formation exposed in north-eastern Arizona.  In total, three specimens, including the holotype were studied and subjected to computed tomographic imaging.  With more anatomical data, the scientists then conducted a series of phylogenetic assessments to see where within the Sauropodomorpha Sarahsaurus should be nested and importantly, how the other sauropodomorphs from North America such as Anchisaurus (A. polyzelus) and Seitaad (S. ruessi) were related to Sarahsaurus.

The Main Fossil Block Associated with Sarahsaurus and a Line Drawing Showing a Layout of the Fossil Material

The main block containing much of the holotype specimen of Sarahsaurus aurifontanalis.

Picture Credit: PLOS One

Sarahsaurus aurifontanalis

All of the Sarahsaurus specimens referred to in this study came from siltstone deposits.  Manual preparation of the fossils was extremely laborious and time consuming.  Many of the bones were encrusted with an extremely hard purple-black oxide coating, hence the use of high-resolution X-ray CT scans to provide more information about the finer details preserved on the fossil material.

In addition, conducting the phylogenetic analysis was made even more problematic than usual as the material used to establish unique characteristics of Sarahsaurus which could then be used to compare with other sauropodomorphs, provided numerous obstacles for the scientists to overcome.  Firstly, a skull used in this study probably came from a much younger individual than the other Sarahsaurus specimens analysed.  Furthermore, not all the specimens shared the same bones so making direct comparisons to establish a unique set of features for Sarahsaurus was challenging.  These factors coupled with some mixing and redistribution of the holotype material in the sediment and the crushed nature of many of the fossil bones made the phylogenetic assessment very tricky, but the researchers were able to conclude that Sarahsaurus aurifontanalis is very probably a member of the Massospondylidae family, which means that this dinosaur is not closely related to the other North American Sauropodomorpha and is more closely related to dinosaurs known primarily from the southern hemisphere (Gondwana).

CT Scans of a Skull Specimen Provisionally Assigned to Sarahsaurus

Skull (MCZ 8893) provisionally referred to Sarahsaurus aurifontanalis, reconstructed from CT data.  Life reconstruction (H) by Brian Engh.

Picture Credit: PLOS One/Brian Engh

Waves of Dinosaur Migration into North America Following the End Triassic Extinction Event

If the three known North American sauropodomorphs are not that closely related and the likes of Sarahsaurus is classified as a member of the Massospondylidae, then this suggests that rather than evolving in North America, these dinosaurs arrived on that part of the super-continent of Pangaea as a result of a number of migrations that took place during the Early Jurassic.  This links with other research that suggests that although Theropods were present in North America during the Triassic transition to the Jurassic, other types of dinosaurs such as the Sauropoda and the Ornithischians populated this part of the world later.

The dinosaurs may not have been the super evolved terrestrial animals that simply outcompeted all the other Tetrapods in the world driving the majority to extinction.  Instead, the Dinosauria may have been opportunists, migrating into areas after the former occupants of key niches in the ecosystem had already died out.

Research Suggests that there were Several Migration Waves into North America During the Early Jurassic

Relative ages of North American sauropodomorphs.

Picture Credit: Everything Dinosaur

To read Everything Dinosaur’s article on the discovery of Seitaad ruessi: Dinosaur Buried Alive is a New Species from Utah

The scientific paper: “Anatomy and Systematics of the Sauropodomorph Sarahsaurus aurifontanalis from the Early Jurassic Kayenta Formation” by Adam D. Marsh and Timothy B. Rowe published in the open access journal PLOS One.

“Powerful Terror Ruler” – Dynamoterror dynastes

A new species of North American Tyrannosaur has been scientifically named.  The newly described “tyrant lizard” joins a plethora of tyrannosaurids known from the Late Cretaceous of Laramidia, but Dynamoterror dynastes stands out from the majority of these fearsome Theropods for some very important reasons.  Firstly, it is quite geologically old for a Late Cretaceous large-bodied Tyrannosaur, its discovery has implications for our understanding of Tyrannosaur evolution.  In addition, its the frontal bones that help make this dinosaur stand out and besides, its scientific name, which means “powerful terror ruler”, is a nod in the direction of the most famous dinosaur of all – Tyrannosaurus rex.

A Life Reconstruction of Dynamoterror dynastes Attacking the Recently Described Invictarx zephyri

Dynamoterror ambushes the armoured dinosaur Invictarx zephyri.

Picture Credit: Brian Engh

The Geological Age – Early Campanian

The fossil bones, representing a single, individual animal were collected in 2012.  They herald from San Juan County, New Mexico, specifically the upper part of the Allison Member of the Menefee Formation.  Although fragmentary, the fossil material consists of asociated bones including left and right frontals (bones from the top of the skull over the eye socket), a right metacarpal (bone from the hand), four broken pieces from the backbone, pieces of rib, a portion of the right ilium and some toe bones, plus several unidenfiable slithers of bone.  It might not sound like much, but this is the first associated tyrannosaurid skeleton reported from the Menefee Formation.  Isolated teeth had been found in this locality before suggesting the presence of tyrannosaurids, but Dynamoterror dynastes is the first to be named and described.  It was probably the dominant predator in the lush, tropical, coastal swamps that covered this part of the southern United States some 80 million years ago.

During the Late Cretaceous, North America was essentially split into two by a wide seaway, the Western Interior Seaway.  To the east lay Appalachia and Tyrannosaurs are known from here, but not many, only two genera have been named to date – Appalachiosaurus montgomeriensis and Dryptosaurus aquilunguis and both of these are only known from a single, partial, associated skeleton.  In the Upper Cretaceous strata to the west that formed the landmass called Laramidia, lots of Tyrannosaurs have been named and described.  However, the tyrannosaurid record for Laramidia is restricted to a period from about 77 million years ago to the K-Pg extinction event some 66 million years ago.  Dynamoterror comes from rocks which are around 3 million years older.  It provides the first fossil record of a Laramidian tyrannosaurid from the Early Campanian of 80 million years ago and, as a result, will help palaeontologists to better understand tyrannosaurid evolution.

The Cool Thing About Frontals

Less than one percent of the skeleton may have been found (field teams were despatched in 2013 and again this year to try and find more remains but without luck), but when it comes to describing a new genus, it is often quality that triumphs over quantity.  The frontal bones, their shape, the groves that they possess and other features including how they knit together with other skull bones, can prove extremely helpful when it comes to identifying a new dinosaur species.  The researchers which included Dr Andrew McDonald (Curator, of the Western Science Centre, California), identified some unique characteristics in the frontal bones, hence the establishment of a new genus.

Photographs and Computer-generated Three-dimensional Models of the Left and Right Frontals of D. dynastes

Photographs and three-dimensional, computer-generated models of the right frontal (A, B) and the left frontal (C, D) of Dynamoterror dynastes (rostal view – viewed from the front of the brain).  Scale bar = 5  centimetres.

Picture Credit: PeerJ/Western Science Centre

A Large Bodied Tyrannnosaur

The researchers cannot be certain whether their fossil discovery represents a fully grown animal or a sub-adult.  However, when the frontal bones of D. dynastes were compared to those of Tyrannosaurus rex, the scientists concluded that Dynamoterror was at least nine metres long.  The armoured dinosaur that features in the illustration (above), Invictarx, was also named and described by Dr McDonald, along with Mr Doug Wolfe (Zuni Dinosaur Institute for Geosciences) who worked together on this Tyrannosaur.  It is likely that more dinosaurs will be described based on fossil discoveries from within the Menefee Formation.  Alton C. Dooley Jr also collaborated in the study of Dynamoterror.

To read about the discovery of the nodosaurid Invictarx: A New Nodosaur from New Mexico

Size Comparison of Selected Late Cretaceous Tyrannosaurs

Size comparison between selected Late Cretaceous Tyrannosaurs.

Picture Credit: Everything Dinosaur

What’s in a Name?

This new taxon provides further, significant insight into the morphology and diversity of tyrannosaurids from the Early Campanian of Laramidia and it’s name is pretty cool too.  The genus name is taken from the Greek word “dynamis” which means “power” and the Latin word “terror”.  The trivial or specific name, is from the Latin word “dynastes” meaning “ruler”.  Hence, the binomial scientific name Dynamoterror dynastes translates to “powerful terrror ruler”, however, the scientific paper also states that this epithet honours the name “Dynamosaurus imperiosus“, from Henry Fairfield Osborn, the American palaeontologist who referred to fossil material later assigned to Tyrannosaurus rex as Dynamosaurus imperiosus in scientific papers published in the early years of the 20th Century.

The Reconstructed Frontal Complex of Dynamoterror dynastes

The reconstructed frontals of D. dynastes.

Picture Credit: PeerJ/Western Science Centre

In the picture above, the left and right frontals have been articulated together to show how they would sit at the top of the skull, in (A) rostral; (B) caudal; (C) right lateral; (D) dorsal; and (E) ventral views.   The illustration (F), shows a view of the reconstructed skull in dorsal view.  Individual bone elements of the skull are colour-coded to show how the top of the skull knitted together: frontals (grey); fused nasals (violet); prefrontals (yellow); lacrimals (red); postorbitals (blue); and parietal (green).  The scale bars represent 5 centimetres and the missing skull bones have been based on the related tyrannosaurid Teratophoneus curriei, a geologically younger Tyrannosaur from the Upper Campanian of southern Utah (Kaiparowits Formation).  T. curriei roamed Laramidia around 76 million years ago, some 4 million years after Dynamoterror dynastes.

The scientific paper: “A New Tyrannosaurid (Dinosauria: Theropoda) from the Upper Cretaceous Menefee Formation of New Mexico” by Andrew T. McDonald, Douglas G. Wolfe and Alton C. Dooley published in PeerJ