Eotrachodon is a genus of herbivorous ornithopod dinosaurs from the Upper Cretaceous family of North America . This is one of the oldest and most primitive hadrosaurids, as well as the only known hadrosaurids from the Appalachian Mountains (modern east of North America) with a preserved skull.
| † Eotrachodon |
|---|
|
|
| {{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{| 1}} | 1}} | 1}} | 1}} | 1}} | 1}} | 1}} | 1}} | 1}} | 1}} | 1}} | 1}} | 1}} | 1}} | 1 }} | 1}} | 1}} | 1}} | 1}} | 1}} | 4}} : | Eukaryotes |
| No rank : | Bilateral symmetrical |
| Infraclass : | Archosauromorphs |
| No rank : | Archosauriformes |
| Infrastructure : | † Ornithopods |
| Parvotryad : | † Iguanodonts |
| Superfamily : | † Hadrosavroids |
|
|
Eotrachodon
Prieto-Marquez et al. , 2016 |
|
† Eotrachodon orientalis
Prieto-Marquez et al. , 2016 |
|
The type and only species Eotrachodon orientalis was named and described by Albert Prieto-Marquez and his colleagues in 2016. The generic name is derived from the Greek words " eos ", which means "beginning", "source" and " Trachodon ", meaning "rough tooth" (a tribute to the first described genus hadrosaurids, which was then considered nomen dubium for a long time). The species name orientalis means “eastern” at the place of hadrosaurid discovery - southeast of North America [1] .
The holotype MSC 7949 was found in the Mooreville Chalk formation layer, which dates back to the Late Santonian , about 83 million years ago [2] , Alabama , USA . The holotype includes a well-preserved, almost full, partially articulated skull and a fragmentary postcranial skeleton. The skull includes both the premaxillary and jaw bones, both the zygomatic, the partial right nasal bone, the left lacrimal, the left prefrontal, both the frontal, the orbital and scaly bones, the left square, the partial cerebral box, the predental bone, both the dental, the arched and the angular bones, hyoid bone and various jaw and dental teeth. The postcranial skeleton is represented by a second cervical and several cervical, thoracic, sacral and caudal vertebrae, a partial left pubic bone, a partial left sciatic, a partial right tibia, and two phalanges of the forelimb. A transverse histological section of the tibia, as well as the absence of complete fusion of the neurocranium and open neurocentral sutures in the cervical and anterior spinal vertebrae indicate that the individual had not reached skeletal maturity at the time of death.
Prieto-Marquez and colleagues do not give a detailed description of the animal in their work. Instead, they focus on osteological features that make it possible to isolate a new taxon, significantly deepen knowledge about the anatomy and distribution of hadrosaurs of Appalachia, draw conclusions about the ancestral morphology and time of appearance of advanced circumnar structures of hadrosaurs, and also establish the geographical place of origin of hadrosaurids [1] .
Eotrachodon had a length of about 7.6 m [2] .
Eotrachodon is characterized by the following autapomorphies : trilateral circummnarial depression is longitudinally divided into the dorsal and ventral fossae, and the latter is divided into caudaventral and slightly incised rostroventral fossa; the dorsal fossa of the circmnarial impression above the bony nostrils extends further, more caudally than the caudoventral fossa; the caudodorsal part of the circumnarial impression above the bone nostrils greatly deepens the lateral surface of the nasal bone in the rostral direction, but gradually smoothes caudally; the premaxillary lateral process is abruptly curved ventrally, forming an angle of 165 degrees with a long axis of circummnarial depression; the sub-triangular articular surface of the jawbone for the zygomatic is oriented more laterally than the dorsal and dorsal zygomatic tubercle clearly protrudes caudally; strongly bent down sagittal crest of the parietal bone is significantly elevated above the temporal arch. In addition, Eotrachodon differs from other hadrosaurids in the following unique combinations of features: thin, pronounced oral margin of the maxilla (convergence with a fluosaurus , prosaurolophus and saurorolophus ); the jawbone combines a wide rostral section; the steeply inclined rostrodarsal margin of the rostroventral apex forms an angle of 45 degrees with the rostral alveolar margin; the dorsal process is located caudally in the middle of the length of the jawbone; large rostrodorsal margin with a wide lateral articular surface with lacrimal bone; short (25% of the length of the jawbone) and a strongly inclined ectopterygoid protrusion [1] .
Based on a cladistic analysis using 273 characters (189 cranial and 84 postcranial) for 61 taxa (22 hadrosaurin (saurolofin according to Prieto-Marquez), 21 lamboseosaurins and 16 taxa external to hadrosaurids), 4 of the most economical trees were obtained. The strict consensus of these trees places Eotrachodon inside hadrosaurids, like a sister taxon to saurolophids (saurolophins (according to Prieto-Marquez) + lambeosaurins) [1] :
| Hadrosauridae |
| | Saaurolophidae |
| Lambeosaurinae |
|
| Saaurolophinae |
|
|
|
| Eotrachodon |
|
|
|
| Hadrosaurus |
|
|
|
Asia has traditionally been regarded as the most probable place of origin of hadrosaurids, since morphologically plesiomorphic taxa preceding hadrosaurid radiation were first discovered in this region. Nevertheless, the subsequent discoveries of new taxa external to hadrosaurids on other continents gave green light to alternative hypotheses for the origin of hadrosaurids, which included Europe and North and South America as ancestral regions.
The recognition of Eotrachodon as one of the oldest and most basal hadrosaurids, the forerunner of the main diversification of the clade, leads to a review of the ancestral areas of hadrosaurids and their main underlining . Biogeographic analysis by Prieto-Marquez and colleagues unambiguously deduces the Appalachian Mountains as the most likely ancestral region of hadrosaurids. The last common ancestor of saurolophid (the main radiation of hadrosaurids) was common in Laramidia and Asia. This reconstruction involves the spread of early hadrosaurids from the Appalachian Mountains to Laramidia and then to Asia. There was a land connection between Laramidia and the Appalachians about 100 million years ago (early Cenomanian ; it should be noted that, with the exception of Eotrachodon, all the known pre-Campanian hadrosaroids of Laramidia are outside the family of hadrosaurids; these are Eolambia caroljonesa , Protohadros byrdi and Huehuecanauhis lus . Perhaps this was the settlement route of the early hadrosaurids, which predicts the presence of these animals in the Appalachian Mountains in the Cenomanian. This allows for the existence of a ghost line of 16 million years before the end of the Cenomanian - the time of the appearance of Eotrachodon . The resettlement from Laramidia to Asia could be carried out through the Bering Isthmus . This probably happened at the late end of Santon, since the oldest saurolophids are dated to the Santonian tier.
Finally, saurolophins and lambaeosaurins arose by way of vicarians in Laramidia (no later than the early Campanian, since the oldest known saurolophins Gryposaurus latidens and Acristavus gagslarsoni date from this time) and in Asia (no later than the end of Santon, since the oldest known lambesaurins Aurusiarisa jarisaris Irisasara sambarasarisambarasaurisambersaurusosaurus tuurusaurusosaurusurus dated by this time) respectively. These conclusions are supported by previous hypotheses, which position North America as the ancestral region for saurofolofin, and Asia for lambeosaurins [1] .