MiniAssignment4_Phylogenetic Trees with Sauropodomorphs (1)

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Apr 3, 2024

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Mini Assignment 4: Phylogenetic Trees with Sauropodomorphs Phylogenetics is a fascinating area of study that traces the evolutionary history and relationships among individuals or groups of organisms. Imagine it like a detective story, where scientists piece together clues from fossils, genetics, and other evidence to unravel the intricate web of life's history. This history is often depicted in a diagram known as a 'phylogeny', a kind of family tree that shows the relationships between different species or groups. For this mini-assignment, we'll be delving into the world of the largest land animals that ever lived: the sauropodomorphs, a group of long-necked dinosaurs that included famous members like Brachiosaurus and Diplodocus. You'll be given an existing phylogenetic tree, a sort of dinosaur family tree, which was created based on the current understanding of how these giants are related to each other. Using character information, you will place additional taxa on the tree and map how size evolved across the sauropodomorphs. This exercise will not only help you understand the fundamentals of phylogenetics but also expose you to the dynamic and ever-evolving nature of scientific knowledge. Below is the phylogenetic tree we will use in this assignment. This diagram corresponds to the current understanding at large about relationships among sauropodomorphs. Each branch leading to a branching node has been labeled with a letter. See the next page for a table that gives the name of the taxonomic group this branch corresponds to, and the derived characters (synapomorphies) that denote a typical member of that group. Figure 1. Phylogeny showing relationships among 12 sauropodomorphs.

Table 1. Character Changes and Branch Details Node Label (Clade) Corresponding Name of Clade Typical Derived Characters of This Clade A Gravisauria Quadrupedal limb proportions (forelimbs as long or longer than hind limbs) B Eusauropoda Metacarpal bones in forelimb become U-shaped cylindrical pillar, wrist becomes locked in place. Most lose most front toe bones except for a single large ‘ungual’ claw. C Neosauropoda Laterally flared ilia (hip bones) to expand gut space. Teeth shift to front of mouth and now occlude (for browsing rather than chewing). Loss of external mandicular fenestra in jawbone. D Diplodocoidea Loss of suture between pre-maxilla and maxilla in skull. Most descendants have flat snout end. E Flagellicaudata A new depression in the skull in front of the eye sockets (Preantorbital fossa). Most members of this group have long whip-like tails, but not all. F Macronaria Very large naris (openings for nostrils) on the top of their skull, larger than their eye orbitals. G Titanosauriformes The large air cavities in presacral neural arch become more finely divided, about 1-cm in scale. H Somphospondlyii Presacral neural arch pneumatization is very spongy and finely divided, less than 1-cm in scale. I Titanosauria Loss of all front toes, extra-wide flared hips, a gracile (skinny) humerus, and many titanosaurs are found with remains of osteoderms, small bony lumps that formed in their skin. The tail becomes short and connections between caudal (tail) vertebrae become rounded (‘procoelus’). J Lithostrota ? Extra wide cervical vertebrae. Most titanosaurs with osteoderms are in this group, but not all.

1) Using the information shown on the labeled phylogeny, and tables 1, add Apatosaurus to the tree on the next page, which is identical to the phylogeny on the first page, but does not have labeled branches. Apatosaurus is a ‘sister taxon’ of one of the sauropods already on the tree, so you can draw it in as a branch attached to the end-branch leading to that taxon. (A clade composed of two taxa are referred to as ‘sister taxa’ in phylogenetics.) Key characters of Apatasaurus, a late Jurassic sauropod:  long-tailed  long snout with a flat end  preantorbital fossa present  unpaired neural spines Which is the sister taxon? 2) Explain what characters (morphological traits) helped you place Apatosaurus. What characters does Apatosaurus share with its closest relative (its sister taxon)? 3) Next, add Argentinosaurus to the tree on the next page. Argentinasaurus is believed to be the largest sauropod yet found, but is known only from fragmentary remains from the Cretaceous of Argentina. Like Apatosaurus, Argentinosaurus is a sister taxon to another sauropod taxon used a ‘tip taxon’ for this tree. Key characters of Argentinasaurus:  Cervical vertebrae show tiny air cavities (less than 1 cm in size)  Long neural spines on posterior vertebrae.  A ‘skinny’ humerus  Fragments of a very wide pelvic girdle Which is the sister taxon you chose? Was there another sister taxon you considered? Which? 4) Explain what characters or morphological traits you used to place Argentinosaurus. What character information led you to place it where you did? What uncertainties did you have in your placement? The Apatasaurus is the sister taxon of the Flagellicaudata clade which has sauropods such as the Diplodocus and Amargasaurus. There were several characteristics of the Apatosaurus that helped me place it with the Flagellicaudata, such as; its long tail, and preantorbital fossa. I chose Titanosauria for the Argentinosaurus. Yes, I considered Somphospondlyii. The long neural spines and 'skinny humerus' are what led me to believe that it was Titanosauria rather than Somphospondlyii. This was mainly due to the Argentinosaurus having more similarities with the Patagotitan than other dinosaurs. I had considered Somphospondlyii as well as Titanosauria due to both their sauropods having the largest mass, however overall, I went with Titanosauria.

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