MiniAssignment4_Phylogenetic Trees with Sauropodomorphs

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Texas A&M University *

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Dec 6, 2023

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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 Diplocoidea 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. In addition, on the next page is a table of ecological and size information about each of these sauropods, including tooth shape.

Table 2. Taxon Information Regarding Size, Dentition, Ecology, Preservation. Genus Estimated Mass Neck Length Teeth Shape Notes Plateosaurus 600–4000 kg ~2 m Leaf-Shaped Vulcanodon 3000–4000 kg 2 – 3 m Unknown Cetiosaurus 9000–18000 kg 5 – 7 m Spoon-like No skull known, but some teeth. Nigersaurus 3000–5000 kg 2.5–3 m Pencil-like No side teeth, front teeth are arranged into teeth battery where they were constantly replaced. Regained mandibular fenestra. Amargasaurus 2600–5000 kg 2–2.5 m Pencil-like Has long, paired neural spines. Diplodocus 10000–15000 kg 6–7.5 m Pencil-like Had a single row of long neural spines. Camarasaurus 15000–23000 kg 5 – 6 m Spoon-like Brachiosaurus 28000–62000 kg 9–12 m Chisel-like Only known from very fragmentary finds – close relative Giraffatitan is more complete and used in its place. Sauroposeidon 40000–60000 kg 11–12 m Unknown Thought to look very similar outwardly to brachiosaurs. Patagotitan 55000–77000 kg 8–11 m Unknown Had long posterior neural spines Alamosaurus 50000–73000 kg 7 – 9 m Peg-like This sauropod is abundantly known from the end of the Cretaceous in North America, but no skull has been found yet. Saltasaurus 6800–7800 kg 3 – 4 m Peg-like Apatosaurus 16000 – 22000 kg 7–8.5 m Chisel-like Argentinosaurus 70000–100000 kg 7–10 m Unknown Very fragmentary remains. Mamenchisaurus 13000–15000 kg 9–11 m Spoon-Like Description of Teeth Shapes Chisel-like: Spatulate teeth with a flat, rounded end, like a chisel. Leaf-shaped: Broad, spatulate teeth with multiple ridges or points, like a pointed leaf. Peg-like: Short and cylindrical teeth. Pencil-like: Long and cylindrical teeth. Spoon-like: Spatulate teeth with a rounded bowl-like depression. Notice that these definitions aren’t very precise. In particular, the line between peg-like and pencil-like is vague. Keep in mind which shapes are probably easier to ‘evolve’ from other sauropod teeth shapes.

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1) Using the information shown on the labeled phylogeny, and tables 1 and 2, 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  peg-like teeth  preantorbital fossa present  unpaired neural spines Which is the sister taxon? Alamosaurus 2) Explain what characters (morphological traits) helped you place Apatosaurus. What characters does Apatosaurus share with its closest relative (its sister taxon)? - Both of them have peg like teeth. 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)  a skinny humerus  long neural spines on posterior vertebrae. Which is the sister taxon you chose? Patagotitan Was there another sister taxon you considered? Which? Not really as there wasn’t many others that had similar traits. 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? - I chose patagotitan as they both had long posterior spines and it might have been close with Sauroposeidon because it’s the one with the longest neck but that wasn’t enough for me to place them together.

5) Add Mamenchisaurus to the above diagram of a phylogenetic tree. Mamenchisaurus was a middle/late Jurassic sauropod known only from China. This sauropod genus could be connected anywhere on the tree, but here’s a clue: the best scientific guess for the placement of Mamenchisaurus is in a polytomy with two sisters. One sister is a clade (a branch with multiple descendants), and a single taxon as its other sister. Key characters of Mamenchisaurus:  18 extremely long cervical vertebrae, giving it a disproportionately long neck  external mandibular fenestra  spoon-like teeth that hadn’t migrated to the front of the jaw  large nares  box-shaped skulls  extremely pneumatized cervical vertebrae, with air cavities less than 1 centimeter in size What is the sister taxon you chose? cetiosaurus Was there an alternative sister taxon you considered? Which? Neosauropoda Mamenchisaurus # Argentinosaurus ? Apatasaurus *

6) Explain what characters or morphological traits you used to place Mamenchisaurus. Does this placement suggest any complicated evolutionary histories (i.e. convergence) for some traits? Which traits might be convergent? The fact that it had spoon like teeth and that the teeth haven’t moved to the front of the jaw. The trait might be that the jaw structure changed which means the skull would have evolved for different food types or something like that. 7) On your phylogenetic tree above, annotate the tooth shape of each dinosaur by marking their name with a symbol or color. However, first add Apatosaurus, Argentinosaurus and Mamenchisaurus just as you did in the previous phylogeny. You may use any media you like: colored pencils, crayons, colored pen, a colored painting tool (on a tablet), emoji, letters, whatever works best for you. How many times did tooth shape change on the phylogeny? Pretty much for every change in clade. 8) You should notice that evolution of sauropod tooth shape seems to follow a particular sequence of change from one tooth morphology to another, with no apparent reversals. This trend can be observed occurring independently in both the Diplocoidea and the Macronaria. What selective pressures might be driving the evolution of tooth shape? The type of food that they would be hunting/eating would require a different type of way to break it down and eat.

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9) Texas sauropod tracks from the Cretaceous (A, on the left) are known for being wide-gauge, with some distance between the left and right trackways, unlike Jurassic trackways (B, right example, from Portugal). Looking at your phylogeny and the character data, what does this suggest the track maker is of the Texas wide-gauge trackways? What additional evidence in the trackways could you look for? Either the titanosaurus or the alamosaurus as both would qualify with the lack of front toes and wide hips which would explain why they are wider apart the Track B and I could look for any identifying markers for the back feet and see if there is any relationship there.

MiniAssignment4_Phylogenetic Trees with Sauropodomorphs

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