MiniAssignment4_Phylogenetic Trees with Sauropodomorphs

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? The Apatosaurus belongs to the same group as Flagellicaudata, which also includes Amargasaurus and Diplodocus. 2) Explain what characters (morphological traits) helped you place Apatosaurus. What characters does Apatosaurus share with its closest relative (its sister taxon)? I think that Flagellicaudata is the sister taxon of Apatosaurus because of their similar genotypic characteristics. Sauropods with equally long whip-like talks and peg-like/pencil-like teeth are typically classified as Flagellicaudata. They are particularly well-known for having a preantorbital fossa, which is a crucial feature of the Apatosaurus. 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? Given its resemblance to the Patagotitan, the "sister taxon" of the Argentinosaurus refers to the group Titanosauria. Was there another sister taxon you considered? Which? Yes, another sister taxon I considered was Somphospondlyii. 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? Character information that led me to place it where I did was its large mass. Because Titanosauria and Somphospondlyii had some of the biggest masses among their sauropods, I

considered them as potential sisters. There were other commonalities, such as a long posterior neural spine, which is exclusive to Titanosauria and not Somphospondlyii.

Table 2. Taxon Information Regarding Size, Dentition, Ecology, Preservation. Genus Estimate d Mass Neck Length Teeth Shape Notes Plateosaurus 600 – 4000 kg ~2 m Leaf-Shape d Bipedal Vulcanodon 3000 – 4000 kg 2 – 3 m Unknown Early quadrupedal sauropodomorph from South Africa. No skull known. Cetiosaurus 9000 – 18000 kg 5 – 7 m Spoon-like No skull known, but some teeth found along other bones. 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 28000–62 000 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. No skull known. Patagotitan 55000 –77000 kg 8–11 m Unknown Had long posterior neural spines. No skull known. Alamosaurus 50000 –73000 kg 7 – 9 m Peg-like Abundantly known from the end of the Cretaceous in North America. However, no skull known. 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 – no skull known, only vertebrae, legs, hip. Mamenchisauru s 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. 7) On the phylogeny labeled ‘Figure 2’, annotate the tooth shape of each dinosaur listed in Table 2 by marking their name with a symbol, color, or some other label. Include the three taxa you added yourself, Apatosaurus, Argentinosaurus and Mamenchisaurus. 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. Note that no teeth are known from some species, so be creative and find a way to visually indicate which have a known tooth shape and which have an unknown ‘mystery’ tooth shape. Based on known tooth shape as mapped onto the phylogeny in figure 2, how many times do you estimate that tooth shape must have changed in the sauropodomorph dinosaurs? I estimate the tooth shape must have changed at least five times in the sauropodomorph dinosaurs. (Please remember to indicate tooth shape on the evolutionary tree in Figure 2.) 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? (Note: Please remember that all sauropods are strongly believed to be herbivores.) I think the main selective pressures that drove the shape of the teeth were the environment and their diet. All of these sauropods lived in very different environments and ate different things. Some may have needed to have sharper teeth while others didn’t need them.