Mini Assignment 4 Phylogenetic Trees with Sauropodomorphs

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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 Table 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 Apatosaurus, 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 is the sister taxa to the clade that contains the Amargasaurus and Diplodocus, which is classified as Flagellicaudata. 2) Explain what characters (morphological traits) helped you place Apatosaurus. What characters does Apatosaurus share with its closest relative (its sister taxon)? Characteristics such as the long tail and preantorbital fossa helped me place the Apatosaurus. Flagellicaudata includes sauropodomorphs with long whip-like tails and depressions in the skull in front of the eye sockets, preantorbital fossa. The characteristics of the Apatosaurus align with those of its sister taxon, Flagellicaudata. 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? I believe that the Argentinosaurus is a part of the Titanosauria clade and sister taxon. Was there another sister taxon you considered? Which? The other 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? When placing the Argentinosaurus in a sister taxon, I used characteristics such as the long neural spines, the ‘skinny’ humerus, and the large size/mass of the species. I was uncertain whether to place the Argentinosaurus in Titanosauria or Somphospondlyii because both clades contain large sauropods. However, the characteristic of a long neural spine is only present in Titanosauria.

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Figure 2. Use this tree to show how you think Apatosaurus, Argentinosaurus, and Mamenchisaurus might relate to other species of sauropodomorph dinosaurs.

5) Add Mamenchisaurus to the phylogenetic tree on the previous page. Mamenchisaurus was a middle/late Jurassic sauropod known only from China. This sauropod genus could be connected anywhere on the tree, but see the characters below. Key characters of Mamenchisaurus:  18 extremely long cervical vertebrae, giving it a disproportionately long neck  External mandibular fenestra  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? The Mamenchisaurus belongs to the sister taxon Somphospondlyii. Was there an alternative sister taxon you considered? Which? The alternative sister taxon that I considered was Titanosauriformes due to the similar characteristics shared with this clade. 6) Explain what characters or morphological traits you used to place Mamenchisaurus. Does this placement suggest any complicated evolutionary histories, such as convergent evolution, for some traits? Which traits might be convergent (i.e. the same traits evolved independently in different lineages)? I placed the Mamenchisaurus into the clade Somphospondlyii due to its defining characteristics. These characteristics include pneumatized cervical vertebrae with air cavities less than 1 centimeter in size and similarities to Sauroposeidon. Some of the traits are likely a result of convergent evolution due to the Mamenchisaurus’s placement. Traits such as the chisel-like teeth and disproportionately long neck could be convergent.

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 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– 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. 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. 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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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 that the tooth shape must have changed in the sauropodomorphs five to six times. (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? Selective pressures such as the habitat/environment and diet might be driving the evolution of tooth shape. The environment in which the individual sauropod species lived influenced the tooth shape due to the different diets that include varying hardness, thickness, material, etc. Different tooth shapes are specialized for certain diets, this explains the evolution of the tooth shapes. (Note: Please remember that all sauropods are strongly believed to be herbivores.)

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). Look over at your phylogeny and the character data, and consider what this implies about the physical traits of different sauropod lineages. Which traits might reflect having a wider body? Which lineages might have those traits, and thus indicate which sauropod lineage is the wide- gauge track maker? Looking over the phylogeny and character data, traits such as wide hips and overall size reflect having a wider body. The lineage that best fits these traits is Lithostrota, specifically the Alamosaurus. What additional evidence in the trackways you might look for to verify your hypothesis of which sauropod makes the wide-gauge trackways? Additional evidence in the trackways that could help verify the hypothesis is the size and shape of the foot, as well as the average length of the species.

Mini Assignment 4 Phylogenetic Trees with Sauropodomorphs

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