Tetrapods Assignment

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Anthropology

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Dec 6, 2023

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Tetrapods 1. Discuss the paleontological evidence for or against this concept. There has been a couple of paleontological evidence for the common ancestor of the tetrapods to have a pentadactyl limb. Paleontologists investigating the Devonian period have discovered that the diversity of vertebrate appendages reveals that paired fins might have evolved twice and that their pectoral fins preceded pelvic fins (Hinchliffe, 1994). Additionally, polydactylous tetrapods around this period had fossils with preserved limbs, with digits ranging from 8 to 6. This would later evolve to be more terrestrially adapted. Another piece of evidence supporting this would be the presence of the autopodium in fossils. The autopodium, an anterior-posterior series of small long bones, is said to be a result of distalization of the zone of polarizing activity (ZPA) (Wagner & Chiu, 2002). Meaning that the sonic hedgehog (shh) expression moves distally as the limb grows. This is common in modern tetrapods and this type of activity is not seen in the fin development of fish as the shh expression remains in one place. The presence of the autopodium is deemed to be the first appearance of additional digits that would form a pentadactyl limb. 2. Discuss embryological evidence for or against this concept. While there has been paleontological evidence of the common ancestor of tetrapods having pentadactyl limbs, there has been conflicting embryological evidence for this case. According to Stewart et al. (2019), it is difficult to test digit homology during embryotic development due to it being unclear if the digit identities of tetrapods represent “distinct and conserved gene regulatory states” (Stewart et al, 2019, p.1). Considering that tetrapods show different patterns of gene expression, it makes it hard to form a clear answer on which common ancestor had pentadactyl limbs. However, Saxena et al. (2017) believe that there is enough evidence to pinpoint a common ancestor, stating that there are enough shared “mechanisms of induction, growth, and pattering” (Saxena et al, 2017, p.1) between tetrapods during embryonic development. It is even stated in this article that the wing buds of certain present bird species exhibit pre-cartilage condensations of five-digit ancestors (Saxena et al, 2017). This contradiction of answers makes it hard to provide proper evidence for this concept. 3. From what ancestor did the modern one-fingered horse originate? The ancestor that the modern-one-fingered horse originated from could have been Hyracotherium .

References: Hinchliffe, J.R. (1994). Evolutionary developmental biology of the tetrapod limb. Development . 1994 , 163-168. https://journals.biologists.com/dev/article/1994/Supplement/163/49479/Evolutionary- developmental-biology-of-the-tetrapod Saxena, A., Towers, M., & Cooper, K.L. (2017). The origins, scaling, and loss of tetrapod limbs. Philosophical Transactions of the Royal Society B. 372(1713), 1-9. https://doi.org/10.1098/rstb.2015.0482 Stewart, T.A., Liang, C., Cotney, J.L., Nooman, J.P., Sanger, T.P., & Wagner, G.P. (2019). Evidence against tetrapod-wide digit identities and for a limited frame shift in bird wings. Nature Communications. 10(3244) . 1-13. https://www.nature.com/articles/s41467-019- 11215-8 Wagner, G.P & Chiu, C.H. (2002) The tetrapod limb: A hypothesis on its origin. J ournal of Experimental Zoology. 291(3), 226-240. https://onlinelibrary.wiley.com/doi/abs/10.1002/jez.1100 Wood, A.R., Bebej, R.M., Manz, C.L., Begun, D.L., & Gingerich, P.D. (2011). Postcranial functional morphology of Hyracotherium (equidae, perissodactyla) and locomotion in the earliest horses. Journal of Mammalian Evolution. 18(1) , 1-32. https://link.springer.com/article/10.1007/s10914-010-9145-7

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Tetrapods Assignment

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