Australopithecus and Paranthropus Lab Activity- Student_updated June 2023

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University of Arkansas *

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Dec 6, 2023

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1 YOUR NAME: Isabella Sanchez LAB SECTION: 3 AUSTRALOPITHECUS and PARANTHROPUS CRANIAL ANATOMY and DIET This lab is designed to introduce you to variation among and between species of Australopithecus and Paranthropus , some of which were possibly our ancestors while others were lineages that became extinct. Using your knowledge of osteology, you will work in a group to develop a list of character states that can be used to distinguish the species and think about what their diets may have been. Lab Objectives • Identify differences among skulls of Australopithecus and Paranthropus species • Compare the major bones and muscles involved with mastication Overview of the Masticatory System The skeletal system that operates the chewing system of the human and ape lineages acts as a lever that moves up and down to accomplish mastication (chewing). The two main muscles in this system are the temporalis muscle and the masseter muscle. The temporalis muscle originates on the lateral side of the cranial vault and marks the cranium with a temporal line in humans. In some apes and early hominins the muscle is so large (and the brain case so small) that this muscle extends onto the top of the skull and forms a sagittal crest . The temporalis muscle then passes through the temporal fossa (the large foramen behind the orbit on the side of the skull that is enclosed by the zygomatic arch), before inserting onto the coronoid process of the mandible. The temporalis muscle functions to elevate (close the mandible). The masseter muscle originates on the zygomatic arch and inserts on the outer mandible at the gonial angle . It elevates the mandible and makes some side-to-side movements. Directions Your task for this lab will be to describe and compare a number of masticatory and dental traits among apes, humans, and fossil hominins belonging to the genera Australopithecus and Paranthropus . BEFORE you answer any of the questions for this lab, review the definitions of the traits and fully complete the observation sheet. Take a few moments to look at the skulls before filling in your observations. This is a group activity so discuss the features among your group.

2 Descriptions of Cranial Traits Facial Prognathism : Hold the cranium with the teeth parallel to the table and then examine how far the incisors project anteriorly in front of the orbits. Character states: pronounced, intermediate, reduced Diastema : In osteological terminology, a diastema is a gap between teeth. The diastemata we are interested in here are the ones adjacent to the canines. In apes, which have large canines, there is a diastema between the maxillary canine and lateral incisor and between the mandibular canine and first premolar. The canine from the opposite arch fits into these gaps when the teeth are in occlusion. Articulate the mandible with the cranium and see for yourself. In species with small canines, like humans, there are no diastemata. Character states: large, small, absent Molar area : Molar area is measured as the product of mediodistal length of the molar (i.e., the length of the tooth from the front to the back of the mouth) and buccolingual width (i.e., the width of the tooth from the cheek to tongue side). In this case we are interested in the area of the mandibular (lower) first molar. This measurement has been provided for you. Canine area : This variable is measured in the same way as molar area, except measured on the mandibular canine. This measurement has been provided for you. Mandibular canine to molar area ratio : This variable is calculated as canine area divided by molar area. This measurement has been provided for you. Incisor area : This variable is measured in the same way as molar area, except measured on both of the maxillary incisors. This measurement has been provided for you. Foramen magnum position : You examined this last lab. An easy way to examine the position of the foramen magnum is to mentally draw a line between the external auditory meati (ear holes) and see if the foramen magnum is between the ears or not. You can also look at the tilt of the foramen magnum by placing a pencil over the foramen in the sagittal plane. If the pencil tilts, the spinal cord would exit the cranium at an angle. If the pencil is flat, the spinal column would be directly below the cranium. Character states: posterior, anterior, intermediate Browridge prominence : Extant apes have quite projecting browridges, while many hominins do not. You can also examine the prominence of the brows with a pencil. A true browridge has two parts, a supraorbital torus (=bar of bone) and a supraorbital sulcus (=groove). If you hold the cranium with the teeth parallel to the table and place a pencil behind the torus, it will rest in the sulcus if the browridge is prominent. If there’s no dip behind the torus, the species lacks a true browridge. Character states: present, absent Position of the zygomatics : This is best tested by placing a pencil transversely across the nasal aperture. If the zygomatics are in front of the nasal aperture, the pencil will rest on them. If the zygomatics are behind the nasal aperture, the pencil will rest on the nasal opening. Character states: forward of or behind the nasal aperture Sagittal crest : This is the crest that is formed by the convergence of the temporal lines. Character states: present, absent

3 OBSERVATION SHEET Species Facial Prognathism Diastema Mandibular First Molar Area Mandibular Canine Basal Area Canine/ Molar Area Ratio Maxillary Incisor Area Foramen Magnum Position Brow ridge prominence Position of Zygomatics Sagittal Crest Pan troglodytes Pronounced Large Species Avg. 103.5 mm 2 Male Avg. 154.8 mm 2 Female Avg. 104.6 mm 2 Male 1.50 Female 1.01 Species Avg. 114.8 mm 2 Intermediate Present Behind Present Gorilla gorilla Pronounced Large Species Avg. 210.5 mm 2 Male Avg. 266.4 mm 2 Female Avg. 142.5 mm 2 Male 1.27 Female 0.68 Species Avg. 153.0 mm 2 Intermediate present behind Present Australopithecus afarensis Intermediate Absent Species Avg. 165.1 mm 2 Species Avg. 94.5mm 2 0.57 Species Avg. 87.98 mm 2 Anterior present behind Absent Australopithecus africanus Intermediate Absent Species Avg. 183.5 mm 2 Species Avg. 96.0 mm 2 0.52 Species Avg. 83.8 mm 2 Anterior present behind Absent Paranthropus aethiopicus Intermediate Absent Species Avg. no data Species Avg. no data No data Species Avg. no data Anterior present forward Present Paranthropus robustus Intermediate Absent Species Avg. 201.4 mm 2 Species Avg. 66.3 mm 2 0.33 Species. Avg. 63.6 mm 2 Anterior present forward Present

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4 Species Facial Prognathism Diastema Mandibular First Molar Area Mandibular Canine Basal Area Canine/ Molar Area Ratio Maxillary Incisor Area Foramen Magnum Position Brow ridge prominence Position of Zygomatics Sagittal Crest Paranthropus boisei intermediate absent Species Avg. 255.8 mm 2 Species Avg. 67.9 mm 2 0.27 Species Avg. 77.0 mm 2 anterior present Forward present Homo sapiens reduced absent Species Avg. 111.2 mm 2 Male Avg. 61.7 mm 2 Female Avg. 54.9 mm 2 Male 0.55 Female 0.49 Species Avg. 66.0 mm 2 anterior absent behind absent

5 *Answer these questions AFTER you’ve completely filled out the observation sheet* Section 1. Early Hominin Skulls- Differences through time 1. Analyze the following proposed evolutionary sequence: Ape ancestor (similar to P. troglodytes ) – > A. afarensis – > P. boisei This sequence suggests that the ape ancestor ( P. troglodytes ) is the most primitive species and P. boisei the most derived. a. Within this lineage, what morphological changes have occurred in the development of the sagittal crest? The sagittal crest is absent in Pan troglodytes and A. afarensis but is present in P. boisei. b. How has molar size changed in this lineage? The molar size is smallest in P. troglodytes, larger in A. afarensis, and largest in P. boisei. Molar size in A. c. How has canine size changed in this lineage? P. troglodytes has large canines that project beyond the occlusal plane. Neither of the hominins, A. afarensis and P. boisei, has projecting canines. P. boisei has even smaller canines than is observed in A. afarensis. d. How has incisor size changed in this lineage? P. Troglodytes has exceptionally broad incisors, especially so in the maxilla. Matching their exceptionally small canines, P. boisei has tiny incisors. 2. How does facial prognathism differ between Pan , Australopithecus , Paranthropus, and Homo ? Prognathism is especially pronounced in Pan, intermediate in Australopithecus, and reduced in Homo. In part, the projection of the lower face of Pan makes room for the large roots of the maxillary canines, which run parallel to the nasal aperture. In the hominins, the midface is more upright than in Pan, though the region below the aperture still projects forward in Australopithecus. In Homo, both midfacial and subnasal prognathism are reduced. 3. How does the prominence of the browridge differ between Pan , Australopithecus , Paranthropus, and Homo ?

6 In Pan, there is a distinct dip (i.e., sulcus) behind the supraorbital torus. Though having thickened tori, neither Australopithecus nor Paranthropus has a sulcus behind the torus. As a result, the brow ridge is much more prominent in Pan than it is in species of Australopithecus or Paranthropus. 4. How does the placement and orientation of the foramen magnum differ between Pan , Australopithecus , Paranthropus and Homo ? In Pan, the foramen is located more posteriorly than in the hominins. The foramen also exits the base of the cranium at an angle. In all Paranthropus, Australopithecus, and Homo, the foramen is shifted forward on the base of the skull so that it lies nearly on a line connecting the ears. 5. How does the size of the diastema differ between Pan, Gorilla, Australopithecus , and Homo ? How does this relate to changes in relative canine size? The canine from the opposite arch fits into a diastema. The hominins, with their small nonprojecting canines, lack a diastema in both dental arches. 6. Look at the characters on your character sheet. What is a (non-dental) feature shared by Paranthropus aethiopicus , Paranthropus robustus, and Paranthropus boisei that is not seen in the other species? Zygomatics that are placed in front of the nasal aperture. The unique placement of the zygomatics in these species creates a "dished" midface in Paranthropus. In all other species, the zygomatics are behind the nasal aperature. Section 2. Sagittal Crests, Muscles, and Diet The hominins Australopithecus and Paranthropus have impressive adaptations to chewing. In these species, we see changes in their cheek teeth (megadontia), reductions in the size of the anterior teeth, and changes in overall shape and size of the head that are likely responses to changes in their diets. Refer to your chart or to the skulls to help answer these questions. 7a. Compare the development of the sagittal crest in the chimpanzee to Australopithecus and to Paranthropus . What differences do you see among these taxa? It's only present in Paranthropus. 7b. What chewing muscle attaches to the sagittal crest? Temporalis 7c. How does the height of the sagittal crest relate to the size of the temporalis muscle?

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7 The height of the sagittal crest is a reflection of the size of the muscle that attaches to it. Examine the skulls of the chimpanzee and Paranthropus boisei . In Paranthropus species, the entire zygomatic bone is moved forward on the face, placing it directly over the first molar, and creates a dish-shaped face. This increases the leverage of the chewing apparatus as a whole. 8. Which chewing muscle attaches to the zygomatic? What does the size of the chewing muscle attachments in Paranthropus tell you about the muscle sizes in this hominin group? Masseter Section 3. Canines and Molars 9a. In terms of the projection of the canines, do the hominins have more or less projecting canines than the extant apes? Less projecting 9b. In terms of the size of the base of the canines, do the hominins have smaller or larger canines than the extant apes? Smaller. Canines of species of Australopithecus, Homo, and Paranthropus are smaller in all dimensions than in the extant apes. 10. The fossil hominins have approximately the same body size as Pan troglodytes (chimps, ~ 150 lbs.) and Gorilla gorilla males weigh about 400lbs. and females about 220lbs. Compared to body size, which species have the largest molars? (Hint: create a ratio of molar area to body size). Paranthropus boisei. It is a chimp-sized critter, but has molars that are nearly the size of a gorilla. 11a. Now examine overall dental proportions. In the table, you calculated the ratio of canine area/molar area. How does this ratio differ between hominins and extant apes?

8 In the extant apes, the canine basal area is larger than the area of the first molar. The opposite is true in the hominins. 11b. Does the ratio change because canine size changes, molar size changes, or both? Both. Compared to a chimp, the hominins have absolutely larger molars and smaller canines. Section 4. The Big Picture 12. How do you think these traits (sagittal crest size, zygomatic position, and relative molar size) relate to diet? What was the toughness of foods ( soft foods, moderately tough foods, extremely tough foods) for the following species? Paranthropus boisei Given their exceptionally large molars and large chewing muscles, indicated by the sagittal crest and forwardly placed masseter origin on the zygomatics, it would be tempting to infer that this hominin had an extremely tough or hard diet. Australopithecus africanus A. africanus is intermediate between Pan troglodytes and Paranthropus boisei. As a result, it would be tempting to infer that the material properties of their diet were also intermediate. Pan troglodytes (chimps) Soft foods, they are frugivores 13. What cultural behavior might have reduced selective pressures for molar size in early members of the genus Homo ? What evidence in archaeology might you seek for evidence of that behavior? Tool use 14. The hominins all have small canines compared to extant apes and other anthropoid primates. Create a hypothesis that explains the reduction of canine size during hominin evolution? Is the change related to diet or social behavior, or both?

9 canine reduction is a result of changes in social behavior or in diet.

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Australopithecus and Paranthropus Lab Activity- Student_updated June 2023

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