Australopithecus and Paranthropus Lab Activity- KEY

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Dec 6, 2023

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YOUR NAME:_______ KEY _____________ LAB SECTION:__________________ 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. 1 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 2 OBSERVATION SHEET Species Facial

Prognathism Diastema Mandibular First Molar Area Mandibular Canine Basal Area Male Avg. Canine/ Molar Area Ratio Male Maxillary Incisor Area Foramen Magnum Position Browridge prominence Position of Zygomatics Sagittal Crest Pan troglodytes Species Avg. 103.5 mm 2 Gorilla gorilla Species Avg. 210.5 mm 2 Species Avg. 154.8 mm 2 Female Avg. 104.6 mm 2 Male Avg. 266.4 mm 2 Female Avg. 142.5 mm 2 Species Avg. 1.50 Female 1.01 Male 1.27 Female 0.68 Species Avg. 114.8 mm 2 Species Avg. 153.0 mm 2 Australopithecus afarensis Australopithecus africanus

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Paranthropus aethiopicus Paranthropus robustus 165.1 mm 2 Species Avg. 183.5 mm 2 Species Avg. no data Species Avg. 201.4 mm 2 94.5mm 2 0.57 Species Avg. 87.98 mm 2 Species Avg. 96.0 mm 2 0.52 Species Avg. 83.8 mm 2 Species Avg. no data No data Species Avg. no data Species Avg. 66.3 mm 2 0.33 Species. Avg. 63.6 mm 2 3 Species Facial Prognathism Diastema Mandibular First Molar Area Species Avg. Mandibular Canine Basal Area Species Avg. Canine/ Molar Area Ratio Maxillary Incisor Area Foramen Magnum Position Browridge prominence

Position of Zygomatics Sagittal Crest Paranthropus boisei 255.8 mm 2 67.9 mm 2 0.27 Species Avg. 77.0 mm 2 Homo sapiens 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 4 *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? Body size is similar in P. troglodytes, A. afarensis, and P. boisei; however, molar size is quite different. Molar size is smallest in P. troglodytes, larger in A. afarensis, and largest in P. boisei. Molar size in A. afarensis is generally described as “megadont,” meaning that they have large teeth relative to body size, and molar size in P. boisei is described as “hypermegadont.” 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. For both hominins, the canines are small and wear down from the apices, which is unlike the chimp in which the canines are honed on their back edge. 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. This reflects their frugivorous diet, which requires lots of incisal preparation of fruits. The hominins both have smaller incisors than chimps. Matching their exceptionally small canines, P. boisei has tiny incisors. The anterior teeth, the canines and incisors, of P. boisei are shoved forward in the mouth and occupy a small space of the arcade. 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. 5 3. How does the prominence of the browridge differ between Pan , Australopithecus , Paranthropus, and Homo ? In Pan, there is a distinct dip (i.e., sulcus) behind the supraorbital torus. The dip is deep enough that you can rest a pen in the sulcus. Though having thickened tori, neither Australopithecus nor Paranthropus has a sulcus behind the torus. As a result, the browridge 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. You can visualize this by holding the cranium with the teeth

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parallel to the table and by placing a pen over the foramen in the sagittal plane. The pen should tilt at approximately 45°. 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. If you do the pencil test with these genera, you’ll see that the foramen is flat on the base of the skull. You can visualize this in one other manner. Hold the cranium with the teeth parallel to the table and look at the angle that the occipital bone makes posterior to the foramen magnum. In chimps, the occipital is angled upward and the nuchal muscles insert high on the posterior portion of the cranium. In the hominins, the occipital is flat and the insertion of the nuchal musculature is now near the base of the skull. 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? In both Pan and Gorilla, 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 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. 6 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? 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? 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 (~ 150 lbs.) and Gorilla gorilla males weigh about 400lbs. and females about 220lbs. Relative 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? 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. 7 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: Your answers will vary. 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. This was the most common interpretation since P. boisei was discovered; however, recent work has revealed that the diet of P. boisei was neither tough nor hard. You’ll have to come to class to find out what they were eating ☺ .

Australopithecus africanus: On a continuum of chewing muscle and molar size, 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 was also intermediate. Pan troglodytes soft foods. They are primarily frugivorous. 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? Hypotheses will fall into two camps: canine reduction is a result of changes in social behavior or in diet. Social behavior: In extant nonhuman primates, canine size is correlated with social behavior. In species where males intensely compete for access to mates, the canines are relatively large. In species where male-male competition is low, canines are relatively small. It is tempting to see canine reduction in hominins as a reflection of low levels of male-male competition. After all, modern humans are mostly monogamous and males do not often physically compete for mates. The problem with this hypothesis is that canine size dimorphism and body size dimorphism are usually similar in extant primates. When male canine size is relatively large compared to female canine size, so is male body size. In early hominins, canines are small and nondimorphic, but body size dimorphism is quite pronounced. This is quite an unusual pattern for a primate, which suggests that reduction in male-male competition may not be the whole story. Diet: There are two subcategories here. The first hypothesis is that the canines themselves were used as an additional incisor in the hominins. This would be unusual for a primate, as canines are rarely used as a dietary tool in living nonhuman primates. This hypothesis has not been tested 8 directly by anyone, but should be straightforward to test by looking at patterns of microscopic wear on canines and incisors. The second hypothesis is that canines reduced as a tradeoff of increasing masticatory robusticity. Look at P. boisei for example. In that species, the incisors and canines are tiny and are shoved forward in the mouth to make room for the exceptionally large postcanine teeth. The problem with this hypothesis is that the earliest hominins to have small canines do not have a robust masticatory apparatus and large molars. As you can see, canine reduction in hominins is complex

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Australopithecus and Paranthropus Lab Activity- KEY

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