11 4 Australopithecus robustus

wt 17000 the enigmatic black skull or

australopithecus ethiopicus is the first

evidence we have at the beginning of

this robust lineage of

australopithecines this lineage that

shows increasing size of the mascotory

apparatus huge teeth and jaws huge

chewing muscles but it actually isn't

the first historically the first

evidence historically we have of this

robust lineage comes from south africa

you'll recall that the cradle of

humankind is an area northwest of

johannesburg and pretoria in south

africa that has dozens of fossil hominin

sites some of which we've already talked

about such as stark fontaine which gave

us the first evidence of ultraviolet

africanus

there are a number of sites including

actually the cave of swartcron's which

is just across the valley from stark

fontaine which have yielded abundant

evidence of these robust austropines

recall so this area of just northwest of

johannesburg and pretoria here in south

africa there are several other locations

in south africa that have yielded some

evidence of these robust ulcerations as

well but the key sites that we'll talk

about are swartcrons chrome dry and

dremelin

now recall that we have evidence of

austropicus africanus extending back

perhaps more than three million years in

south africa and continuing to at least

two and a half million years with

specimens such as this sts-5 from the

cave of stark fontaine

however shortly after that period or at

least some time after that period we get

specimens such as this on the left sk-48

from the cave of swartcons again these

are separated by just a few miles

geographically and they might be

separated by a few hundred thousand

years in terms of their geological

position but we can see significant

differences between these two specimens

if we look at sk-48 compared to sts-5 we

see that that breadth across the midface

has expanded even more than it was in

sts-5 we have these huge flaring

zygomatics like sts-5 they're rooted

very deep on the maxilla just above the

actual dentition themselves but they've

gotten much larger and we can see this

hollowing out actually of the face

associated with this expansion of the

zygomatic laterally so sk-48 it turns

out is representative of a different

species in south africa not

australopithecus africanus but

australopithecus robustus it's the first

named lineage we have for these robust

australopithecines and the robustity

refers to the size of the mascatory

apparatus the fact that these specimens

have even larger teeth than osteopathic

as africanus even larger chewing muscles

even larger structures associated with

this whole process of mastication the

earliest specimen that we actually

assigned to ultra previous robustus is

this jaw from the side of chrome dry

again just a short distance from strict

fontaine and sport crons tm 1517

looking at the specimen you can see it

has nice elongated molars expanding

actually in length as we move down the

molar row from m1 to m3 they're also

quite broad and again they expand in

their breadth from m1 to m3 as well in

this context but note that the p4 is

almost as large as the first molar in

other words again we have evidence of

these molarized premolars one of the

trademark characteristics of these

robust austropines as we move later in

the evolution of the robust ulcerations

though we can see these features

exaggerated to a high degree here's a

later specimen from sport crohn's sk23

where we can see that now those

premolars have gotten even larger so in

this case the breadth of the second

premolar is even larger than that of the

first molar and we can see the same

process extending all the way up

actually into the first premolar the p3

here which again shows expanded size

moving down the molar row we can see

that the m2 and m3 remain incredibly

long and large teeth note also that the

enamel in these specimens is very thick

meanwhile if we look at the anterior

dentition the canine and incisors on

these specimens we can see that the

space available for them is very minimal

in fact in lots of australopithecus

robust specimens we see evidence of

dental crowding you can see that here

where the second lateral incisor is kind

of tucked in behind the central incisors

this is oftentimes what we see in the

robust austropines again indicating the

prioritization of the post canine teeth

the prioritization of this huge chewing

teeth the premolars and the molars

located laterally on the jaw here we see

that specimen in lateral view and you

can see again corresponding with this is

this very very tall ascending ramus it

has this very tall vertical ramus again

perhaps associated with large chewing

muscles the masseter muscles connect

here on the side of the jaw and are

associated again with the primary up and

down lever movements of the jaw itself

again we don't have much of a chin like

all of these early hominins but you can

see the long molar teeth and you can see

in fact that the m3 is tucked away here

actually fully behind the ascending

ramus

looking again at a few of these

specimens in lateral view here again is

sk48 on the left sk83 on the right we

can see in lateral view actually the

dramatic expression of these features

the zygomatic arch is this big almost

wave-like structure reflective of the

again perhaps large muscular forces

placed upon it also notice that if the

masseter muscles here attaching to the

bottom of the zygomatic

and then connecting into the jaw down

inferiorly from this notice that the

angle of action extends right through

these molars the angle of action in

other words of the masseter is directly

in line with the molars giving these

organisms maximum efficiency and chewing

through the molar teeth the same thing

is true with sk83 here where again if

the jaw is situated just inferiorly

right here the angle of action of that

masseter is bringing you right in line

with the molars these expanded chewing

teeth again suggesting a certain

prioritization of efficiency in chewing

especially associated with these big

buckle chewing teeth looking anteriorly

we can see more variation here in these

specimens again sk48 on the left a

specimen from the side of dremelin here

on the right this specimen from dremelin

dnh7 is thought to be a female

sk-48 is more ambiguous in its sexual

classification it could possibly be

female suggesting a large range of

female variation in the specimens or it

could be a small male suggesting that

there's a lot of variation in the male

specimens or it could simply represent

something intermediate either male or

female suggesting that both sexes are

variable and that austropithicus

robustus in general has a large amount

of sexual dimorphism but again you can

see the breath across the face and the

dremeline specimen you can see this huge

jaw that corresponds with it it almost

looks like it doesn't fit onto the

specimen given its huge size given the

small size of the skull itself looking

at the dremeline specimen more closely

here you can see in lateral view again

this wave-like big wide zygomatic arch

again associated with this chewing

muscle going straight through the buccal

teeth of the jaw

notice that the zygomatic comes forward

anteriorly to the point where it

actually extends anterior to the profile

of the face itself so that it projects

out here in front of the face itself

that gives the face itself kind of a

concave look it's sort of shoveled out a

little bit again this is characteristic

of all of the robust australopithecines

this dished out appearance of the face

is something that again reflects the

anterior movement of the zygomatics

moving that chewing muscle forward and

the posterior movement of the jaw

expanding and creating additional space

for these big molar teeth looking at the

superior view again you'll notice that

there's this huge space for the temporal

fossa a huge space for the muscle of the

temporalis to occupy filling in this

entire space within the zygomatic arch

again indicative of the huge muscle

forces with muscle fibers extending

posteriorly and anteriorly onto the

skull occupying essentially this entire

region lateral region of the skull we

can look at more variability of these

specimens from dremelin dnh7 the

mandible associated with that specimen

we just looked at on the left dna h a

larger specimen associated on the right

this possibly represents male and female

variation with a larger dna h specimen

representing a male and the smaller dna

seven specimen representing a female

notice in both cases the huge amount of

space occupied again by the premolars

and molars and the tiny space available

for those anterior chewing teeth again

there's a huge prioritization of that

chewing ability clearly australopithecus

robustus is prioritizing its ability to

chew to chew effectively and to chew

frequently

so these large teeth suggest some kind

of evolutionary selection for long

duration or long extended chewing life

again when you wear through your teeth

you pretty much lose the ability to

survive now there's other interesting

remains from the austropicus robustus

specimens aside from the fossils

themselves

this specimen sk 1585 is one of a number

of preserved endocasts from the south

african cave sites recall that we've

already seen an earlier endocast from

australopithecus africanus from the tong

child this one comes from the cave of

sport krons and there are a number of

features that we can notice about this

specimen

first of all it shows a slight expansion

in brain size australopithecus robustus

if we look across the sample actually

shows larger brain size than australicus

africanus this might be the reflection

of simply evolution of increasing brain

size across a single lineage australia's

robustness may in fact simply be a later

derived version of ostropica's africanus

or at least descend from australia's

africanus but there are a number of

features of this endocast in addition to

its large size which suggests some

derivation a little bit more development

to the frontal lobe some increasing

complexity on the lateral margin of this

endocast as well as perhaps the

formation of increased convolutions on

the posterior side all of which might

indicate increased complexity in

increased neurological capabilities of

these specimens regardless australia's

robustus again contributes to the large

amount of variation that we have in

these early hominins from south africa

australopithecus robustus historically

is the earliest of the robust lineages

that we have seen although geologically

it might fall after australopithecus

africanus and likewise after

australopithecus atheists that we've

already seen in east africa now we're

about to look at the robust lineage that

exists in east africa perhaps at the

same time period perhaps slightly later

in time as austropicus robustus those

are specimens that refer to in a moment

as australopithecus boisei