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rsos.royalsocietypublishing.org
Research
Cite this article: López-Torres S, Schillaci MA,
Silcox MT. 2015 Life history of the most
complete fossil primate skeleton: exploring
growth models for Darwinius. R. Soc. open sci.
2: 150340.
http://dx.doi.org/10.1098/rsos.150340
Received: 13 July 2015
Accepted: 11 August 2015
Subject Category:
Biology (whole organism)
Subject Areas:
palaeontology/evolution/developmental
biology
Keywords:
Adapoidea, Strepsirrhini, Haplorhini,
Anthropoidea, Eocene
Life history of the most
complete fossil primate
skeleton: exploring growth
models for Darwinius
Sergi López-Torres, Michael A. Schillaci and
Mary T. Silcox
Department of Anthropology, University of Toronto Scarborough, 1265 Military Trail,
Toronto, Ontario, Canada M1C 1A4
Darwinius is an adapoid primate from the Eocene of Germany,
and its only known specimen represents the most complete
fossil primate ever found. Its describers hypothesized a close
relationship to Anthropoidea, and using a Saimiri model
estimated its age at death. This study reconstructs the ancestral
permanent dental eruption sequences for basal Euprimates,
Haplorhini, Anthropoidea, and stem and crown Strepsirrhini.
The results show that the ancestral sequences for the basal
euprimate, haplorhine and stem strepsirrhine are identical,
and similar to that of Darwinius. However, Darwinius differs
from anthropoids by exhibiting early development of the lower
third molars relative to the lower third and fourth premolars.
The eruption of the lower second premolar marks the point
of interruption of the sequence in Darwinius. The anthropoid
Saimiri as a model is therefore problematic because it exhibits a
delayed eruption of P2 . Here, an alternative strepsirrhine model
based on Eulemur and Varecia is presented. Our proposed model
shows an older age at death than previously suggested (1.05–
1.14 years), while the range for adult weight is entirely below
the range proposed previously. This alternative model is more
consistent with hypotheses supporting a stronger relationship
between adapoids and strepsirrhines.
Author for correspondence:
Sergi López-Torres
e-mail: [email protected]
1. Background
Electronic supplementary material is available
at http://dx.doi.org/10.1098/rsos.150340 or via
http://rsos.royalsocietypublishing.org.
Adapoids were medium-sized, arboreal euprimates, widespread
throughout portions of Europe, Asia, Africa and North America
from the Early Eocene to the Late Miocene. Adapoids were
a very diverse group, comprising six families and more than
100 species [1–3]. Despite receiving considerable attention in the
literature, the evolutionary relationships of adapoids to modern
strepsirrhines or haplorhines remain unclear. Currently, two
opposing hypotheses predominate: (i) the Adapoid–Anthropoid
hypothesis [4–17] and (ii) the Adapoid–Strepsirrhine hypothesis
[18–25].
2015 The Authors. Published by the Royal Society under the terms of the Creative Commons
Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted
use, provided the original author and source are credited.
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Life-history analysis assesses the chronology of development and reproduction throughout a species’
lifetime, from conception to death. The pattern and timing of tooth emergence known for extant primates
can be used to broadly infer the life history of fossil groups [37–39]. This is particularly useful for fossil
taxa, in which the dentition is the primary and most reliable gauge of ontogeny [39]. The relevance of
the dentition as a source of information to explain life history relies on the fact that it is not strongly
influenced by environment [40]. There is also a relationship between the sequence of dental eruption and
the overall pace of growth, maturation and other aspects of primate life history [41]. For example, M1 is
the first permanent tooth to erupt in primates, and the timing of its emergence is highly correlated with
adult brain weight, body weight and probably reflects infant precociality [40,42]. In order to use data
from dental eruption sequences in modern taxa to form inferences about a fossil, it is necessary to define
the point of interruption of the sequence. Defining this point allows for modern growth trajectory data
to be used to determine not only age at death but also other variables such as projected adult weight
(e.g. [17]).
1.2. Dental eruption and life history of Darwinius
The Darwinius specimen is split into two parts: PMO 214.214 and its counterpart WDC-MG-210 [17].1 The
specimens, altogether nicknamed ‘Ida’, come from the site of Messel (near Darmstadt, Germany; 47.5 Ma
[43]). The individual was a weaned and independently feeding juvenile, with an erupted M1 [17]. Based
on the absence of a baculum, the specimen has been interpreted as female [17], although it is possible
that this element was lost (e.g. along with the left lower limb below the knee) or had not fully developed
at the time of the animal’s death.
The preserved dentition (figure 1) allows for several inferences about the permanent dental eruption
sequence. We follow Gingerich & Smith’s [44] inference that P2 is an adult tooth, and Franzen et al.
[17] in considering it to be the last tooth to have emerged before death. It is known that variability in
the premolar eruption sequence is common in some New World monkeys, such as in Callicebus, Alouatta,
Saimiri, Callithrix, Mico, Saguinus and Cebus [45–47]. However, there is no intraspecific variability reported
in the presence of P2 in adapoids [48,49], and no reported variation in eruption sequences [44]. Therefore,
a priori, there is no empirical basis for believing the eruption of P2 in Darwinius is variable, or that P2
erupted unusually early or late in ‘Ida’. We are additionally assuming that dP2 is replaced by P2 in
‘Ida’, as it is in modern primates that retain this tooth. It is possible that this is an incorrect assumption,
and that P2 emerged without having a deciduous precursor, in a manner similar to P1 in many living
mammals [50]. However, loss of P1 replacement is inferred to be a very ancient feature in mammalian
evolution, based on its absence in even some non-eutherians (e.g. Didelphis [51]), whereas in all adapoids
for which there are adequate data, P2 is known to be replaced [18,44]. As such, arguing for a lack of
a deciduous precursor for P2 would require the assumption that this particular lineage developed a
peculiar homoplasy, not observed in living primates. Although this is of course possible, in the absence
of any data it seems more parsimonious to assume that Darwinius was a typical adapoid and replaced its
P2 . It is worth noting that in applying a Saimiri model, or indeed a model based on any living primate
(e.g. [17,44]), this same assumption is being made.
1
PMO: Geological Museum, Natural History Museum, University of Oslo; WDC-MG: Wyoming Dinosaur Center, Messel Grube
collection.
................................................
1.1. Relevance of dental eruption sequences in primate life history
2
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The Adapoid–Strepsirrhine hypothesis has been more broadly accepted for the past two decades,
based on the fact that it is recovered in all broadly sampled, recent phylogenetic analyses [22,26–29].
However, the debate surrounding the phylogenetic position of adapoids was thrust back into prominence
with the description of the most complete fossil primate skeleton, the caenopithecid adapoid Darwinius
masillae. Franzen et al. [17] proposed a stronger relationship between Darwinius and haplorhines,
and Gingerich et al. [30] later united it specifically with anthropoids. Since then, the biology and
the evolutionary relationships of Darwinius and adapoids to modern primates have been discussed
extensively in the literature (e.g. [23,24,31–36]). An area that has received much less treatment than the
question of Darwinius’ phylogenetic position is the age model used [17] to reconstruct its body mass and
age at death. This paper considers the appropriateness of anthropoids generally, and Saimiri specifically,
with respect to choosing an appropriate model for Darwninius’ development, and also assesses the
relevance of growth data to the phylogenetic debates.
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3
M2
P3
P4
M1
M2
M3
0
1
2
3
4
M1
dP4
dP3
I2
dP3
dP4
P3
P4
dC1
I1
dC1
dI2 I1
I2
P2
C1
5 mm
deciduous crown
permanent crown
crown shadow
visible root
root shadow
Figure 1. Radiograph of the right side of the skull of Darwinius masillae showing the deciduous (indicated with a ‘d’) and permanent
teeth. Adapted from Franzen et al. [17, fig. 5].
Following from this inference, the teeth of Darwinius can be divided in two sets: (M1 M2 I1 P2 ), which
are adult and erupted, and (I2 M3 C P4 P3 ), which are in various stages of development and have yet to
erupt, with the point of interruption in the sequence occurring after the eruption of P2 [17]. Using the
dental eruption sequence of the New World anthropoid Saimiri as a model, Franzen et al. [17] estimated
an age at death between 9 and 10 months based on the interruption of the dental eruption sequence,
and a projected adult weight between 650 and 900 g. For the unerupted teeth, it is also possible to make
certain inferences about their likely place in the sequence based on their degree of development. For
example, the crown of M3 is completely formed and the tooth is in process of erupting. It was probably
only covered by soft tissue, although it still lacks mineralized roots [17]. Therefore, this would place the
timing of eruption of M3 early in the sequence.
The goals of the present study are to reassess the age estimate for Darwinius by using ancestral
reconstruction of dental eruption sequences, and to further explore the life history of Darwinius.
Specifically, we assess the validity of using the anthropoid (Saimiri) model for inferring the age and
life history of Darwinius.
2. Material and methods
Ancestral state reconstruction has been embraced by both systematists and evolutionary biologists for
many years [52] as a way of making inferences about the pattern of evolution of particular traits or
character complexes. Indeed its implementation can be traced back almost 30 years to the work of
Felsenstein [53]. Since then, this method has been applied extensively by many authors in the field of
palaeontology (e.g. [54–64]). For the ancestral state reconstruction analysis in this study, dental eruption
sequences for 97 fossil and extant taxa were used, including Primates, Scandentia and Soricomorpha
(table 1). A matrix of 14 characters related to eruption sequences was created (table 2). The 14 characters
used in our matrix constitute the minimum number of informative characters needed to allow the
reconstruction of eruption sequences. These characters code for presence or absence of certain teeth, and
relative timing of eruption of teeth in different positions. By reconstructing the ancestral states for every
character, it is possible, from the information they provide, to infer the order of dental eruption at any
particular node on the tree. Although not all of the characters are independent, because of the way they
are defined, it is impossible for them to generate conflicting reconstructions. For example, in electronic
supplementary material, table S2, the ancestral reconstruction for the euprimate node for character 6
‘Eruption of I1 relative to the earliest premolar’ recovers both states (i.e. I1 erupting before or after
the earliest premolar), instead of one ancestral state. However, because all incisors erupt unequivocally
before M2 (character 8, state 3) and all premolars erupt unequivocally after M2 (character 12, state 0), the
ancestral state for character 6 resolves as I1 erupting before the earliest premolar.
................................................
M3
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permanent dental eruption sequence
(M1 M2 I1 P2 ) (I2 C M3 P4 P3 )a
Dymecodon pilirostris [65]
M1 M2 M3 [P2 P3 P4 ] I1 C P1
Tupaia glis [65]
M1 M2 M3 P2 I3 P4 [I1 C] P3 I2
Acidomomys hebeticus [66]
M1 M2 P3 (I1 M3 ) (I2 P4 )
Plesiadapidae [67]
[M1 M2 P2 ] [M3 P3 P4 ]
Microcebus murinus [68]
M1 M2 I1 -I2 -C P2 M3 P4 P3
Mirza coquereli [68]
M1 I1 -I2 -C M2 P2 M3 P4 P3
Cheirogaleus major [68]
[M1 I1 -I2 -C M2 ] P2 M3 P4 P3
Cheirogaleus medius [68]
[M1 I1 -I2 -C M2 ] P2 M3 P4 P3
Allocebus trichotis [68]
[M1 I1 -I2 -C M2 ] P2 M3 P4 P3
Megaladapis edwardsi [69]
M1 I1 -I2 -C M2 P M3 PP
Lepilemur mustelinus [68]
M1 M2 I1 -I2 -C P4 M3 P3 P2
Archaeolemur majori [68,69]
M1 M2 I1 -I2 M3 P4 P3 P2
Archaeolemur edwardsi [69]
M1 M2 PP M3 IIP
Hadropithecus stenognathus [68,69]
M1 M2 I1 -I2 M3 P4 P3 P2
Avahi laniger [68,69]
M1 I1 -I2 P4 P3 M2 M3
Propithecus verreauxi [68,69]
M1 I1 -I2 M2 P4 M3 P3
Propithecus diadema [68,69]
M1 I1 -I2 M2 P4 P3 M3
Hapalemur griseus [68]
M1 I1 -I2 -C M2 P4 M3 P3 P2
Lemur catta [42,70]
M1 M2 I1 -I2 -C P4 P3 P2 M3
Eulemur mongoz [71]
M1 I1 -I2 -C M2 M3 P2 P4 P3
Eulemur rufus [42,70]
M1 I1 -I2 -C M2 P2 M3 P4 P3
Eulemur macaco [71,72]
M1 I1 -I2 -C M2 P2 M3 P4 P3
Varecia sp. [42,73]
M1 I1 -I2 -C M2 P2 M3 P4 P3
Otolemur crassicaudatus [74]
M1 I1 -I2 -C M2 M3 P2 P4 P3
Sciurocheirus alleni [74]
M1 I1 -I2 -C M2 P2 M3 P4 P3
Galago senegalensis [74]
M1 I1 -I2 -C M2 P2 M3 P4 P3
Galago gallarum [74]
M1 I1 -I2 -C M2 M3 P2 P4 P3
Galago moholi [74]
M1 I1 -I2 -C P2 M2 M3 P4 P3
Galagoides demidovii [74]
M1 M2 I1 -I2 -C M3 P2 P4 P3
Loris tardigradus [74]
I1 -I2 -C M1 M2 P2 M3 P4 P3
Nycticebus javanicus [74]
I1 -I2 -C M1 M2 P2 M3 P4 P3
Nycticebus coucang [74]
I1 -I2 -C-M1 M2 P2 M3 P4 P3
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Perodicticus potto [74]
I1 -I2 -C M1 P2 M2 M3 P4 P3
Notharctus tenebrosus [18,44]
M1 P1 M2 M3 P2 P4 P3
Adapis parisiensis [2,44,75]
M1 P1 M2 M3 P4 P3 P2
Sivaladapis nagrii [1]
M1 I1 P2 (C P4 ) P3
Tarsiidae [76]
[M1 P2 ] I1 M2 M3 -C P4 P3
Homunculus patagonicus* [77]
[M1 I1 I2 ] M2 P2 P4 P3 M3 C
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(Continued.)
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taxon
Darwinius masillae [17]
4
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Table 1. List of lower permanent dental eruption sequences for 97 taxa. Taxa marked with an asterisk (*) had published information on
only upper dentition. The time of eruption between lower and upper dentition differs, but the sequence of eruption is usually the same
for both dentitions. Parentheses () group teeth that in a fossil are either all emerged or all have not emerged yet. Square brackets [ ]
surround teeth when actual sequence has not yet been resolved. Simultaneous eruptions are indicated with teeth united by hyphens, i.e.
toothcombs.
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Table 1. (Continued.)
5
Saguinus fuscicollis [78]
M1 I1 I2 M2 P4 P2 P3 C
Saguinus oedipus [46]
M1 I1 [I2 M2 ] [P4 P2 ] P3 C
Saguinus midas [46]
M1 I1 I2 M2 P4 P2 P3 C
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Saguinus mystax [46]
M1 I1 I2 M2 [P4 P2 ] P3
Saguinus bicolor [46]
M1 I1 [I2 M2 ] P4 P2 P3
Leontopithecus sp. [79]
M1 I1 I2 M2 P2 P3 P4 C
Callimico goeldii [79]
M1 [M2 I1 I2 ] [P4 P2 P3 M3 C]
Cebuella pygmaea [79]
M1 [M2 I1 I2 ] [P4 P2 P3 ] C
Callithrix jacchus [80,81]
M1 M2 I1 [I2 P4 ] P3 [P2 C]
Mico argentatus [46,81]
M1 [M2 I1 ] [I2 P4 ] P3 P2 C
Mico humeralifer [46,81]
M1 M2 [I1 I2 ] P2 P4 P3 C
Aotus trivirgatus [65]
M1 M2 I1 M3 I2 P4 P3 P2 C
Cebus capucinus [79]
M1 I1 I2 M2 P4 P3 P2 [C M3 ]
Cebus albifrons [79]
M1 I1 I2 M2 P2 P3 P4 ? [C M3 ]
Sapajus apella [79]
M1 I1 I2 M2 P4 P2 P3 C M3
Saimiri sciureus [70]
M1 M2 I1 I2 M3 P4 P2 P3
Alouatta sp. [79]
M1 I1 I2 M2 P2 P3 [P4 M3 ] C
Stirtonia victoriae* [47]
(M1 I1 I2 ) M2 (P2 P4 P3 M3 C)
Lagothrix sp. [79]
M1 I1 I2 M2 P2 P3 ? P4 ? M3 C
Ateles sp. [79]
M1 I1 I2 P2 M2 P4 P3 C M3
Brachyteles sp. [79]
M1 I1 I2 M2 P2 P3 P4 ? C M3
Chiropotes sp. [79]
M1 I1 I2 M2 P2 P3 P4 ? C M3
Cacajao sp. [79]
M1 M2 I1 I2 P2 ? P3 ? P4 ? M3 C
Pithecia sp. [79]
M1 M2 I1 I2 M3 P4 P2 P3 C
Callicebus sp. [79]
M1 I1 I2 M2 P2 P4 P3 M3 C
Apidium phiomense [66]
M1 M2 P2 P4 (P3 M3 ) C
Parapithecus grangeri [72]
M1 M2 P2 P4 (P3 M3 ) C
Chlorocebus pygerythrus [65]
M1 I1 I2 M2 P4 P3 C M3
Cercopithecus ascanius* [82]
M1 I1 I2 M2 [P3 P4 ] M3 C
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Macaca nemestrina [83]
M1 I1 I2 M2 P4 P3 C M3
Macaca mulatta [83]
M1 I1 I2 M2 P3 P4 C M3
Macaca fascicularis [84]
M1 II M2 PP M3
Paradolichopithecus arvernensis [74]
M1 I1 I2 M2 P3 P4 C M3
Papio anubis [83]
M1 I1 I2 M2 C [P4 P3 ] M3
Papio cynocephalus [83]
M1 I1 I2 M2 P4 P3 C M3
Papio hamadryas hamadryas [85]
M1 M2 I1 P3 P4 I2 C M3
Theropithecus gelada [86]
M1 I1 I2 M2 P3 P4 C M3
Lophocebus albigena* [82]
M1 I1 I2 [P3 P4 ] M2 C M3
Mandrillus sphinx [83]
M1 I1 I2 M2 P4 P3 C M3
Kuseracolobus aramisi [87]
M1 I1 I2 M2 M3
Mesopithecus pentelicus [87]
M1 I1 M2 I2 PP C M3
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(Continued.)
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permanent dental eruption sequence
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Table 1. (Continued.)
6
Piliocolobus badius [87]
M1 [I1 I2 M2 ] [P4 P3 ] [C M3 ]
Procolobus verus [87]
M1 I1 M2 I2 P3 P4 C M3
Colobus angolensis [83]
M1 M2 I1 I2 P4 P3 M3 C
Colobus guereza [83]
M1 I1 I2 M2 P4 P3 M3 C
Presbytis sp. [83]
M1 M2 I1 I2 M3 P4 P3 C
Semnopithecus priam [85]
M1 I1 I2 P4 C P3 M2 M3
Trachypithecus sp. [88]
M1 I1 [I2 M2 ] P4 P3 [C M3 ]
Nasalis larvatus [87,89]
M1 I1 I2 M2 [PP C] M3
Pygathrix sp. [88,89]
M1 M2 I1 I2 M3 P4 P3
Victoriapithecus macinnesi [87,90]
M1 M2 PP
Hylobates lar [85]
M1 I1 I2 M2 P3 P4 M3 C
Symphalangus syndactylus [85]
M1 I2 I1 M2 P4 P3 C M3
Pongo sp. [41]
M1 I1 I2 M2 P4 P3 C M3
Gorilla sp. [41]
M1 I1 I2 M2 P4 P3 C M3
Pan troglodytes [65]
M1 I1 I2 M2 [P3 P4 ] M3 C
Australopithecus africanus [91]
M1 I1 I2 M2 P3 P4 C M3
Homo sapiens (Australian aboriginal) [65]
[M1 I1 ] I2 C P3 [M2 P4 ] M3
Homo sapiens (White American) [65]
[I1 M1 ] I2 [C P3 P4 M2 ] M3
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a I to P are not erupted.
2
3
Table 2. Description of the characters used in the ancestral state reconstruction analysis. Characters are treated as unweighted and
unordered.
no.
character
states
1
eruption of replacement teeth
0: after molar eruption; 1: first erupted replacement tooth
erupts before the last erupted molar
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2
premolar eruption sequence
0: 2-3-4; 1: 2-4-3; 2: 4-2-3; 3: 4-3-2; 4: absence of P2
3
premolar eruption sequence (if no P2 )
0: 3-4; 0: 4-3
4
eruption of P2 relative to M3
0: P2 erupts after M3 ; 1: P2 erupts before M3
5
eruption of P3 relative to M3
0: P3 erupts after M3 ; 1: P3 erupts before M3
6
eruption of I1 relative to the earliest premolar
0: I1 erupts after the earliest premolar; 1: I1 erupts before
the earliest premolar
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7
simultaneous eruption of I1 , I2 , and C (or I1 and I2 only)
0: not simultaneous; 1: simultaneous
8
number of incisors erupting after M2
0: 3; 1: 2; 2: 1; 3: 0
9
number of premolars erupting after M3
0: 3; 1: 2; 2: 1; 3: 0
10
eruption of the incisors relative to M3
0: all incisors erupt after M3 ; 1: M3 erupts between two
incisors; 2: all incisors erupt before M3
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.........................................................................................................................................................................................................................
11
eruption of the incisors relative to the premolars
0: the earliest incisor erupts after the latest premolar;
1: intermediate situation; 2: the latest incisor erupts
before the earliest premolar
.........................................................................................................................................................................................................................
12
eruption of the premolars relative to M2 *
0: all premolars erupt after M2 ; 1: at least one premolar
erupts before M2 . (*) Coded as inapplicable if P1 is
present
.........................................................................................................................................................................................................................
13
eruption of M1
0: first tooth to erupt; 1: not the first tooth to erupt
14
eruption of P4 relative to M3
0: P4 erupts after M3 ; 1: P4 erupts before M3
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
................................................
permanent dental eruption sequence
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taxon
.........................................................................................................................................................................................................................
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Euprimates
Haplorhini
Anthropoidea
Figure 2. Phylogenetic relationships of the 97 fossil and extant taxa used in this analysis. The ancestral nodes for Euprimates, stem
Strepsirrhini, crown Strepsirrhini, Haplorhini and Anthropoidea are indicated. Combined cladogram from Marivaux et al. [92], Gunnell
[93], Arnold et al. [94], Silcox et al. [95], Steiper & Seiffert [27], Kay [96], Kistler et al. [97] and Seiffert et al. [29].
The cladogram (figure 2) used in this analysis is a supertree based on Marivaux et al. (used for
placing Sivaladapis [92]), Gunnell (for Notharctus [93]), Arnold et al. (for all living primates [94; v. 3],
Silcox et al. (for plesiadapiforms and Tupaia [95]), Steiper & Seiffert (for Victoriapithecus [27]), Kay (for
................................................
stem
Strepsirrhini
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crown
Strepsirrhini
Dymecodon pilirostris
Tupaia glis
Acidomomys hebeticus
Plesiadapidae
Microcebus murinus
Mirza coquereli
Allocebus trichotis
Cheirogaleus major
Cheirogaleus medius
Lepilemur mustelinus
Archaeolemur majori
Archaeolemur edwardsi
Hadropithecus stenognathus
Avahi laniger
Propithecus verreauxi
Propithecus diadema
Megaladapis edwardsi
Hapalemur griseus
Lemur catta
Eulemur mongoz
Eulemur rufus
Eulemur macaco
Varecia sp.
Otolemur crassicaudatus
Schiurocheirus alleni
Galago senegalensis
Galago moholi
Galago gallarum
Galagoides demidovii
Loris tardigradus
Nycticebus javanicus
Nycticebus coucang
Perodicticus potto
Notharctus tenebrosus
Adapis parisiensis
Sivaladapis nagrii
Tarsiidae
Apidium phiomense
Parapithecus grangeri
Saguinus fuscicollis
Saguinus bicolor
Saguinus midas
Saguinus oedipus
Saguinus mystax
Leontopithecus sp.
Callimico goeldii
Cebuella pygmaea
Mico argentatus
Mico humeralifer
Callithrix jacchus
Aotus trivirgatus
Cebus capucinus
Cebus albifrons
Sapajus apella
Saimiri sciureus
Alouatta sp.
Stirtonia victoriae
Lagothrix sp.
Brachyteles sp.
Ateles sp.
Chiropotes sp.
Cacajao sp.
Pithecia sp.
Callicebus sp.
Homunculus patagonicus
Chlorocebus pygerythrus
Cercopithecus ascanius
Macaca nemestrina
Macaca fascicularis
Macaca mulatta
Paradolichopithecus arvernensis
Papio anubis
Papio hamadryas hamadryas
Papio cynocephalus
Lophocebus albigena
Theropithecus gelada
Mandrillus sphinx
Kuseracolobus aramisi
Mesopithecus pentelicus
Piliocolobus badius
Procolobus verus
Colobus angolensis
Colobus guereza
Presbytis sp.
Semnopithecus priam
Trachypithecus sp.
Nasalis larvatus
Pygathrix sp.
Victoriapithecus macinnesi
Hylobates lar
Symphalangus syndactylus
Pongo sp.
Gorilla sp.
Pan troglodytes
Australopithecus africanus
Homo sapiens (Austr. aborig.)
Homo sapiens (White American)
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Stirtonia and Homunculus [96]), Kistler et al. (for Hadropithecus and Megaladapis [97]) and Seiffert et al. (for
Adapis, Apidium and Parapithecus [29]) (see also electronic supplementary material, text S1). Although the
trees that we used are considered widely accepted, we acknowledge that there are other phylogenies
that might contradict some of the positions in the tree [28,98]. The supertree also reflects discussions
on phylogenetic relationships from Strasser & Delson (on Paradolichopithecus and Mesopithecus [99]),
Shoshani et al. (on Archaeolemur [100]) and Frost (on Kuseracolobus [101]). Dental eruption sequences
exist for two other platyrrhine species that are not included in the analysis, Saguinus nigricollis [102]
and S. graellsi [46], but Arnold et al.’s [94] tree did not include them. Because all Saguinus species
have very similar dental eruption sequences, we are confident that the exclusion of these two species
would not alter the reconstruction of the ancestral anthropoid condition. The outgroups used for the
ancestral euprimate node were plesiadapiform taxa. Plesiadapiforms were a group of small, arboreal,
archaic mammals widespread during the Palaeocene and the Eocene throughout North America, Europe
and Asia. Representatives of plesiadapiforms in our tree include plesiadapids and paromomyids. Here,
plesiadapiforms are considered stem primates (following [103–114]). A representative of Scandentia,
Tupaia glis, as outgroup for Primates (including plesiadapiformes) has been used. Although ideally
we would have also included a dermopteran taxon, no permanent dental eruption sequences for
dermopterans have been published. Although the lack of dermopterans in the cladogram would have
been problematic if the ancestral primate node was reconstructed, the lowest node reconstructed is the
ancestral euprimate node. Because plesiadapiforms are inferred to be closer to Euprimates than any
other groups (following Silcox et al. [95]), under the Outgroup Algorithm [115] they have the greatest
impact on polarizing the euprimate node, so it is very unlikely for dermopterans to produce a new
unequivocal resolution at that node. Also, the most comprehensive relevant study supports Sundatheria
(the combined clade of Scandentia and Dermoptera) as the living sister group of Primates [51], rather
than Dermoptera alone [116] or Scandentia alone [117]. This makes Dermoptera no more relevant than
the included scandentian in reconstructing the basal euprimate node. The dental eruption sequence of
Dymecodon pilirostris (Soricomorpha) has been added as a primitive representation of a mammal dental
eruption sequence to further constrain the basal euprimate node. Temporal branch length information
for living taxa was taken from Arnold et al. [94]. The branch lengths for fossil taxa originate from many
sources and the choices of dates of appearance of lineages are reported and explained in electronic
supplementary material, table S4, and electronic supplementary material, figure S2.
Ancestral state reconstructions were executed in the MESQUITE v. 3.01 software package [118], using
parsimony. The generalized parsimony algorithm can be applied to optimization of a character of
unknown polarity onto a rooted tree, and no additional algorithmic complications are presented by trees
containing polytomies [55]. MESQUITE also allows missing data when using the parsimony algorithm,
but cannot do likelihood calculations with gaps or soft polytomies. Because the data include a significant
proportion of relevant fossil taxa, which sometimes produce partially complete eruption sequences,
it is important to apply software and an algorithm that can accommodate these limitations, making
parsimony the best option. Ancestral permanent dental eruption sequences were reconstructed for five
hypothetical ancestors: (i) Euprimates, (ii) stem Strepsirrhini, (iii) crown Strepsirrhini, (iv) Haplorhini,
and (v) Anthropoidea. For the ancestral state reconstruction, only lower permanent dental eruption
sequences were used, because they are more often reported in the literature, thus increasing the number
of taxa available for analysis. Canines were not included in the reconstruction because their time of
eruption appears to be influenced by sexual dimorphism [42,82]. Although some primitive primates
retain P1 , eruption data on first premolars are not included in this analysis because the loss of this tooth
early in primate evolution renders this character ambiguous at several nodes. To generate the ancestral
eruption sequences, all hypothetical ancestors were assumed to have a P2 . The ambiguities in ancestral
state reconstructions for the characters ‘Eruption of P2 relative to M3 ’, ‘Eruption of I1 relative to the
earliest premolar’ and ‘Eruption of incisors relative to premolars’ are resolved with restrictions implied
by other characters. For the five nodes studied here, the ancestral state reconstructions result in one
permanent dental eruption sequence for each hypothetical ancestor. The ancestral reconstruction analysis
was also run without including fossil data (see electronic supplementary material, figure S1 and text S2),
to evaluate the effect of fossils on the reconstruction.
As discussed in detail below, several lemurids that were similar to Darwinius in body mass and lifehistory variables were chosen as sources of comparative data. Age-specific body mass data for Eulemur
macaco, Eulemur rufus and Varecia variegata (V. v. variegata) were taken from the Duke Lemur Center (DLC)
database [119]. Although ‘Ida’ has been inferred to be female [17], because this inference is based on
negative evidence (see above), we have followed the conservative course of including both male and
female data in our analysis. None of the living species are known to be sexually dimorphic, and use of
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ancestral node
dental eruption sequence
Euprimates
M1 I1 I2 M2 P2 M3 P4 P3
stem Strepsirrhini
M1 I1 I2 M2 P2 M3 P4 P3
crown Strepsirrhini
M1 I1 -I2 -C M2 P2 M3 P4 P3
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
Haplorhini
M1 I1 I2 M2 P2 M3 P4 P3
Anthropoidea
M1 I1 I2 M2 P2 P4 P3 M3
Darwinius masillae
(M1 M2 I1 P2 ) (I2 C M3 P4 P3 )a
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
a I to P are not erupted.
2
3
just the female data led to extremely similar results (not shown). The percentage of adult body mass
achieved at ‘Ida’s’ age at death was determined using the age-specific mass estimated from the Lowess
regressions from the three lemurid species, and adult mean body mass estimates from each regression.
Lowess regressions were obtained using PAST [120]. Dental eruption times for the three lemurids are
taken from Smith et al. [42].
It is important to note that some of the specimens from which the data were collected in Eaglen [73],
and subsequently reported in Smith et al. [42] are currently assigned to other taxa. What Eaglen [73]
classifies as Eulemur fulvus is currently ascribed to E. rufus according to the DLC, and consequently will
be referred to as E. rufus in this paper. Also, the V. variegata sample in Eaglen [73] is composed of a mixture
of individuals of V. variegata and hybrids of V. variegata and V. rubra [119]. Therefore, in this paper we are
assigning the dental eruption sequence for these specimens to the genus Varecia in general.
The dental eruption sequence that we use for Saimiri (M1 M2 I1 I2 M3 P4 P2 P3 ) is the same one as
that used by Franzen et al. [17], as the purpose of this paper is to reassess the viability of the original
anthropoid model. However, another eruption sequence is known for Saimiri [79], which differs in the
relative time of eruption of the third molar (M1 M2 I1 I2 P4 P2 P3 M3 ). While we use Franzen et al.’s [16]
eruption sequence throughout the paper, we discuss in the conclusion how Henderson’s [79] eruption
sequence would influence conclusions about the Saimiri model.
3. Results and discussion
3.1. Ancestral reconstruction of permanent dental eruption sequences
Ancestral dental eruption sequences were reconstructed for five nodes (euprimates, stem strepsirrhines,
crown strepsirrhines, haplorhines and anthropoids; table 3), based on the ancestral state reconstruction
for the 14 characters (see electronic supplementary material, table S2).
The earliest members of Adapoidea and Omomyoidea are very similar in dental morphology [49], so
unsurprisingly there is little variation in eruption sequence inferred for the ancestral nodes. Our ancestral
reconstruction suggests two clear trends in the evolution of eruption sequences. The strepsirrhine
line is characterized by a primitive dental eruption sequence at the base of stem Strepsirrhini that
matches the one inferred for the basal euprimate. Subsequently, this primitive sequence is modified in
crown strepsirrhines by the simultaneous eruption of the incisors, along with the canine, in association
with the evolution of the toothcomb. The haplorhine line is similarly marked by a primitive basal
eruption sequence that resembles the basal euprimate sequence, and then it is characterized by a late
eruption of M3 at the base of anthropoids. There are several genera of anthropoids in which M3 erupts
comparatively early (Saimiri, Aotus, Pithecia, Pygathrix and Presbytis [65,70,79,83,88,89]), but based on the
distribution of this trait on this tree this is inferred here to represent evolutionary events occurring in
the context of Anthropoidea, like the loss of M3 in callitrichines. In contrast to the inferred primitive
state for anthropoids, Darwinius exhibits early eruption of M3 suggesting that it was more strepsirrhinelike. These results therefore make it difficult to determine the relationship of adapoids to either stem
strepsirrhines or basal haplorhines, because they both present the same dental eruption sequence.
However, these results are less consistent with the Adapoid–Anthropoid hypothesis because adapoids
appear to lack the delay of M3 eruption, a synapomorphic characteristic of primitive anthropoids.
................................................
ancestral permanent
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Table 3. Reconstructed ancestral permanent dental eruption sequences for five primate nodes (see electronic supplementary material,
table S2 for the nodal reconstructions on which these sequences were based). Parentheses () group teeth that in a fossil are either all
emerged or all have not emerged yet.
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Based on the contrasts between anthropoids and Darwinius in the ancestral state reconstruction of dental
eruption patterns, Saimiri may not be a good model for the growth of Darwinius, as previously proposed
by Franzen et al. [17]. But, contrary to the general anthropoid trend, Saimiri is a fast-growing platyrrhine
that, like Darwinius, exibits an early eruption of M3 [70]. The ancestral state reconstruction analysis,
however, indicates that this early eruption of the third molar appears secondarily in Saimiri. Because
the hypothesized relationships of Darwinius are to stem anthropoids [30], not Platyrrhini generally, or
Saimiri specifically, this similarity would necessarily be a case of homoplasy. Also, cebids generally show
a late eruption of P2 [70,79], with the exception of Cebus albifrons [79], in which it appears to be quite
variable [72]. Saimiri especially stands out among the Cebidae for having one of the latest eruptions of the
second premolar. Therefore, this pattern contrasts markedly with that observed in Darwinius, in which
this tooth is already erupted, with five teeth still remaining unerupted. This has profound implications
for calculating the age at death, because it is after the eruption of P2 when the sequence is interrupted
by death in Darwinius [17,44]. It is worth noting that Henderson [79] provides another dental eruption
sequence for Saimiri in which the relative time of eruption of the M3 is markedly later. One of the most
convincing arguments in favour of the Saimiri model is that squirrel monkeys have, according to the
sequence used in Franzen et al. [17], one of the most strepsirrhine-like dental eruption sequences among
anthropoids, precisely because of an earlier relative eruption of M3 . In the light of this fact, if Henderson’s
[79] sequence is correct, Saimiri would make an even less appropriate model.
By contrast, both stem strepsirrhines and basal haplorhines would make good models for the
growth of Darwinius, because of their primitive-looking dental sequences. The only non-anthropoid
haplorhine taxa in our sample are tarsiids, which exhibit a dental eruption sequence (table 1) which
differs from that inferred for the basal haplorrhine (i.e. extremely early eruption of P2 , lack of I2 , and
simultaneous eruption of M3 and C), making tarsiers a poor choice as model taxa. On the other hand,
stem strepsirrhines and Darwinius also share early eruption of M3 and P2 . Given that stem strepsirrhines
are known only from extinct taxa, without direct information available about their age-specific growth
and development, a new growth model based on living strepsirrhines is needed for Darwinius.
Three families of strepsirrhines primitively share a dental eruption sequence similar to that of
Darwinius: Lemuridae, Galagidae and Cheirogaleidae [42,68,73,74]. Galagidae and Cheirogaleidae are
significantly smaller than caenopithecids [3]. Generally, in mammals, most life-history variables are
correlated to body mass [121], making these very small primates inappropriate models for Darwinius.
On the other hand, lemurids exhibit similar body masses to caenopithecids [3], which makes them a
more reasonable model. Dental eruption sequences are known from six lemurids: Lemur catta, Hapalemur
griseus, E. rufus, E. macaco, E. mongoz and Varecia sp. [73]. However, L. catta and H. griseus would make
poor models for the growth of Darwinius because the eruption of P2 in these two species occurs much
later in the sequence, and, as discussed above, this tooth is of critical importance in determining the age
at death of this particular specimen. Lemur catta and H. griseus also possess a premolar eruption sequence
of 4-3-2, which is derived in the context of Lemuridae, instead of the primitive 2-4-3 pattern found in the
rest of lemurids and stem strepsirrhines. Among the three Eulemur species, E. mongoz differs the most
from Darwinius in having a late eruption of P2 . It would be preferable to apply a model based on species
with earlier P2 eruptions. Like E. rufus and E. macaco, Varecia has a 2-4-3 premolar eruption pattern and
an early P2 eruption. Additionally, these fast-growing primates are fairly well studied, making them the
best living models available for the growth of Darwinius.
................................................
3.2. Reassessment of the Saimiri model
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The crown strepsirrhines show another distinctive feature: the presence of a toothcomb. The eruption
of a toothcomb results in the almost simultaneous emergence of the incisors and the canine. The eruption
of the toothcomb is generally early for crown strepsirrhines [42,68,69,71,73,74], with the exception of
Archaeolemur edwardsi [69]. However, the pattern of eruption for the toothcomb does not differ markedly
from the eruption pattern of incisors for euprimates, stem strepsirrhines or Darwinius, all of which share
an early and contiguous eruption of incisors.
The ancestral reconstruction analysis not including fossil taxa provides similar results (electronic
supplementary material, table S3), but there is a substantial difference in the final reconstruction. In
the analysis that excludes fossils it is not possible to unequivocally reconstruct the primitive premolar
eruption sequence for Anthropoidea, with four different states being inferred to be equally parsimonious.
This is particularly problematic because the time of eruption of P2 is crucial for the study of life history of
Darwinius specifically. Therefore, the inclusion of fossil data in the analysis is required to resolve relevant
ancestral state reconstructions.
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(a)
11
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3600
3200
2800
weight (g)
2400
2000
1600
1200
800
400
0
(b)
3600
3200
2800
weight (g)
2400
2000
1600
1200
800
400
0
(c)
6400
5600
weight (g)
4800
4000
3200
2400
1600
800
0
0
0.8
1.6
2.4
3.2
4.0
age (years)
4.8
5.6
6.4
7.2
8.0
Figure 3. Lowess regressions illustrating patterns of ontogeny for individuals of three lemurid species from birth to the age of 8 years.
Smoothing factor of 0.1 for all regressions. (a) Eulemur macaco; (b) Eulemur rufus; and (c) Varecia variegata. Vertical lines indicate the
supposed interruptions of the sequence in Darwinius.
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Our ancestral state reconstruction infers the same dental eruption sequences for basal Euprimates, stem
Strepsirrhini and basal Haplorhini. These hypothesized primitive sequences resemble that of Darwinius
in the early eruption of M3 and the non-simultaneous eruption of I1 -I2 -C in contrast to anthropoids and
crown strepsirrhines, respectively. The late eruption of M3 in anthropoids and the fact that M3 seems to
be the next tooth to erupt in Darwinius at the moment of her death suggests that anthropoids likely do
not provide the most appropriate model for estimating growth in adapoids, including Darwinius. The
eruption of P2 is important for defining the interruption of the sequence in this particular specimen, and
the late eruption of P2 in Saimiri suggests that this genus in particular does not represent a good model
for Darwinius.
Our results also suggest that eruption sequences carry useful phylogenetic information. Although
variable to some extent, higher level primate taxa (e.g. crown Strepsirrhini, Anthropoidea) can be
grouped based on different trends in eruption sequences. Therefore, the study of eruption sequences
can contribute to our understanding of primate phylogenetic relationships, in a way that allows for the
incorporation of fossil material. In this case, the contrast between the inferred late eruption of M3 in the
common ancestor of Anthropoidea, and the advanced stage of development of this tooth in Darwinius,
could be interpreted as conflicting with the Adapoid–Anthropoid hypothesis.
The lemurid model for the development of Darwinius proposed in this study does not categorically
invalidate the Saimiri model. However, it is an alternative in closer agreement with the more similar
dental eruption sequences found in strepsirrhines. Also, it agrees with the currently most widely
supported hypothesis for adapoid relationships: the Adapoid–Strepsirrhine hypothesis. This model
suggests an older age at death (1.05–1.14 years, depending on the model used) than previously proposed
(0.75–0.83 years [17]). Our model also suggests a narrower range for the projected adult weight (622–
642 g) and entirely below the previously proposed (650–900 g [16]), consistent with caenopithecid range
of body masses. Although the current data on lemurid growth are sufficient for certain species of
lemurids, better documentation of data on growth, and development, and eruption sequences for more
lemurid species would certainly improve the quality of potential new models.
Data accessibility. All supporting data are available as the electronic supplementary material.
Authors’ contributions. S.L.T. collected the data and drafted the manuscript. All authors conceived of the study,
participated in the design of the study, participated in data analysis, interpreted the data, revised the content of the
article and gave final approval for publication.
Competing interests. The authors declare that they have no competing interests.
Funding. This work was supported by an NSERC Discovery grant to M.T.S.
Acknowledgements. We are grateful to S. R. Leigh and J. G. Fleagle for providing access to unpublished data. We thank
K. D. Rose, M. A. O’Leary and M. Godinot for their insight on adapoid P2 variability. We thank C. L. Makowski for
observations relevant to the assessment of the sex of Darwinius. Thanks to K. Padian and anonymous reviewers for
comments that substantially improved this paper.
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The age of eruption of P2 in E. rufus is 1.14 years, 1.05 years in E. macaco and 1.06 years in Varecia.
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