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About the alleged record of the Rheidae Diogenornis in the Cenozoic of Argentina: new
interpretations
Carolina Acosta Hospitaleche
a
and Mariana B.J. Picasso
b
a
División Paleontología de Vertebrados, Museo de La Plata. Paseo del Bosque s/n B1900FWA, La Plata, Facultad de Ciencias Naturales y, La Plata,
Universidad Nacional de La Plata, Argentina;
b
División Zoología Vertebrados, Museo de La Plata, Paseo del Bosque s/n B1900FWA, La Plata, Facultad de
Ciencias Naturales y, La Plata, Universidad Nacional de La Plata, CONICET, Argentina
ABSTRACT
A tibiotarsus (MACN-A 12195) collected by Ameghino in the Sarmiento Formation (Eocene–lower Miocene)
outcropping in the ‘Río Chico locality’ (Chubut Province, Argentina) assigned to Diogenornis (Rheiformes,
Palaeognathae), and a second limb bone (MPEF-PV 1411) from early Miocene levels of the Sarmiento
Formation in Bryn Gwyn (Chubut Province, Argentina) assigned to a bird that resembles Diogenornis, were
re-examined here. After a thorough comparison, we established that MACN-A 12195 corresponds to an
indeterminate Palaeognathae morphologically close to Rheidae, Struthionidae, and Casuariidae, whereas
MPEF-PV 1411 belongs to a Sphenisciformes, probably Palaeospheniscus. The presence of Diogenornis
outside of Brazil cannot be conrmed, and therefore, its biochron and distribution area should be still
restricted to the early Eocene of Itaboraí (Brazil).
ARTICLE HISTORY
Received 27 May 2022
Accepted 1 July 2022
KEYWORDS
Rheiformes; Palaeognathae;
Sarmiento Formation;
Patagonia; South America
Introduction
The fossil record of South American Cenozoic birds includes
rich assemblages of Palaeognathae and Neognathae birds
among which Rheiformes are widely represented (e.g.,
Noriega et al. 2017; Tambussi et al. 2021). During the last
decades, the known taxonomic diversity of this group
increased due to the description of several new species for
Argentina, some of them highly questionably or pending
further revision (Picasso et al. 2022). Moreover, besides the
new taxa recently erected, the assignment of several indeter-
minate remains coming from Patagonia to the monotypic
genus Diogenornis, increased the number of taxa present in
Argentina (see for example Agnolin 2017).
Although currently assigned to the Rheidae (Mayr 2009,
2017), Diogenornis fragilis was described first as
a Opisthodactylidae (Alvarenga 1983) and considered even
a Casuariidae (Alvarenga 2010). Diogenornis fragilis
Alvarenga, 1983, from the early Eocene Itaborai Formation at
the Bacia de São José de Itaboraí (Rio de Janeiro) is one of the
oldest Rheiformes known so far. It is worth clarifying here that
previous contributions listed this species among the
Palaeocene taxa (e.g., Tambussi 1995; Taranto 2012;
Tambussi and Degrange 2013; Agnolin 2017; Noriega et al.
2017; Tambussi et al. 2021), but the updated stratigraphic
information does not rise doubts to its regarding its Eocene
age (Picasso et al. 2022 and references cited there).
The holotype of Diogenornis fragilis consists of two tibio-
tarsi (left and right) and a right tarsometatarsus without the
distal end (DGM 1421 R; see acronyms below) that exhibit an
immature condition (Alvarenga 1983). Besides, other referred
materials belonging to adults were designated as paratype and
include two incomplete tarsometatarsi (DGM 422/DGM
423 R), four fragments of tarsometatarsi (MNRJ 4033/MNRJ
4036 V), a proximal end of humerus (DGM 1424 R), vertebrae
and fragment of vertebrae (DGM 1425 R-DGM 1427, DGM
1429, DGM 1437, MNRJ 4037 V/MNRJ 4038 V), a fragment of
beak (DGM 1428 R), and a proximal phalanx (DGM 1430 R)
(Alvarenga 1983).
Besides, two specimens collected in the Argentinian
Patagonia (Figure 1) were identified as putative Diogenornis
during the last decade. The first one, a hindlimb remain
(MPEF-PV 1411) from the early Miocene of the Sarmiento
Formation that outcrops in the Bryn Gwyn locality (Chubut
Province, Argentina) was identified as a tibiotarsus that resem-
ble Diogenornis (Agnolin and Cenizo 2014). This record, pub-
lished in a short abstract without any picture, would expand
the biochron and distribution area of Diogenornis to the early
Miocene of Patagonia.
The second, a portion of tibiotarsus MACN-A 12195 (52–
195) collected in undetermined levels of the Sarmiento
Formation (Eocene–lower Miocene) outcropping in the ‘Río
Chico locality’ (Chubut Province, Argentina) was determined
as Diogenornis (Agnolin and Cenizo 2014) and later described
as a Rheidae cf Diogenornis (Agnolin 2017). An incomplete
trochlea was inventoried together with the distal tibiotarsus,
under the same repository number of the Ameghino collection,
which is the reason why it was included in the present
revision.
Encouraged by the stratigraphic and geographic connota-
tions that the presence of Diogenornis would have in the
Miocene of Patagonia, we revised these records. The aim of
the present contribution is to re-study specimens MPEF-PV
1411 and MACN-A 12195, previously determined as
a tibiotarsus reminiscent of a Diogenornis and a Rheidae cf
Diogenornis, respectively. A detailed description of the fossils,
accompanied by updated and precise stratigraphic informa-
tion, and comparative figures is provided in this context.
CONTACT Carolina Acosta Hospitaleche acostacaro@fcnym.unlp.edu.ar CONICET. División Paleontología de Vertebrados, Museo de La Plata. Paseo del Bosque s/n
B1900FWA, La Plata, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, La Plata, Argentina
HISTORICAL BIOLOGY
https://doi.org/10.1080/08912963.2022.2098489
© 2022 Informa UK Limited, trading as Taylor & Francis Group
Material and methods
Specimens under study belong to the Ameghino Collection of
the Museo Argentino de Ciencias Naturales ‘Bernardino
Rivadavia’ (MACN-A), Ciudad de Buenos Aires, Argentina,
and Museo Paleontológico Egidio Feruglio (MPEF-PV),
Trelew, Argentina. Comparative materials belong to the Museo
de La Plata (MLP, MLP-O), Ciudad de La Plata, Argentina,
Smithsonian National Museum of Natural History (USNM),
and are listed in Appendix 1.
Unfortunately, the recent tragedy, in September 2018, in
which the National Museum of Brazil caught fire resulted in
the destruction of all the materials with the acronym MNRJ
mentioned above. Consequently, the only remains of
Diogenornis still available are those housed at the Division of
Geology and Mineralogy in Rio de Janeiro, Brazil, identified
with the prefix DGM. Our efforts to localise and see the mate-
rials were totally unsuccessful, and we could only determine
that they are still on loan outside their permanent repository.
For that reason, our only option was to compare Diogenornis
from the characters and images published in Alvarenga (1983:
Figure 1; 2010), Berqvist (2006: fig. 6.2) and Mayr (2009: fig.
5.2; 2022: fig. 3.5).
Anatomical nomenclature follows Baumel and Witmer (1993). All
measurements were taken with a Vernier Caliper of 0.01 of increment.
Systematic palaeontology
Infraclass Palaeognathae Pycraft, 1900
Order ?Rheiformes
Figure 2
Referred material
Distal end of left tibiotarsus MACN-A 12195 (52–195) (Figure 2(a-
e)), undetermined distal fragment of trochlea (Figure 2(f-h)).
Preserved measurements(Figure 2(a-d)
Measurements (taken and expressed in mm): 1, proximo-distal
height of the condylus medialis measured on the cranial face
14.67; 2, Preserved latero-medial distal width 26.76; 3, distance
between the medial tuberositas retinaculi extensoris and the medial
face of the condylus medialis, 9.49; 4, sulcus extensorius maximum
width 7.32; 5, proximo-distal distance between the incisura inter-
condylaris and the sulcus extensorius 8.95; 6, minimum proximo-
distal intercondylaris height 6.12; 7, cranio-caudal width of the
condylus medialis 26.76; 8, proximo-distal height of the condylus
medialis 16.77; 9, depressio epicondylus medialis major axis 11.11;
10, depressio epicondylus medialis minor axis 9.14; 11, cranio-
caudal width of the area intercondylaris in distal perspective 12.37.
Provenance
Indeterminate levels of the middle Eocene–lower Miocene
Sarmiento Formation outcropping in the Río Chico locality,
Chubut Province, Argentina (Figure 1). We note here that this
locality where Carlos Ameghino collected the fossils was erro-
neously assigned to the no longer existing Río Chico Formation
in previous reports and its age was also mistakenly assigned to the
Riochican mammal age (Agnolin 2017). An updated stratigraphical
scheme can be found in Raigemborn et al. (2010) and Woodburne
et al. (2014), and a further explanation about this locality can be
found in Picasso et al. (2022).
Description and comparison
Given that the shaft is lacking, only part of the distal condyles could
be compared. The sulcus extensorius (Figure 2(a)), although
slightly obscured by a transversal diagenetic fracture, seems well
delimited from the incisura intercondylaris (anterior intercondylar
fossa sensu Cracraft 1974). The sulcus extensorius is longitudinally
delimited by the tuberositas retinaculi extensoris like in
Opisthodactylus (Noriega et al. 2017: Figure 1), Rhea (Figure 4(e))
(particularly the lateral bony ridge, which, however, is weakly
developed in sub-adult specimens), Casuarius, and Emeus (Figure
4(u)) (in which there is also an ossified short pons supratendineus),
which are slightly marked in Dromaius (Figure 4(i)) (the medial
ridge is higher and continuous with the transverse ligament scar),
and absent in Aepyornis and Struthio (Figure 4(q‒M) respectively).
Figure 1. Map indicating the fossiliferous localities in a more regional context. a, location of Argentina in South America. b, the Chubut Province in Patagonia (Argentina).
c-d, the palaeontological localities Bryn Gwyn and ‘Río Chico’.
2C. ACOSTA HOSPITALECHE AND M. B. J. PICASSO
The incisura intercondylaris is more extended than in
Diogenornis but less than in Rhea, Emeus (Figure 4(e-u)) and
Casuarius. It is also shallower than in Struthio and Dromaius
(Figure 4(i-m)), and deeper than in Emeus (Figure 4(u)), in which
it is barely marked and restricted to the most lateral area of the
facies cranialis.
Although the sulcus extensorius does not reach the incisura
intercondylaris, the attachments of the transverse and internal
ligaments are separated (Figure 2(a)). The scars are continuous
in Opisthodactylus (see, however, remarks below regarding
Opisthodactylus cf. O. kirchneri MLP 68-III-14-2),
Pterocnemia (Agnolin and Noriega 2012: fig. 5) and presum-
ably in Diogenornis, whereas in Rhea and Dromaius (Figure 4
(e-i)) both surfaces are interrupted by the distal end of the
sulcus extensorius. In Diogenornis and Struthio (Figure 4(m)),
the attachment area of the transverse ligament is notably wide.
The narrowing at the intercondylar area is similar to the
condition of Rhea (Figure 4(a)) and Opisthodactylus. In Emeus
(Figure 3(u)), this narrowing is more pronounced and in
Diogenornis, Casuarius, Dromaius, Struthio, and Aepyornis
(Figure 4(i-q)) both condyles are further undercut by the inci-
sura intercondylaris.
The condylus medialis is cranially expanded (Figure 2(c)) and
distally truncated (in a medial perspective). The medial margin
of the condylus medialis is thicker and prominent at the cranial
edge, and becomes weaker towards the caudo-distal part
(Figure 2(c,d)). We notice, however, that the caudal and distal
margins are partially eroded. The condylus medialis of Aepyornis
(Figure 4(r)) is characterised by a distal narrowing that consti-
tutes almost a notch. The outlines of this edge are similar in the
fossil and Rhea (Figure 4(e)), in which the maximum distal
extension is near the axis of the shaft, whereas in
Opisthodactylus, Dromaius, Emeus (Figure 4(i-v)), the maximum
distal extension corresponds to the cranial portion. The condy-
lus lateralis is only partially preserved, the lateral rim is lacking,
and the outline is not clearly distinguished, preventing more
detailed comparisons (Figure 4(c-x)).
The epicondylus medialis (Figure 2(b,c)) is rounded and
prominent, more medially extended than in Diogenornis
(Mayr 2022: fig. 3.5) and slightly more than in
Opisthodactylus; as a result, the epicondyle is more medially
extended than the condylus medialis. Unlike any other speci-
men compared, the epicondyle becomes gradually lower in its
caudo-distal part, and it is not clearly separated from the
medial surface of the condyle. A deep and elongated fossa
Figure 2. MACN-A 12195 Palaeognathae indeterminate. a-f: distal end of left tibiotarsus in different views (a, cranial; b, caudal; c, medial; d, distal; and e, lateral). f-h,
indeterminate distal fragment of trochlea. Abbreviations: bs, broken surface; cl, condylus lateralis; cm, condylus medialis; cmr, condylus medialis rim, d, diaphysis; em,
epicondylus medialis; fem, fossa epicondylaris medialis; se, sulcus extensorius; ii, incisura intercondylaris; nia, narrowing intercondylar area; tla, transverse ligament
attachment; tne, tuberositas retinaculi extensoris. Measurements (taken and expressed in mm): 1, proximo-distal height of the condylus medialis measured on the cranial
face 14.67; 2, Preserved latero-medial distal width 26.76; 3, distance between the medial tuberositas retinaculi extensoris and the medial face of the condylus medialis,
9.49; 4, sulcus extensorius maximum width 7.32; 5, proximo-distal distance between the incisura intercondylaris and the sulcus extensorius 8.95; 6, minimum proximo-
distal intercondylaris height 6.12; 7, cranio-caudal width of the condylus medialis 26.76; 8, proximo-distal height of the condylus medialis 16.77; 9, depressio epicondylus
medialis major axis 11.11; 10, depressio epicondylus medialis minor axis 9.14; 11, cranio-caudal width of the area intercondylaris in distal perspective 12.37. Scale bars
represent 10 mm.
HISTORICAL BIOLOGY 3
epicondylaris medialis occupies the cranial portion of the con-
dyle and becomes shallower caudaly, like in Opisthodactylus.
In Rhea, Dromaius, and Struthio, this fossa is more caudaly
expanded (Figure 4(f-n)).
Remarks
The assignment to a flightless Palaeognathae is based on the
absence of an ossified pons supratendines, the narrowing of the
intercondylar area, and the proximo-distally shortened condyles,
features not found in other large-sized birds such as
Pelagornithidae, Falconiformes, Teratornithidae, Sphenisciformes,
and Phorusrhacidae. Even when direct comparisons with the holo-
type of Diogenornis fragilis were not possible given that the material
is for the moment not available at the repository, a number of
differences were noticed between both specimens. In MACN-A
12195, the condylus medialis is proximo-distally more extended,
the epicondylus medialis is prominent and more medially projected
than the condylus medialis in a cranial perspective, the sulcus
extensorius (although partially obscured by diagenesis) is more
clearly delimited by the tuberositas retinaculi extensorius, and the
fossa intercondylaris is extended medially and distal to the sulcus
extensorius.
We also notice that the attachment scars for the insertion of
the internal and transverse ligament are described as continuous
in the diagnosis of Opisthodactylus kirchneri (Noriega et al.
2017), but MLP 68-III-14-2 assigned to Opisthodactylus cf.
O. kirchneri (Tambussi et al. 2021) has two separated attach-
ments. This feature, however, could be variable according to the
development of the attachment areas, that at least in living
flightless Palaeognathae depends on ontogenetic parameters
among other sources of variation.
Figure 3. MPEF-PV 1411 Sphenisciformes cf. Palaeospheniscus, distal end of right femur in different views: a, cranial; b, caudal; c close-up of the fossa poplitea indicated in
b; d, medial; e, lateral; f, distal. Abbreviations: cf, corpus femori; cl, condylus lateralis; cm, condylus medialis; csm, crista supracondylaris medialis; fp, fossa poplitea; ilcc,
impressio lig. cruciati cranialis; pb, pachyostosic bone; si, sulcus intercondylaris; sp, sulcus patellaris; vf, vascular foramina. Measurements: 1, Preserved maximum length,
25.4; 2, latero-medial distal width: 9.77; 3, cranio-caudal width of the trochlea medialis, 12.09; 4, width to the level of fossa poplitea area: 10.00. Scale bar represents 10 mm.
4C. ACOSTA HOSPITALECHE AND M. B. J. PICASSO
Figure 4. Comparison of the tibiotarsal distal end of different flightless Palaeognathae and the fossil described here: a-d, MACN-A 12195 (a, cranial; b, medial; c, caudal; d,
distal); e-h, Rhea americana MLP 880 (a, cranial; b, medial; c, caudal; d, distal); i-l, Dromaius novaehollandiae MLP-O 14497 (i, cranial; j, medial; k, caudal; l, distal); m-p,
Struthio camelus MLP-O 14522 (m, cranial; n, medial; o, caudal; p, distal); q-t, Aepyornis maximus cast MLP 7–67 (q, cranial; r, medial; s, caudal; t, distal); u-x, Emeus crassus
MLP 86-VI-15-1 (u, cranial; v, medial; w, caudal; x, distal). Abbreviations: cl, condylus lateralis; cm, condylus medialis; cmr, condylus medialis rim; em, epicondylus medialis;
fem, fossa epicondylaris medialis; se, sulcus extensorius; ii, incisura intercondylaris; nia, narrowing intercondylar area; ps, pons supratendeus. Scale bars represent 10 mm.
HISTORICAL BIOLOGY 5
Order Sphenisciformes
Palaeospheniscus?
Referred material
Right femur without the proximal end MPEF-PV- 1411.
Preserved measurements (in mm)(Figure 4(a-d)
1, Preserved maximum length, 25.4; 2, latero-medial distal width:
9.77; 3, cranio-caudal width of the trochlea m edialis, 12.09; 4, width
to the level of fossa poplitea area: 10.00.
Provenance
Early Miocene (Coluehuapean) levels of the Sarmiento Formation.
Bryn Gwyn locality, Chubut Province, Argentina (Figure 1). We
note that, in previous reports, these levels have been erroneously
assigned to the Oligocene (Agnolin and Cenizo 2014), when they
correspond in fact to the early Miocene (Paredes et al. 2008).
Description and comparison
MPEF-PV- 1411 is a partial distal end of a right femur with the
condylus lateralis and a portion of the trochlea fibularis missing
(Figure 3(a-e)). Regarding their size, the MPEF-PV- 1411 is con-
siderably smaller than Diogenornis, according to the measurements
provided by Alvarenga (1983). In fact, MPEF-PV- 1411 is slightly
smaller than the femora of the modern penguin Spheniscus magel-
lanicus and the polyborine falcon Caracara.
The shaft is partially preserved, the lateral half is lacking (Figure 3
(d)) and the periosteal bone is exposed, showing a thickness of the
walls consistent with a moderate degree of pachyostosis. Although
medially eroded, the condylus medialis appears narrow (Figure 3(b)).
The region that corresponds to the fossa poplitea is shallow and the
lateral and medial margins are poorly or not defined (Figure 3(b)).
Within this fossa, three small foramina of different sizes can be
observed (Figure 3(b)). The crista supracondyli medialis is thin and
extended from the condylus medialis to the shaft (Figure 3(b)). The
sulcus patellaris is medially and laterally delimited in the preserved
portion, although its real depth cannot be observed due to the partial
erosion of the edges. The sulcus patellaris is slightly shallower than the
sulcus intercondylaris (Figure 3(a)), and the impressio lig. cruciati
cranialis is rounded and shallow (Figure 3(b-f)).
Remarks
In comparison with Rheiformes, MPEF-PV 1411 is notably smaller,
the condylus medialis and the lateralis are more equally extended,
the fossa poplitea is shallower and not so clearly delimited, the
a sulcus patellaris is also shallower and moderately defined, the
crista tibiofibularis is subparallel to the shaft (oblique in rheas),
and the sulcus patellaris is aligned and connected with the sulcus
intercondylaris. This is true not only for adults, all the features
described here are already developed in chicks between one and
three months old (Picasso 2010).
Conclusions
After the examination of the two specimens from the Sarmiento
Formation (Chubut Province, Argentina) assigned to Diogenornis
(MACN-A 12195) and to bird that resembles Diogenornis (MPEF-
PV 1411), our own observations and the comparisons with other
extinct and extant taxa, the following can be pointed out.
Specimen MPEF-PV 1411 is not a distal fragment of tibiotarsus,
but a distal femur, assigned to Sphenisciformes, by the notable
pachyostosis, the similar extension of both condyles, the shallower
fossa poplitea, and the crista tibiofibularis subparallel to the shaft. It
was probably a representative of Palaeospheniscus, a genus that
matches in size, and is frequently found in the Miocene of
Patagonia, like for example in the overlying Gaiman Formation.
A more detailed discussion of the features included in the previous
published description is not possible given that it was described as
a completely different hindlimb skeletal element (see above)
The previously published description of MACN-A 12195 was
based on mirrored images (Agnolin 2017: Figure 2), this specimen
is assigned to a flightless Palaeognathae due to the absence of an
ossified pons supratendineus, a condylus medialis cranio-distally
expanded and delimited by a thick trochlear margin, and a deep
depressio epicondylaris medialis. A more accurate assignment is
not recommended here given the poor preservational state of the
material.
This way, the biochron of Diogenornis fragilis must be still
restricted to the Ypressian (early Eocene) and should not be
expanded on the basis of the specimens MPEF-PV 1411 and
MACN-A 12195. The same way, D. fragilis should be still consid-
ered as an endemic taxon of Brazil. More complete material, as
always, could provide new information about the identity of the
materials here described, particularly about MACN-A 12195,
assigned here to a flightless PalaeognathaeGiven that erroneous
systematic assignments lead to misinterpretation of the main evo-
lutive pathways and generate confusions regarding past biodiver-
sity, biogeographical and corological patterns, we prefer to be
cautious for the moment and until new evidence is available.
Acknowledgments
To Martín Ezcurra and Agustín Martinelli (MACN), M. Encarnación Pérez and
Eduardo Ruigómez (MPEF), Diego Montalti, and Yanina Herrera (MLP) for the
access to the specimens under study and comparative material. We also thanks
to two anonymous reviewers and the editor in chief for their helpful comments.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Funding
This work was partially supported by Universidad Nacional de La Plata (PI
N953) and CONICET (PIP 0096).
ORCID
Carolina Acosta Hospitaleche http://orcid.org/0000-0002-2614-1448
Mariana B.J. Picasso http://orcid.org/0000-0003-4306-8921
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Appendix 1
Comparative specimens: femur and tibiotarsus of Rhea americana (adults MLP
273, MLP 903, MLP 880, MLP 881; five months old MLP 887; eight months old
MLP 882); Struthio camelus MLP-O 14522; Dromaius novaehollandiae MLP-O
14497; Emeus crassus MLP 86-VI-15-1; Aepyornis maximus cast MLP 7–67;
Casuarius casuarius USNM 429823 (consulted in http://n2t.net/ark:/65665/
36dfeae3a-dc9c-4074-9dfb-8437129f2691); Opisthodactylus cf. O. kirchneri
MLP 68-III-14-2; Spheniscus magellanicus MLP 473; Palaeospheniscus patagoni-
cus MLP 20–28, MLP 20–58.
HISTORICAL BIOLOGY 7
