10.5061/DRYAD.7VG5FD0
Syme, Caitlin E.
University of Queensland
Salisbury, Steven W.
University of Queensland
Data from: Taphonomy of Isisfordia duncani specimens from the Lower
Cretaceous (upper Albian) portion of the Winton Formation, Isisford,
central-west Queensland
Dryad
dataset
2018
crocodyliformes
concretion
Albian
Winton Formation
decay
Isisfordia duncani
2018-02-12T19:59:08Z
2018-02-12T19:59:08Z
en
https://doi.org/10.1098/rsos.171651
175656 bytes
1
CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
Taphonomic analysis of fossil material can benefit from including the
results of actualistic decay experiments. This is crucial in determining
the autochthony or allochthony of fossils of juvenile and adult Isisfordia
duncani, a basal eusuchian from the Lower Cretaceous (upper Albian)
distal-fluvial-deltaic lower Winton Formation near Isisford. The
taphonomic characteristics of the I. duncani fossils were documented using
a combination of traditional taphonomic analysis alongside already
published actualistic decay data from juvenile Crocodylus porosus
carcasses. We found that the I. duncani holotype, paratypes, and referred
specimens show little signs of weathering and no signs of abrasion.
Disarticulated skeletal elements are often found in close proximity to the
rest of the otherwise articulated skeleton. The isolated and
disarticulated skeletal elements identified, commonly cranial, maxillary,
and mandibular elements, are typical of lag deposits. The holotype QM
F36211 and paratype QMF34642 were classified as autochthonous, and the
remaining I. duncani paratypes and referred specimens are
parautochthonous. We propose that I. duncani inhabited upper and lower
delta plains near the Eromanga Sea in life. Their carcasses were buried in
sediment-laden floodwaters in delta plain overbank and distributary
channel deposits. Future studies should refer to I. duncani as a brackish
water tolerant species.
TABLE 4. Completeness (Cp.) and articulation (Art.) scores for I. duncani
specimens from Isisford (following Beardmore et al. (2012); Syme and
Salisbury (2014)).Completeness (Cp.) and articulation (Art.) scores for I.
duncani specimens from Isisford. As per Beardmore et al. [40], Xp denotes
skeletal units that are not visible due to truncation with the edge of the
block or blocks missing, and Xh denotes skeletal obscured by matrix or
overlying skeletal elements. Beardmore et al. [40] propose that specimens
with 3 instances of either Xp or Xh in the dataset should be omitted from
further analysis: in this instance, all specimens except the holotype QM
F36211 were excluded from further analyses (denoted by ‘--’). The average
completeness and articulation for QM F36211 was calculated both with and
without the dermal skeletal units: with the dermal skeletal units,
compared to a maximum total score of 36 (4 for each of the 9 of 11 units
omitting Xp/Xh data); without the dermal skeletal units, compared to a
maximum total score of 28 (4 for each of the 7 of 9 units omitting Xp/Xh
data). The data was then compared with the trend lines proposed by
Beardmore et al. [40] where possible.TABLE 4.docxTABLE 3. NISP, MNE, and
MNI of Isisfordia duncani specimens from Isisford.Summary of the
Isisfordia duncani fossil elements recovered from the Isisford locality,
including the number of identified specimens—where ‘specimens’ refers to
the number of bone fragments—recovered per taxon (NISP), minimum number of
elements (MNE) which may comprise multiple fragments each, and minimum
number of individuals (MNI). We have included totals both without dermal
skeletal units (as NISP and MNE), and with dermal skeletal units such as
osteoderms included (as NISP inc. ost. and MNE inc. ost.). The relatively
high NISP and MNE compared to MNI for I. duncani indicates that the
majority of individuals comprise a multitude of articulated and/or closely
associated skeletal elements. Approximate size range taken from and
estimated using Salisbury et al. [66].TABLE 3.docxTABLE 5. Table
incorporating all other taphonomic classification systems used in this
study of the I. duncani specimens from Isisford
[13,16,20,26–28,32,101–104,106–114] that were used in conjunction with
Table IV in Dodson [19] and trends classified by Beardmore et al.
[40].Table incorporating all other taphonomic classification systems used
in this study of the I. duncani specimens from Isisford
[13,16,20,26–28,32,101–104,106–114] that were used in conjunction with
Table IV in Dodson [19] and trends classified by Beardmore et al. [40]. We
also chose to include counts of number of individual specimens (NISP) and
minimum number of elements (MNE) with and without osteoderms, for ease of
comparison in future with other tetrapods lacking dermal skeletons. The
maximum number of skeletal elements in I. duncani, both excluding and
including osteoderms, are listed next to MNE. Specimens QM F58793 and QM
F58794 were found in close proximity, therefore NISP, MNE, and MNI are
listed both grouped together (MNI = 2), and separately, to demonstrate the
variation in NISP and MNE per individual. Where possible, life stage was
determined by identifying closure neurocentral sutures in vertebrae
(closure proceeds caudally to cranially throughout ontogeny, with ‘adult’
defined as possessing closed neurocentral sutures in all vertebrae) and
size comparison to the holotype that has been identified as an adult (see
Salisbury et al. [66]). By comparing across specimen numbers, we
determined whether disarticulated elements showed greater degrees of bone
surface modifications. Degrees of articulation and completeness are the
sum total of scores for each skeletal unit. The Dodson [19] classification
scheme assumes absences are true absences; we selected the most likely
class per specimen based on other taphonomic indicators (such as
truncation with block, articulation versus completeness, and so on).TABLE
5.doc
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