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.docx
TABLE 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.docx
TABLE 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
