Tale that started small.
Ozarkodina? sp. nov., was first reported by Aldridge (1985) from the Silurian Much Wenlock Limestone Fm. of Britain as probably having a seximembrate apparatus, with a P1 element with short high blade, a central cavity below an inconspicuous cusp, anterior denticles slender & tall, and posterior denticles slender, lower and gently inclined posteriorly; all elements possessing tiny delicate elements with confluent slender denticles.
The seximembrate apparatus was confirmed by Jeppsson in Calner et al. (2008) when he described & named Aldridgeodus minimus from the Silurian of Gotland, Sweden. The M element with radiating denticles was described as “unique” & “strange” regarding it as the defining element of the new genus. The species name ‘minimus’ referred to having the tiniest ramiform element[s] of any Silurian conodont of which Jeppsson was aware.
Aldridge & Jeppsson both stressed the fine denticulation, Jeppsson describing the white matter roots as well developed & sharply delimited, and the slender needle shaped denticles as set very closely and deeply rooted.
Jeppsson also noted not only the under-representation of the M element in older collections, but also that A. minimus was diminutive compared to other Silurian conodonts, therefore suffering higher rate of taphonomic loss, & complicating its recovery using standard processing techniques.
In N.A., Aldrigeodus elements were first recovered from Ontario, Canada, about 2000 from the Eramosa Formation at Hepworth, but kept as Ozarkodina ? sp. nov. by von Bitter et al., (2007) prior to formal description & naming of Aldridgeodus minimus by Jeppsson in Calner et al. (2008).
A. minimus clusters subsequently recovered from Hepworth.
Difficulty in finding the M element at Hepworth.
Began finding rare elongated P1 elements among shorter A. minimus P1 elements.
More or less simultaneously with Calner et al.’s publication, Bancroft (2008) identified A. minimus from the Eramosa Fm. in s. Ont. in two cores from Guelph, Ont.
In s. Ont., as in Gotland, the diminutive Aldridgeodus elements derived from acid residue were only recovered using buffered formic acid techniques.
In last few years, what is likely new species of Aldridgeodus has increasingly been recovered, mostly from Hepworth W., both as discrete elements, as well as natural assemblages & clusters. Characteristics of potential new species: elongated P1 & P2 elements. Again lack of, or difficulty in, identifying 'M' element.
The species has exceedingly fine denticulation, i.e. 'needle-like' denticles with the 'deep' basal extension of the denticles. In connection with the exceedingly fine denticulation, we note Jones et al.’s recent paper entitled “The sharpest tools in the box”, on conodont elements being the sharpest teeth ever. Jones & his colleagues worked on Ozarkodina excavata from Hepworth; however, the sharpness of Aldrideodus denticles from there may break the record even for conodonts.
Paleoenvironments:
On Gotland, abundant Aldridgeodus minimus was recovered from atypical middle Silurian shale postulated to have been deposited in possibly sediment-starved lagoonal, or analogues environments, in the interior area of a carbonate platform. The Eramosa Fm. has similarly been interpreted as a restricted marine lagoonal facies with sabkha-like cycles, ranging to more open ramp, deeper lagoonal facies influenced by storm pulses.
This suggests that Aldridgeodus, like its associated Ctenognathodus, was very much an environmentally controlled taxon on both Gotland & in Ontario, Canada, being more common in shallow-water, lagoonal restricted carbonates & shales.
Correlation & Age
On Gotland with its superb & tight biostratigraphic controls, A. minimus is long ranging, occurring from the Kockelella ortus ortus Zone through to the main part of the Polygnathoides siluricus Zone, i.e. the latest Sheinwoodian through to early Ludfordian.
In the Guelph, Ont. cores, its co-occurrence with Kockelella walliseri below the last occurrence of Ozarkodina sagitta rhenana, allows the range of A. minimus to be extended lower, i.e. into the middle Sheinwoodian Lr. Kockelella walliseri Zone.
However, in the Eramosa Lagerstaette at Hepworth biostratigraphically diagnostic conodonts have not been found, making it more difficult to correlate & date the Eramosa Fm., and its contained Aldrigeodus minimus from there. The Eramosa at Hepworth is dominated by the long ranging Ozarkodina excavata, a species whose strong environmental preference, while still lagoonal, defines a more open marine conodont biofacies.
In the Eramosa Lagerstaette at Hepworth West, even the long ranging Ozarkodina excavata is either missing, or is exceedingly rare. Instead, Aldrigeodus minimus & Aldrigeodus ? n. sp., associated with Ctenognathodus cf. murchisoni define a more restricted lagoonal conodont biofacies.
Similar to Ctenognathodus Association of Aldridge & Jeppsson (1999) & Viira and Einasto (2003), a shallow-water, nearshore, lagoonal fauna.
The key to the age & correlation of the Eramosa at and around Hepworth., and thus to the age & range of the Aldridgeodus from there, appears to lie in the co-occuring Ctenognathus cf. murchisoni.
Some years ago,Viive Viira of Talinn Technical University identified our Ctenognathus cf. murchisoni as a predecessor of Ctenognathodus murchisoni, the latter a species that defines a younger zone by that name in the Baltic, and that occurs in part, in rocks with the stable carbon isotope signature of the younger Mulde Excursion.
This as yet unnamed older predecessor is likely Ctenognathodus n. sp. A of Viira and Einasto (2003) from the Viita & Kuusnomme beds in three boreholes on Saaremaa Island in Estonia, and is likely restricted to the Rootsikula Stage of the upper Wenlock.
Such a restriction, and an older (pre-murchisoni) age for Ct. cf. murchisoni (and therefore its associated Aldridgeodus) from Hepworth, while needing to be confirmed, seems more likely and would agree with co-author Bancroft’s conodont-based age-determinations of a [mostly] Sheinwoodian, rather than Homerian age for the Eramosa at Wiarton & Guelph. It would also be in better agreement with her findings that the carbon stable isotope signature in the Eramosa elsewhere in Ontario records the tail end of the older Ireviken Excursion rather than a part of the younger Mulde Excursion.