Сыктывкар, Республика Коми, Россия
УДК 551.734.5 Верхний девон Фран. Фамен. Сенекские слои. Шотокские слои. Брадфорд. Таган. Фингер-Лейкс. Шеманг. Кассадага. Коневанго. Интумесценсовые слои.Ибергский и адорфский известняки. Орогения: Бретонская фазаскладчатости
Данное исследование посвящено изучению онтогенетических (связанных с процессами роста) трофических сдвигов франских конодонтов Mehlina gradata из разрезов Восточно-Европейской платформы. Несмотря на то что морфология P1-элементов этого вида оставалась практически неизменной на протяжении всего жизненного цикла, изотопный анализ выявил значимую корреляцию между размером элемента и его изотопным составом углерода. С увеличением размеров элементов наблюдается обогащение изотопом 13C. Это позволяет предположить, что при сохранении стабильной морфологии ротового аппарата трофическая позиция организма претерпевала изменения. Результаты указывают на то, что по мере роста Mehlina gradata, вероятно, переходила на более высокие трофические уровни (например, к активному хищничеству или потреблению падали), демонстрируя тем самым, что морфологическая стабильность не обязательно влечет за собой экологический стазис.
конодонты, онтогенез, изотопы углерода, экология
Introduction
Conodonts are a group of small free-swimming, probably nektic or nekto-planktic marine animals (Briggs et al., 1983) that flourished during the Palaeozoic and became extinct during the earliest Jurassic (Du et al., 2023). The only mineralized parts of conodonts are tooth-like phosphate elements (conodont elements) organized into a bilaterally subsymmetrical feeding apparatus. The apparatus organization is a trait of genus level, but the morphology of P1 elements is a trait of species level in most of the middle and late Palaeozoic conodonts.
The conodont genus Mehlina Youngquist, 1945 is characterized by simple blade-like morphology of P elements (Figure 1), and robust S and M elements (Youngquist, 1945). P1 elements of this genus are composed of laterally compressed fused denticles, and are slightly curved in both the dorso-ventral and rostro-caudal direction. The basal pit is small and has narrow flanks transforming into a slightly inverted keel in the anterior and posterior processes. The cusp is commonly reduced in mature specimens (Figure 1). The morphology of P1 elements resembles Class II symmetry of Lane (1968). The symmetrical sinistral and dextral elements compose a pair with supposed scissor-like biomechanics characteristic of the blade-shaped P1 elements (e. g. Nicoll, 1987; Purnell and von Bitter, 1992).
Due to accretional growth of elements, their size is proportional to the individual age (e. g. Muller and Nogami, 1972). Consequently, ontogenetic changes are frequently investigated based on element length (e. g. Zhuravlev, 1995; Plotitsyn and Zhuravlev, 2016).
Blade-form elements generally show minimal size-shape dependence. Small elements possess a similar general morphology as large elements of the same species. Given this observed morphological stability, this study aims to test a hypothesis: Does the trophic position of Mehlina gradata undergo significant changes during its ontogeny despite the lack of clear P1 element morphology shifts?
Material and methods
The studied specimens originated from shallow-water nearshore Frasnian sequences deposited on the Northwest East European Platform, specifically within the Main Devonian Field (Figure 2). The analyzed stratigraphic interval encompasses conodont zones MN4 and MN6 across three geographically distinct localities: Shelon River, Tchudovo, and Ilmen Lake (Figure 3). The stratigraphical framework follows Zhuravlev (2022). The upper portion of the Lower Frasnian, comprising the Tchudovo and Dubnik beds, is biostratigraphically equivalent to the upper part of the MN4 conodont Zone (Polygnathus reimersi local Zone). The Tchudovo Beds are characterized by thin, rhythmic alternations of clayey limestone and limy clay, while the overlying Dubnik Beds consist of limy clays containing marl lenses and layers. The Middle Frasnian encompasses the Porkhov and Svinord beds, which correlate with the MN5 conodont Zone (Polygnathus pollocki local Zone), as well as the Ilmen and Buregi beds, corresponding to the MN6 conodont Zone (Polygnathus ilmenensis local Zone and Polygnathus efimovae local Zone, respectively). The Porkhov and Svinord beds consist of alternating clayey limestone and limy clays. In contrast, the Ilmen Beds are composed of silty clays with intercalated thin limestone and silt layers, whereas the Buregi Beds consist of shelly limestones. The study specimens of Mehlina gradata came from the Tchudovo, Dubnik, Ilmen, and Buregi beds (Figure 3).
The P1 elements of Mehlina gradata were studied with a number of analytical techniques, including morphological investigations with scanning electron microscopy (SEM) and optic microscopy, simple biometry (7 conodont elements), trace element analysis (8 conodont elements), and C-isotope geochemistry (20 conodont elements). Because most of the isotopically analysed conodont elements are fragmented, it is impossible to accurately determine their original length. SEM imaging in secondary electron mode was employed for morphological assessment (Figure 1). Standardized measurement of P1element includes element length and number of denticles (Figure 1).
The trace element analysis was performed on albid tissue that demonstrates most homogenous composition among conodont hard tissues. Strontium-to-Calcium ratios (Sr/Ca) as well as content of P and F in the albid tissue were determined using energy-dispersive X-ray spectroscopy (EDX) coupled with a VEGA3 TESCAN microprobe. Four chemical elements (Ca, Sr, P, and F) were analyzed per counting cycle at an accelerating voltage of 20 kV, emission current of 78—88 mA, and a probe diameter of 190—290 nm. Sample standards include wollastonite for Ca, GaP for P, fluorite for F, and SrF2 for Sr.
Analysis of carbon isotope composition (d13Ccon) was performed on P1 elements (CAI = 1) following method described earlier (Zhuravlev, 2023). The d13Ccon values were determined by a DELTA V Advantage mass spectrometer equipped with a Thermo Electron continuous flow interface (ConFlo III) and an elemental analyser (Flash EA 1112) (CKP Geonauka, Syktyvkar). Carbon dioxide produced by high-temperature (900 °C) combustion of conodont elements in oxygen was analysed. At this temperature, both carbon from organic matter and carbon from hydroxyapatite are converted to carbon dioxide (Zhuravlev, 2023). The d13C values are given in ppm relative to the VPDB standard. The international standard USGS-40 (L-glutamic acid) is used for calibration. The error of the d13C determination is ±0.15‰.
Results
P1 element length of the study specimens of Mehlina gradata range from 0.4 mm to 1.0 mm. The number of denticles varies from 11 to 18 that correspond to original description of the species (Youngquist, 1945). Morphological analysis indicates that the general blade-form morphology and small basal pit are maintained throughout ontogeny, while the P1 element shows minimal variation in shape — only an increase in size and number of denticles occur between early and late developmental stages (Figure 1). The number of denticles is proportional to the element length (Figure 4) (n=7, R2=0.805).
Mean Sr/Ca (mol/mol) in albid tissue is about 0.007 (n=8, Standard deviation 0.005) (Table 1). It is slightly higher than mean Sr/Ca of other Frasnian conodonts from the same localities (about 0.004, n=17, Standard deviation 0.002) (Figure 5).
C-isotope composition of P1 elements of Mehlina gradata varies from –31.9 ‰ to –24.7 ‰. Mean d13Ccon value is of –27.4 ‰ (n=20, Standard deviation 1.993). The biometrically studied seven specimens show d13Ccon values ranging from -30.1‰ to –24.7 ‰ with the mean value is of –27.3 ‰ (n=7, Standard deviation 1.881) (Table 2). Element length and C-isotope composition show significant positive correlation (n=7, R² = 0.8689, p=0.0022) that is independent on stratigraphic position of the study specimens (Figure 6). Mean values for MN4, MN5, and MN6 zones vary insignificantly (Figure 7).
The combination of a relatively high Sr/Ca ratio paired with lower mean d13Ccon suggests that Mehlina gradata may have occupied a comparatively low trophic level (Shirley et al., 2018; Zhuravlev, 2023).
Discussion
The range of d13Ccon variations in the studied specimens of Mehlina gradata is about 7 ‰, but Sr/Ca ratio ranges from 0.002 to 0.016 mol/mol (Tables 1, 2). Notably, these variations occur in rather small parts of the paleobasin and in the short time intervals. For example, variations of d13Ccon values within a single sample (sample 5110/3) reach approximately 4 ‰ (Table 2). The positive correlation between d13Ccon values and P1 element length suggests a clear ontogenetic trend: larger elements are associated with higher d13C values. Due to a small number of studied specimens (seven P1 elements) this is a preliminary finding that warrants further investigation with larger sample sets.
By analogy with modern low vertebrates, it can be supposed that the ontogenetic 13C enrichment of mineralized tissue in Mehlina gradata was caused by ontogenetic dietary shifts (e. g., moving to higher trophic levels), habitat transitions (e. g., migrating from inshore nurseries to offshore adult habitats), and metabolic changes that alter the balance of respired and environmental carbon incorporated into the tissue (e. g. Carlisle et al., 2015; Sakamoto et al., 2023; Díaz-Delgado et al., 2026).
The lack of locality and facies control on the ontogenetic variations of d13Ccon in Mehlina gradata in study material suggests that habitat and migration patterns probably played an insufficient role in these variations. Metabolic effects remain a theoretical possibility. High metabolic rates in younger, faster-growing individuals could lead to a greater proportion of metabolic (respired) carbon being incorporated into the mineralized tissue compared to environmental carbon. Because respired carbon is typically 13C-depleted, this could result in lower d13C values in early life stages. However, as the metabolic peculiarities of conodont biomineralization remain unknown, we cannot definitively evaluate “vital effects” on d13Ccon values. The most probable cause of the observed variations is dietary diversity, a hypothesis further supported by the wide range in Sr/Ca ratios.
The positive correlation found between element length (as individual age proxy) and d13Ccon values indicates ontogenetic dietary shifts that transcend simple morphological stability. Early life stages were probably associated with feeding at a lower trophic level than those observed in mature individuals, suggesting an adaptive shift toward a predatory or scavenging lifestyle as the conodont grew. These findings support the hypothesis that despite maintaining a consistent shape of P1 elements, Mehlina gradata exhibited dynamic trophic plasticity throughout its life cycle. In general, Mehlina gradata shows wide range of d13C variations among individuals and ontogenetic trends similar to modern small fishes occupying low trophic level (Sakamoto et al., 2023).
Similar changes in trophic position during ontogeny have been reconstructed for blade-form Ozarkodina confluens (Branson et Mehl, 1933) on the basis of Sr/Ca ratio (Shirley et al., 2018). In contrast, conodonts bearing coniform elements in the apparatus, such as Panderodus equicostatus (Rhodes, 1953) and Proconodontus muelleri Miller, 1969, show lack of substantial variations in trophic level during ontogeny (Leonhard et al., 2021). Probably, an ontogenetic trend in trophic position may be a common feature of conodonts possessing blade-form elements in P1 position.
These findings suggest that P element morphology cannot be used as a main trait for reconstructing the position of conodonts in a trophic web. The study of the Late Devonian Palmatolepis provided further evidence of the weak relationship between the morphology of P elements and trophic position (Goudemez et al., 2026). Notably, the calcium stable isotope ratio in conodont elements (as a trophic level proxy) remained unchanged in Palmatolepis, despite significant modifications in element morphology during the Famennian (Goudemez et al., 2026).
Conclusions
The substantial range of d13Ccon values strongly suggests that Mehlina gradata utilized a diverse and varied food source throughout its life despite of morphological stability of P1 elements. This trophic variability challenges the interpretations of feeding ecology based only on morphology of P1 elements. Element size may play a principal role in trophic specialization of conodonts.
The research was conducted as part of government-funded project 122040600008-5.
1. Briggs D. E. G., Clarkson E. N. K., Aldridge R. The conodont animal. Lethaia. 1983;16:1—14.
2. Carlisle A. B., Goldman K. J., Litvin S. Y., Madigan D. J., Bigman J. S., Swithenbank A. M., Kline Jr T. C., Block B. A. Stable isotope analysis of vertebrae reveals ontogenetic changes in habitat in an endothermic pelagic shark. Proceedings of the Royal Society: Biological Sciences. 2015; 282:20141446. http://dx.doi.org/10.1098/rspb.2014. 1446
3. Díaz-Delgado E., Chung M.-T., Magozzi S., Willis T. J., Trueman C. N. Potential metabolic records in isotope signals of chondrichthyan hard tissues. Journal of Fish Biology. 2025;107(3):1001—1017. https://doi.org/10.1111/jfb.70109
4. Du Y., Onoue T., Tomimatsu Y., Wu Q., Rigo M. Lower Jurassic conodonts from the Inuyama area of Japan: implications for conodont extinction. Frontiers in Ecology and Evolution. 2023;11:1135789. https://doi.org/10.3389/fevo.2023.1135789
5. Goudemez C., Assemat A., Thiery G., Girard C. 3D topography as an indicator of change in food processing ability in elements of the conodont genus Palmatolepis. Lethaia. 2026;59(3):1—16. https://doi.org/10.18261/let.59.3.2
6. Lane H. R. Symmetry in conodont element-pairs. Journal of Paleontology. 1968;42:1258—1263.
7. Leonhard I., Shirley B., Murdock D. J. E., Repetski J., Jarochowska E. Growth and feeding ecology of coniform conodonts. PeerJ. 2021;9:e12505 https://doi.org/10.7717/peerj.12505
8. Müller K. J., Nogami Y. Growth and function of conodonts. Int. Geol. Congr. Rep. 24th Sess. Montreal. Sect.7;1972. p. 20—27.
9. Nicoll R. S. Form and function of the Pa element in the conodont animal. In: Aldridge R.L. ed., Palaeobiology of conodonts. Chichester: Ellis Horwood; 1987. p. 77—90.
10. Plotitsyn A. N., Zhuravlev A. V. Morphology of the early ontogenetic stages of advanced siphonodellids (conodonts, Early Carboniferous). Vestnik of Institute of Geology Komi SC UB RAS. 2016;8:3—8.
11. Purnell M. A., Von Bitter P. H. Blade-shaped conodont elements functioned as cutting teeth. Nature. 1992;359:629—631.
12. Sakamoto T., Kodama T., Horii S., Takahashi K., Tawa A., Tanaka Y., Ohshmio S. Geographic, seasonal and ontogenetic variations of 15N and 13C of Japanese sardine explained by baseline variations and diverse fish movements. Progress in Oceanography. 2023;219:103163. https://doi.org/10.1016/j.pocean.2023.103163
13. Shirley B., Grohganz M., Bestmann M., Jarochowska E. Wear, tear and systematic repair: testing models of growth dynamics in conodonts with high-resolution imaging. Proceedings of the Royal Society: Biological Sciences. 2018;285(1886):20181614. https://doi.org/10.1098/rspb.2018.1614.
14. Youngquist W. L. Upper Devonian conodonts from the Independence Shale (?) of Iowa. Journal of Paleontology. 1945;19(4):355—367.
15. Zhuravlev A. V. Lower–middle Frasnian organic carbon isotope record of conodonts in East European Platform. Palaeoworld. 2022;31(2):249—257. https://doi.org/10.1016/ j. palwor.2021.07.003
16. Zhuravlev A. V. Ontogeny and trophic types of some Tournaisian Polygnathacea (Conodonta). Courier Forschungsinstitut Senckenberg. 1995;182:307—312.
17. Zhuravlev A. V. Carbon isotope study of conodont elements: Applications and limitations. Marine Micropaleontology. 2023;178:102200. https://doi.org/10.1016/j.marmicro.2022.102200
