Docodonta, Kretzoi, 1946

Averianov, Alexander O., Lopatin, Alexey V., Skutschas, Pavel P., Martynovich, Nikolai V., Leshchinskiy, Sergei V., Rezvyi, Anton S., Krasnolutskii, Sergei A. & Fayngertz, Alexey V., 2005, Discovery of Middle Jurassic mammals from Siberia, Acta Palaeontologica Polonica 50 (4), pp. 789-797 : 791-793

Docodonta indet.

Figs. 2–4 .

Locality and horizon: Berezovsk Quarry, Krasnoyarsk Territory, Russia; Upper Member of Itat Formation, Bathonian, Middle Jurassic.

Material.—PIN 5087/1, posterior fragment of a right dentary with alveoli for the ultimate molar, posterior alveolus for the penultimate molar and partially preserved coronoid and pseudangular processes. PM TGU 200/3−BR−1, posterior fragment of a right dentary with alveoli for the two last molars and partially preserved coronoid process.

Description.—PIN 5087/1 ( Figs. 2A , 3 ). The alveolus for the ultimate molar is 1.5 mm long. The alveolus for the posterior root is smaller than that for the anterior root. Only posterior alveolus from the penultimate molar is preserved, with remnant of the posterior root. It seems that the penultimate molar does not differ much in size from the ultimate molar. Breakage of the anterior side of the fragment reveals a relatively large mandibular canal. Along the medial side and just adjacent to the alveoli there is a groove for the replacement dental lamina that posteriorly deviates ventrally and terminates in the coronoid area. It is possible that eruption of molars was not completed at death of the individual and at least one more posterior molar could be added later in the ontogenesis. But there is no replacement pit at the end of the replacement groove. On the medial side closer to the ventral border there is a continuous Meckel’s groove. On the fragment preserved the Meckel’s groove is remarkably straight, without any tenmandibular canal dency to converge upon the ventral border of the dentary. It is quite deep and terminates near the mandibular foramen. There is a distinct ridge just anterior to the mandibular foramen that overhangs the Meckel’s groove dorsally. A narrow area ventral to this ridge is interpreted here as a facet for the dorsal flange of the prearticular. If this is correct, at least the portion of the Meckel’s groove ventral and posterior to this ridge was housing the prearticular. The Meckel’s groove continues posteriorly into a shallow facet, tapering posteriorly and reaching the anterior end of the pseudangular process. This facet is most probably for the prearticular. It is adjacent and just ventrally to the angular facet. The mandibular foramen is placed deeply at the anterior border of the postdentary trough at the level of the posterior root of the ultimate molar. The foramen is facing posteriorly. Posteriorly to the mandibular foramen the postdentary groove markedly widens dorsoventrally. The trough is deepest anteriorly, where it is partly overhanging by the thin bony lamina, and becomes shallower posteriorly. The trough is separated into two unequal parts by a longitudinal diagonal ridge. The deeper but smaller dorsal part of the trough is analogous to the structure in Morganucodon interpreted as adductor fossa by Kermack et al. (1973). However, according to Luo (personal communication, 2005), this interpretation is wrong as adductor muscle could not have inserted into this space because it is block off by the coronoid. As we do not know for sure the function of this depression and there is no other available name for this structure, it is referred here to as “adductor fossa” in quotation marks. The shallower but larger ventral part of the trough is the angular facet. The latter is not clearly subdivided, so it is not clear if the surangular facet is present on this fragment. Dorsally to the “adductor fossa” there is quite a weak medial ridge overhanging the postdentary trough. The pseudangular process (= angular process of other authors, see discussion in Kielan−Jaworowska et al. 2004) is prominent and directed posteroventrally. It lacks a minor part of the ventral margin but its posterior margin is intact and very thin. From the medial side the pseudangular process is subdivided by a strong vertical ridge. Area posterior to this ridge is deeply concave and continuous with the angular facet. It is interpreted here as the facet for reflected lamina of angular. On the medial side dorsal to the postdentary trough and posterior to the last tooth there is a marked rugosity, which is interpreted as an attachment area for the coronoid. This facet does not extend on to the coronoid process. On the lateral side there is an extensive but shallow masseteric fossa reaching anteriorly the posterior end of the ultimate molar. The coronoid crest is quite robust.

PM TGU 200/3−BR−1 ( Figs. 2B , 4 ). This specimen is more water−worn compared with PIN 5087/1. It is similar in size to the latter specimen, having alveoli for the ultimate and penultimate molars, 1.5 and ~ 1.8 mm in diameter respectively. The ultimate molar is slightly smaller than the penultimate molar and its long axis is set somewhat obliquely to the axis of the latter. On the medial side the dorsal border of the postdentary trough is partially destroyed, but a weak medial ridge overhanging the trough is visible. Meckel’s groove is straight and deep anteriorly (as in PIN 5087/1) and continuous to the shallower prearticular facet posteriorly without any tapering. Moreover, the floor of the prearticular facet rotates to an almost vertical position compared with the more inclined position of the floor of Meckel’s groove, which gives impression that the groove widens dorsoventrally posteriorly. The mandibular foramen is placed in the deep antefacet for reflected lamina of angular rior portion of the postdentary trough and facing posteriorly. Ventral to the mandibular foramen there is a faint ridge which possibly delimits the dorsal border of the prearticular dorsal flange facet. The postdentary trough is deep throughout the fragment preserved, without a diagonal ridge on the floor. Possibly all this part of the postdentary trough is represented by “adductor fossa”. There is no groove for the replacing dental lamina, or coronoid attachment area visible. There is breakage ventral to the anterior alveolus of the ultimate molar which exposes the molar root. There is no pseudangular process preserved and the ventral border of the dentary is convex throughout the entire fragment, without a concavity which should be anterior to the pseudangular process. This might mean that the pseudangular process was located relatively far posterior from the preserved posterior end. The masseteric fossa is shallower and the coronoid crest is weaker than in PIN 5087/1, but this might be related to a prominent water abrasion of PM TGU 200/3−BR−1.

Mammalia indet.

Fig. 5 .

Locality and horizon: Berezovsk Quarry, Krasnoyarsk Territory, Russia; Upper Member of Itat Formation, Bathonian, Middle Jurassic.

Material.—PM TGU 200/3−BR−2, tooth.

Description.—PM TGU 200/3−BR−2. The crown is 0.6 mm long, 0.3 mm width, and 0.6 mm high. The complete tooth is 1.5 mm high. The crown is single−cusped, with a small posterior denticle and without an anterior denticle. The bulbous anterior side of the crown projects anteriorly beyond the root. One crown side (labial?) is little more convex than the opposite side. There is no cingulum. There is a single long and robust root, without any trace of subdivision. Its distal end is narrowed before a distalmost widening.

Figures

Fig. 2. Docodonta indet., Berezovsk Quarry, Krasnoyarsk Territory, Russia; Itat Formation, Middle Jurassic. Damaged bone is shown in grey. A. PIN 5087/1, right dentary fragment, occlusal view (A1, stereopair) and outline drawing (A2). B. PM TGU 200/3−BR−1, right dentary fragment, occlusal view (B1, stereopair) and outline drawing (B2). Anterior is to the top.

Fig. 3. Docodonta indet., Berezovsk Quarry, Krasnoyarsk Territory, Russia; Itat Formation, Middle Jurassic. Damaged bone is shown in dark grey, area occupied by Meckel’s cartilage and postdentary is shown in light grey. PIN 5087/1, right dentary fragment, labial view (A, stereopair), lingual view (C, stereopair), and outline drawings (B, D). Anterior is to the bottom (A, B) and top (C, D).

Fig. 4. Docodonta indet., Berezovsk Quarry, Krasnoyarsk Territory, Russia; Itat Formation, Middle Jurassic. Damaged bone is shown in dark grey, area occupied by Meckel’s cartilage and the postdentary bones is shown in light grey. PM TGU 200/3−BR−1, right dentary fragment, labial view (A, stereopair), lingual view (C, stereopair), and outline drawings (B, D). Anterior is to the bottom (A, B) and top (C, D).

Fig. 5. Mammalia indet., Berezovsk Quarry, Krasnoyarsk Territory, Russia; Itat Formation, Middle Jurassic. PM TGU 200/3−BR−2, a tooth in lingual? (A) and labial? (B) views.