These two tiny monotremes tell on each other
Ideally, traditionally and currently
related taxa make their interrelationships known in phylogenetic analysis. Once known, each can then continue to inform on the other, helping to understand the dark corners of vague data.
Figure 1. Late Triassic Brasilitherium compared to Early Cretaceous Chaoyangodens. Two marsupials with odd tooth shapes and few molars.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/brasilitherium-588.jpg?w=188″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/brasilitherium-588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-88490″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/brasilitherium-588.jpg” alt=”Figure 1. Late Triassic Brasilitherium compared to Early Cretaceous Chaoyangodens. Two marsupials with odd tooth shapes and few molars.” width=”584″ height=”930″ />
Figure 1. Late Triassic Brasilitherium compared to Early Cretaceous Chaoyangodens. Two marsupials with odd tooth shapes and few molars.
Today
Late Triassic (225mya) Brasilitherium (Fig 1) informs on Early Cretaceous (140 mya) Chaoyangodens (Fig 1), where the canine is smaller than the canine-like first premolar (cyan). Note the double roots on all the premolars. The odd tooth shapes in late Triassic Brasilitherium are derived traits.
In that regard, humans and primates have traditional tooth shapes, in which the canine is the longest tooth and molar numbers exceed premolar numbers.
The above changes to scoring in the LRT did not affect monotreme tree topology.
According to Hou and Meng 2014,
the canine in Chaoyangodens was double-rooted. That hypothesis of identification comes into question here, especially in such a primitive (= Monotreme) taxon.
Brasilitherium riograndensis
(Bonaparte et al., 2003; Early Norian, Late Triassic, 208 mya) was considered a stem mammal, but here nests near the base of the monotreme mammals despite the not-quite mammalian jaw joint. This may explain why monotreme ear bones are below and medial to the jawline, but in other mammals they are posterior to the jaw joint. The long rostrum includes as many as seven premolars. It could use a good µCT scan to understand the buried skull.
Figure 1. Chaoyangodens lii in situ and restored skull in lateral view. This taxon is a monotreme basal to both the echidna and platypus.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/06/chaoyangodens_lii_insitu588.jpg?w=144″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/06/chaoyangodens_lii_insitu588.jpg?w=491″ tabindex=”0″ role=”button” class=”size-full wp-image-31743″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/06/chaoyangodens_lii_insitu588.jpg” alt=”Figure 1. Chaoyangodens lii in situ and restored skull in lateral view. This taxon is a monotreme basal to both the echidna and platypus.” width=”584″ height=”1218″ />
Figure 2. Chaoyangodens lii in situ and restored skull in lateral view. This taxon is a monotreme basal to both the echidna and platypus. It could use a good µCT scan to understand the buried skull.
Chaoyangodens lii
(Hou and Meng 2014; early Cretaceous, 120mya) was considered a eutriconodont mammal with a reduced post-canine tooth formula and four incisors. The fifth tooth is a tiny canine. Here this specimen nests in the monotreme clade with Brasilitherium. The premolars are large for the skull. The scapula is perforated. The pelvis is unfused and simple.
Figure 3. The dentition of Lactodens and Feredocodon here reidentified with DGS colors. Now there is no canine.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/lactodens.feredocodon.teeth588.jpg?w=272″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/lactodens.feredocodon.teeth588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-88499″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/lactodens.feredocodon.teeth588.jpg” alt=”Figure 3. The dentition of Lactodens and Feredocodon here reidentified with DGS colors. Now there is no canine. ” width=”584″ height=”644″ />
Figure 3. The dentition of Lactodens and Feredocodon here reidentified with DGS colors. Now there is no canine.
Revised thinking on the dentition of Lactodens and Feredocodon.
Since canines have only one root, the ‘double root canines’ of Lactodens and Feredocodon are here (Fig 3) re-idenitified as premolars. Canines are absent.
References
Bonaparte JF, Martinelli AG, Schultz CL and Rubert R 2003. The sister group of mammals: small cynodonts from the Late Triassic of Southern Brazil. Revista Brasileira de Paleontologia 5:5-27.
Hou S and Meng J 2014. A new eutriconodont mammal from the early Cretaceous Jehol Biota of Liaoning, China. Chinese Science Bulletin. 59 (5–6): 546–553
wiki/Chaoyangodens
wiki/Kuehneotherium
Source: https://pterosaurheresies.wordpress.com/2024/09/02/these-two-tiny-monotremes-tell-on-each-other/
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