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We report a systematic study of all known methyl carbon chains toward TMC-1using the second data release of the GOTHAM survey, as well as a search forlarger species. Using Markov-Chain Monte Carlo simulations and spectral linestacking of over 30 rotational transitions, we report statistically significantemission from methylcyanotriacetylene (CH$_3$C$_7$N) at a confidence level of4.6$\sigma$, and use it to derive a column density of ${\sim}$10$^{11}$cm$^{-2}$. We also searched for the related species, methyltetraacetylene(CH$_3$C$_8$H), and place upper limits on the column density of this molecule.By carrying out the above statistical analyses for all other previouslydetected methyl-terminated carbon chains that have emission lines in oursurvey, we assess the abundances, excitation conditions, and formationchemistry of methylpolyynes (CH3C$_{2n}$H) and methylcyanopolyynes(CH3C$_{2n-1}$N) in TMC-1, and compare those with predictions from a chemicalmodel. Based on our observed trends in column density and relative populationsof the A and E nuclear spin isomers, we find that the methylpolyynes andmethylcyanopolyynes families exhibit stark differences from one another,pointing to separate interstellar formation pathways, which is confirmedthrough gas-grain chemical modeling with nautilus.

CH3-Terminated Carbon Chains in the GOTHAM Survey of TMC-1: Evidence of Interstellar CH3C7N